A unit converter rescales a measurement from one unit to another without changing the physical amount. For length, 1 inch = 2.54 cm, 1 foot = 0.3048 m and 1 mile = 1.609344 km. For weight, 1 pound = 0.45359237 kg and 1 kg ≈ 2.20462 lb. Both are linear — convert through a base unit (the metre or the kilogram). This platform converts 24 length and 18+ weight units instantly, with tables, ranges, bulk conversion, modes and a reference library.
Key Takeaways
- A unit converter rescales a measured quantity from one unit to another without changing the underlying physical amount.
- Length and weight are both linear: convert by going through a base unit — the metre for length, the kilogram (or gram) for weight.
- 1 inch = 2.54 cm, 1 foot = 0.3048 m, 1 mile = 1.609344 km, and 1 nautical mile = 1852 m — all exact by definition.
- 1 pound = 0.45359237 kg exactly, 1 kg ≈ 2.20462 lb, 1 ounce = 28.349523125 g, and 1 stone = 14 lb.
- The SI base unit of length is the metre and of mass is the kilogram; the metric system is decimal, so units scale by powers of ten.
- The imperial/US customary system uses inches, feet, yards and miles for length and ounces, pounds and tons for weight.
- Astronomical distances use the astronomical unit (Earth–Sun distance), the light year and the parsec (~3.26 light years).
- A light year measures distance, not time — it is how far light travels in one year (~9.46 trillion km).
- This platform converts 24 length units and 18+ weight units with exact factors, plus tables, ranges, bulk conversion, modes and a reference library — all in your browser.
People Also Ask
How many feet are in a meter?
There are about 3.28084 feet in one metre. To convert metres to feet, multiply by 3.28084; to convert feet to metres, multiply by 0.3048.
How many centimeters are in an inch?
Exactly 2.54 centimetres are in one inch. To convert inches to cm multiply by 2.54; to convert cm to inches divide by 2.54.
How many pounds are in a kilogram?
One kilogram equals about 2.20462 pounds. To convert kg to lb multiply by 2.20462; to convert lb to kg multiply by 0.45359237.
How many kilometers are in a mile?
One mile equals exactly 1.609344 kilometres. To convert miles to km multiply by 1.609344; to convert km to miles multiply by 0.621371.
What is the SI unit of length and mass?
The SI base unit of length is the metre (m) and the SI base unit of mass is the kilogram (kg).
How many grams are in an ounce?
One ounce equals 28.349523125 grams. To convert ounces to grams multiply by 28.3495; to convert grams to ounces divide by 28.3495.
What is a nautical mile?
A nautical mile is exactly 1852 metres — originally one minute of latitude. It is used in marine and air navigation and is about 1.15 statute miles.
How many kilograms are in a stone?
One stone equals 14 pounds, which is about 6.35029 kilograms. To convert stone to kg multiply by 6.35029.
What Is Unit Conversion?
Unit conversion is the process of expressing a measured quantity in a different unit while keeping the physical amount it represents exactly the same. When you say a table is 1.5 metres long or 59 inches long, you are describing the identical length with two different rulers. Conversion is simply the arithmetic that lets you move between those rulers reliably and accurately, and it is one of the most common calculations in science, engineering, trade and everyday life.
Every conversion rests on a known relationship between the two units — a conversion factor. Because length and weight are linear quantities, that factor is a single multiplier: to turn metres into feet you multiply by 3.28084, and to turn feet back into metres you multiply by 0.3048. The cleanest way to handle many units at once is to route everything through one agreed base unit (the metre for length, the kilogram or gram for mass), convert the input into that base, then convert the base into whatever target you want. That is exactly how this platform computes every result, which keeps it accurate to roughly fifteen significant figures.
The Metric System Explained
The metric system is a decimal system of measurement built around a small set of base units and a consistent ladder of prefixes. For length the base is the metre; for mass it is the gram (with the kilogram being the SI base unit). Prefixes such as kilo- (×1000), centi- (÷100), milli- (÷1000), micro- (÷1,000,000) and nano- (÷1,000,000,000) scale those base units up and down by powers of ten, so converting within the system is as simple as moving a decimal point.
This decimal regularity is the metric system’s great strength and the reason it is used by science and by almost every country in the world. There are no awkward factors of 12 or 16 to remember — a kilometre is exactly 1000 metres, a centimetre is exactly one hundredth of a metre, and a kilogram is exactly 1000 grams. The system was born of the French Revolution in the 1790s and has since evolved into the International System of Units (SI), the modern, globally agreed standard for measurement.
The Imperial & US Customary Systems Explained
The imperial system, and the closely related US customary system, descend from older English units and remain in everyday use in the United States and, partially, in the United Kingdom. For length they use the inch, foot (12 inches), yard (3 feet) and mile (5280 feet); for weight they use the ounce, pound (16 ounces), stone (14 pounds, UK only) and ton. Unlike the metric system these units scale by irregular factors, which is why a reliable converter is so useful.
Although imperial and US customary units share most names, a few differ — most famously volume units like the gallon, and the ton (the US short ton is 2000 pounds while the UK long ton is 2240 pounds). Crucially, the core length and weight units were harmonised with the metric system in 1959: the international inch was defined as exactly 2.54 cm and the pound as exactly 0.45359237 kg. That international agreement is what makes precise imperial-to-metric conversion possible, and it is the basis for every factor in this tool.
SI Units Explained
The International System of Units (SI) is the modern form of the metric system and the world’s most widely used measurement framework. It defines seven base units, of which the metre (length) and the kilogram (mass) are the two most relevant here, alongside the second, ampere, kelvin, mole and candela. Every other unit — speed, force, energy and so on — is derived from these base units, giving science a single coherent language.
Modern SI definitions are remarkably profound: since 2019 the kilogram has been defined in terms of the Planck constant rather than a physical metal cylinder, and the metre is defined by the speed of light (the distance light travels in 1/299,792,458 of a second). These definitions are reproducible in any properly equipped laboratory on Earth, which is what guarantees that a metre measured in Tokyo is identical to a metre measured in Toronto. When a converter advertises “SI accuracy”, this is the standard it is honouring.
Length Units Guide
Length units span an astonishing range, from the picometre (a trillionth of a metre, the scale of atoms) up to the parsec (tens of trillions of kilometres, the scale of the galaxy). The metric ladder — millimetre, centimetre, metre, kilometre — covers most everyday and scientific needs, while micrometre and nanometre serve microscopy and electronics. The imperial family — inch, foot, yard, mile — dominates American daily life and construction.
Beyond these are specialised units worth knowing. The nautical mile (1852 m) is used at sea and in the air because it corresponds to one minute of latitude. Surveyors use the rod, chain and furlong, and the United States retains a slightly different “survey foot” (1200/3937 m) for legacy land records. Historical units such as the cubit and the hand survive in niche contexts — the hand, exactly four inches, is still how horse height is measured. This converter supports all of these so you can move freely between scientific, navigational, surveying and historical measures.
Weight Units Guide
Weight (more precisely, mass) units also cover a wide spectrum. The metric ladder runs from the milligram through the gram and kilogram to the tonne (metric ton, 1000 kg). The avoirdupois system used in commerce gives us the grain, dram, ounce, pound, stone, hundredweight and ton. Because the pound is defined as exactly 0.45359237 kg, all of these convert to metric with full precision.
Two specialised families deserve mention. The troy system — troy ounce, troy pound and pennyweight — is used for precious metals; a troy ounce (31.1035 g) is noticeably heavier than the ordinary avoirdupois ounce (28.3495 g), which is why gold prices can confuse the unwary. The carat (exactly 200 mg) measures gemstones, and the atomic mass unit (dalton) measures atoms and molecules. Engineers also meet the slug, the imperial unit of mass in the foot-pound-second system. All are included here, grouped by system so you always pick the right one.
Scientific Units & Precision
Scientific work demands both an extreme range of magnitudes and careful control of precision. At the small end, the nanometre and picometre describe wavelengths and atomic spacings, while the atomic mass unit captures the mass of individual particles. At the large end, astronomical units, light years and parsecs describe distances that would be meaningless in metres. Scientific notation — writing 9.461 × 10¹⁵ m instead of a sixteen-digit number — is essential for readability, and this tool offers it alongside engineering notation.
Precision matters as much as range. A measurement reported to too few digits loses information; one reported to too many implies a false certainty. This converter lets you choose anywhere from zero to fifteen-plus significant figures and switch between standard and scientific notation, so a laboratory result can carry its full precision while an everyday answer stays clean and readable. Because the engine uses exact internationally-defined factors and IEEE-754 double-precision arithmetic, the only meaningful limit on accuracy is the precision you ask for.
Engineering & Construction Units
Engineering and construction sit at the boundary between metric and imperial, and getting conversions right is a safety as well as a cost issue. Builders routinely convert between feet-and-inches and millimetres, architects translate drawing scales, and civil engineers work with surveying units like the chain and rod that are baked into old property records. A single misplaced conversion can ruin a cut list or misposition a foundation.
The classic engineering pitfalls are mixing the US survey foot with the international foot (they differ by about two parts per million, which matters over miles of road), and confusing the short ton with the metric tonne when ordering materials. This platform keeps these units clearly labelled and grouped, generates printable conversion tables for site use, and shows the exact factor and step-by-step working behind every result so an engineer can verify it. The engineering and construction modes preset the most common pairings to speed up day-to-day work.
Astronomical Measurements
Astronomy works at scales where ordinary units break down. Distances within the Solar System are measured in astronomical units (AU), where one AU is the average Earth–Sun distance, defined as exactly 149,597,870,700 metres. Distances to stars and galaxies are measured in light years — the distance light travels in one year, about 9.46 trillion kilometres — and in parsecs, the unit professional astronomers prefer, equal to about 3.26 light years.
A common misconception is that a light year measures time; it does not — it is purely a distance. The parsec, meanwhile, comes from “parallax of one arcsecond” and arises naturally from how astronomers actually measure stellar distances. Converting between kilometres, AU, light years and parsecs makes the scale of the cosmos tangible: Neptune orbits at about 30 AU, the nearest star is roughly 4.24 light years (1.3 parsecs) away, and the Milky Way spans tens of thousands of light years. The astronomy mode in this tool is built to explore exactly these conversions.
Healthcare & Fitness Measurements
In health and fitness, weight conversion is a daily necessity. Much of the world records body weight in kilograms, the United States uses pounds, and the United Kingdom often uses stones and pounds — so a single person’s weight might be 70 kg, 154 lb or 11 stone 0 lb. Accurate conversion matters for tracking progress, setting goals and, in clinical settings, calculating weight-based medication doses where errors can be dangerous.
This is why the fitness and medical modes present the most relevant units together and to sensible precision. Body weight rarely needs more than one decimal place, while medication references may need milligram precision. (Any medical figures here are for general education only — clinical dosing must always follow professional guidance.) Pairing these conversions with related tools such as a BMI calculator gives a complete picture, which is why the platform links the two.
Shipping & Logistics Measurements
Logistics lives and dies by accurate measurement. Parcel and freight weights must be converted between kilograms and pounds for international shipping, and the difference between a metric tonne (1000 kg), a US short ton (2000 lb ≈ 907 kg) and a UK long ton (2240 lb ≈ 1016 kg) can change a freight quote significantly. Cargo dimensions and distances likewise cross between metric and imperial as goods move between countries.
Carriers also use volumetric (dimensional) weight, billing on whichever is greater of actual and volumetric weight — so converting dimensions and masses correctly directly affects cost. Sea and air navigation add the nautical mile and the knot. The shipping mode in this converter groups the metric, imperial and US units logistics teams need, and the bulk converter can process an entire manifest of weights or dimensions at once, exporting the results for use in a shipping system.
Conversion Formulas
Every length and weight conversion follows the same simple template because both quantities are linear. To convert a value, multiply by the source unit’s factor (its size in the base unit) and divide by the target unit’s factor: result = value × (fromFactor ÷ toFactor). For example, to convert 5 miles to kilometres, the mile’s factor is 1609.344 m and the kilometre’s is 1000 m, so 5 × (1609.344 ÷ 1000) = 8.04672 km.
The most useful factors are worth committing to memory: 1 in = 2.54 cm, 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 mi = 1.609344 km, 1 lb = 0.45359237 kg, 1 oz = 28.349523125 g, and 1 stone = 6.35029 kg. The Formula Explorer in this tool shows the exact factor, the human-readable formula and a step-by-step calculation for any pair you choose, so you can both get the answer and understand precisely how it was derived.
Common Conversion Examples
A handful of conversions come up constantly. In length: 1 metre ≈ 3.28 feet, 30 cm ≈ 11.8 inches, 6 feet = 1.8288 metres, 5 km ≈ 3.11 miles, and a marathon’s 42.195 km ≈ 26.22 miles. In weight: 1 kg ≈ 2.2 lb, 100 g ≈ 3.53 oz, 70 kg ≈ 154 lb ≈ 11 stone, and a 1-tonne car ≈ 2205 lb. Knowing a few anchor values lets you sanity-check any converter at a glance.
Cooking, travel and DIY produce their own staples: a recipe’s 250 g of flour ≈ 8.8 oz, a 5-foot-9 person is about 175 cm, and a 26-inch suitcase is about 66 cm tall. The reference library and conversion-table generator in this platform turn these into printable charts, while the range converter can produce a complete table — say, every kilogram from 1 to 100 in pounds — in a single step for repeated use.
Metric vs Imperial
The metric and imperial systems represent two different philosophies of measurement. Metric is decimal and coherent: units relate by powers of ten, prefixes are consistent across quantities, and arithmetic is easy. Imperial is rooted in human-scale, historical units — the foot, the pound, the inch — that are intuitive for everyday estimation but awkward to compute with because of their irregular ratios (12 inches to a foot, 16 ounces to a pound, 5280 feet to a mile).
Most of the world standardised on metric during the twentieth century, while the United States remains the most prominent holdout for everyday use, with the UK in a hybrid state (metric for most trade, imperial for road distances, beer and body weight). Neither system is “more accurate” — both are exactly defined — but metric’s decimal structure makes it dominant in science and international trade. The practical upshot is a constant need to convert between them, which is exactly what this platform is built to do instantly and exactly.
A Short History of Measurement
Measurement is as old as civilisation. Early units were based on the human body — the cubit (forearm), the foot, the hand, the digit — which made them intuitive but inconsistent, since bodies differ. Ancient empires from Egypt to Rome standardised units for trade and construction, and the Roman mile (mille passus, “a thousand paces”) is the ancestor of the modern mile. For millennia, though, units varied from town to town, hampering commerce and science.
The metric system, introduced in France in the 1790s, was a deliberate break from this chaos: a rational, decimal system tied to nature rather than to a king’s body. Over the next two centuries it spread worldwide and matured into the SI. The twentieth century brought international agreements — notably the 1959 definition of the yard and pound in metric terms — that finally made imperial and metric units exactly interconvertible. Today’s definitions, anchored to fundamental physical constants, are the culmination of this long quest for a universal, reproducible standard.
Global Measurement Standards
Measurement standards are maintained by an international infrastructure most people never see. The General Conference on Weights and Measures (CGPM) and the International Bureau of Weights and Measures (BIPM) in France govern the SI, while national metrology institutes — NIST in the United States, the NPL in the United Kingdom, and their counterparts worldwide — realise and disseminate the standards within each country. This system ensures a measurement is comparable anywhere on Earth.
For everyday and commercial purposes, this matters because it underpins fair trade, safety and interoperability: a bolt machined to a specification in one country fits a nut made in another, and a kilogram of produce is the same kilogram at every border. A trustworthy converter respects these standards by using the exact, internationally-agreed factors rather than rounded approximations — which is the commitment behind every number this platform produces.
Measurement Accuracy & Significant Figures
Accuracy and precision are different things. Accuracy is how close a value is to the true amount; precision is how finely it is expressed. A conversion can be perfectly accurate (using an exact factor) yet reported to any precision you choose. Significant figures communicate that precision: stating 2.2 lb implies less certainty than 2.20462 lb, and reporting more digits than your original measurement justifies is misleading.
Good practice is to carry full precision through a calculation and round only the final answer to match the precision of the input. This converter does exactly that — it computes with double-precision arithmetic and exact factors, then formats the result to the precision and notation you select. For most everyday use, three or four significant figures are plenty; for scientific or engineering work, the high-precision and scientific-notation options preserve every digit that matters.
Best Practices for Converting Units
A few habits prevent the most common and costly conversion mistakes. Always confirm which unit you actually have — an ounce can be avoirdupois or troy, a ton can be short, long or metric, and a foot can be international or US survey. Keep track of precision and round only at the end. Sanity-check results against a known anchor (1 kg ≈ 2.2 lb, 1 m ≈ 3.3 ft) to catch order-of-magnitude errors.
When converting many values, use a tool rather than doing each by hand: this platform’s bulk converter and table generator eliminate transcription errors and are far faster. For shareable or repeatable work, save presets and export the results. And when accuracy is critical — in engineering, medicine or trade — verify the factor and the step-by-step working, both of which this converter exposes, so you are never trusting a black box. Following these practices turns unit conversion from a source of bugs into a routine, reliable step.
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Frequently Asked Questions
What is a unit converter?
How do I convert length units?
How do I convert weight units?
How many feet are in a meter?
How many meters are in a foot?
How many centimeters are in an inch?
How many inches are in a centimeter?
How many kilometers are in a mile?
How many miles are in a kilometer?
How many millimeters are in an inch?
How many inches are in a foot?
How many feet are in a yard?
How many feet are in a mile?
How many yards are in a mile?
How many meters are in a kilometer?
How many centimeters are in a meter?
How many millimeters are in a meter?
What is a nautical mile?
What is the difference between a nautical mile and a mile?
What is a light year?
Is a light year a measure of time?
What is an astronomical unit (AU)?
What is a parsec?
How many kilometers are in a light year?
How many AU are in a light year?
How many light years are in a parsec?
What is a chain, furlong and rod?
What is a hand in measurement?
What is a cubit?
What is a league?
What is the US survey foot?
What is the difference between the survey foot and the international foot?
How many pounds are in a kilogram?
How many kilograms are in a pound?
How many grams are in an ounce?
How many ounces are in a gram?
How many ounces are in a pound?
How many grams are in a pound?
How many pounds are in a stone?
How many kilograms are in a stone?
What is a metric ton (tonne)?
What is the difference between a short ton, long ton and metric ton?
How many kilograms are in a metric ton?
How many pounds are in a metric ton?
What is a carat?
What is a troy ounce?
What is the difference between a troy ounce and a regular ounce?
What is a grain in weight?
What is a dram?
What is a slug?
What is an atomic mass unit?
What is a hundredweight?
What is the SI unit of length?
What is the SI unit of mass?
What is the difference between mass and weight?
Why do different countries use different measurement systems?
Which countries use the imperial system?
Is the metric or imperial system more accurate?
What is the metric system?
What is the imperial system?
How do I convert cm to feet and inches?
How do I convert feet and inches to cm?
How many cm is 5 feet 9 inches?
How do I convert kg to stones and pounds?
How many pounds is 70 kg?
How many kg is 150 pounds?
How many meters is a 100-yard distance?
How many km is a marathon?
How many feet is 10 meters?
How many cm is 12 inches?
How accurate is this unit converter?
Can I convert very large or very small numbers?
Can I convert many values at once?
Can I generate a conversion table?
Does this converter work offline and keep my data private?
Can I save my favourite conversions?
What export formats are supported?
What is scientific notation?
What is engineering notation?
What does “significant figures” mean?
How do I convert square feet to square meters?
Why is the kilogram the SI base unit and not the gram?
What is the difference between a US gallon and other volume issues?
How many grams are in a kilogram?
How many milligrams are in a gram?
How many ounces are in a kilogram?
How many pounds are in a stone in kg terms?
How do I convert miles per hour to km per hour?
What is the easiest way to estimate km to miles in my head?
What is the easiest way to estimate kg to pounds in my head?
How many cm are in 6 feet?
How many kg is 200 pounds?
How many feet is 1.8 meters?
How do I convert grams to pounds?
How do I convert tonnes to kg?
How do I convert mm to cm?
How do I convert km to meters?
What units does this tool support?
Can I use this converter for school or exams?
Is the kilogram-to-pound factor exact?
What is the difference between weight in physics and in everyday use?
How do I convert between metric and imperial quickly?
Built by the Omnitool Measurement Team
Our converters are used for millions of conversions a month, with a focus on scientific accuracy and clarity.
SI-standard factors
Every factor uses exact internationally-agreed definitions (1 in = 2.54 cm, 1 lb = 0.45359237 kg), giving ~15 significant figures of accuracy.
Last updated June 2026
Units, factors and content are reviewed regularly to stay accurate and aligned with current standards.
Accuracy & privacy: All conversions run entirely in your browser using exact, internationally-defined factors and double-precision arithmetic — nothing you enter is sent to a server. Medical and healthcare references are for general education only; always follow professional guidance for clinical decisions.
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