Metric conversion means one of two things: changing between metric units, which only moves the decimal point because every step is a power of 10 (1 km = 1000 m, 1 m = 100 cm), or changing between metric and imperial/US units, which needs a fixed factor (1 inch = 2.54 cm, 1 kg = 2.2046 lb). The two converters below handle both, and the charts underneath cover the conversions people look up most.
Metric Prefix Converter
Metric to Imperial Converter
For engineering quantities such as pressure, torque, power, flow or viscosity, use the full engineering unit converter.
Metric prefix chart
Every metric unit is built from a base unit (metre, gram, litre, second and so on) plus a prefix that multiplies it by a power of 10. Learn the prefixes once and you can convert any metric unit.
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| quetta | Q | 1030 | mass of Jupiter ≈ 1.9 quettagrams |
| ronna | R | 1027 | mass of Earth ≈ 6 ronnagrams |
| yotta | Y | 1024 | |
| zetta | Z | 1021 | global data stored, in zettabytes |
| exa | E | 1018 | exaflop supercomputers |
| peta | P | 1015 | petabyte |
| tera | T | 1012 | 1 TB drive |
| giga | G | 109 | 1 GW power station, 2.4 GHz Wi-Fi |
| mega | M | 106 | 1 MW, 1 MPa |
| kilo | k | 103 = 1000 | 1 km, 1 kg |
| hecto | h | 102 = 100 | hectare (100 m × 100 m) |
| deca | da | 101 = 10 | rarely used |
| (base unit) | 1 | metre, gram, litre | |
| deci | d | 10−1 = 0.1 | decibel, decilitre |
| centi | c | 10−2 = 0.01 | 1 cm |
| milli | m | 10−3 = 0.001 | 1 mm, 1 mL, 1 mg |
| micro | µ | 10−6 | 1 µm (micron), 1 µF |
| nano | n | 10−9 | 3 nm chip process, 1 ns |
| pico | p | 10−12 | 100 pF capacitor |
| femto | f | 10−15 | femtosecond lasers |
| atto | a | 10−18 | |
| zepto | z | 10−21 | |
| yocto | y | 10−24 | |
| ronto | r | 10−27 | |
| quecto | q | 10−30 | mass of an electron ≈ 911 quectograms |
The four newest prefixes (ronna, quetta, ronto, quecto) were adopted by the General Conference on Weights and Measures in November 2022, the first additions since 1991. Prefix symbols are case-sensitive: m is milli but M is mega, so 5 mW and 5 MW differ by a factor of a billion.
How to convert metric units: the ladder method
For the everyday prefixes, picture a ladder with kilo at the top and milli at the bottom:
kilo → hecto → deca → base unit → deci → centi → milli
(memory aid: King Henry Died By Drinking Chocolate Milk)
Count the steps between the two units. Going down the ladder, move the decimal point right that many places (the number gets bigger, because the units are smaller). Going up, move it left.
| Convert | Steps | Working | Answer |
|---|---|---|---|
| 3.5 km to m | 3 down | 3.5 → 35 → 350 → 3500 | 3500 m |
| 250 mg to g | 3 up | 250 → 25.0 → 2.50 → 0.250 | 0.25 g |
| 4.2 L to mL | 3 down | 4.2 × 1000 | 4200 mL |
| 75 cm to m | 2 up | 75 ÷ 100 | 0.75 m |
| 0.075 kg to g | 3 down | 0.075 × 1000 | 75 g |
Beyond the ladder, use exponents instead of counting steps. To convert 5 µm to mm, subtract the exponents: micro is 10−6 and milli is 10−3, so the difference is −6 − (−3) = −3. Multiply by 10−3: 5 µm = 0.005 mm. The prefix converter above shows this exponent working for every conversion.
Square and cubic metric units
The ladder rule changes for area and volume. Each step for area moves the decimal point two places, and each step for volume moves it three. So 1 m² = 10,000 cm² (2 steps × 2 = 4 places), and 1 m³ = 1,000,000 cm³ (2 steps × 3 = 6 places).
How litres, cubic centimetres and kilograms connect
The metric system was designed so that volume and mass link neatly through water:
- 1 litre = 1 cubic decimetre (dm³) = 1000 cm³
- 1 millilitre = 1 cm³ = 1 cc, so a 150 cc engine displaces 150 mL
- 1 m³ = 1000 litres
- 1 litre of pure water has a mass of almost exactly 1 kg (about 0.99997 kg at 4 °C), so 1 mL of water ≈ 1 g
This link is why a 1000-litre overhead tank full of water weighs about one tonne, a figure you need before placing it on a roof slab.
Metric to imperial conversion charts
Length
| Metric to imperial | Imperial to metric |
|---|---|
| 1 mm = 0.03937 in | 1 in = 25.4 mm (exact) |
| 1 cm = 0.3937 in | 1 in = 2.54 cm (exact) |
| 1 m = 39.37 in = 3.281 ft = 1.094 yd | 1 ft = 30.48 cm = 0.3048 m (exact) |
| 1 km = 0.6214 mi | 1 yd = 0.9144 m (exact) |
| 1 mi = 1.609344 km (exact) |
Mass (weight)
| Metric to imperial | Imperial to metric |
|---|---|
| 1 g = 0.03527 oz | 1 oz = 28.35 g |
| 1 kg = 2.2046 lb | 1 lb = 0.45359237 kg (exact) |
| 1 tonne = 1000 kg = 1.1023 US (short) tons | 1 stone = 14 lb = 6.350 kg |
| 1 tonne = 0.9842 UK (long) tons | 1 US ton = 907.2 kg; 1 UK ton = 1016 kg |
Volume
| Metric to US / UK | US / UK to metric |
|---|---|
| 1 mL = 0.0338 US fl oz | 1 US fl oz = 29.57 mL; 1 UK fl oz = 28.41 mL |
| 1 L = 0.2642 US gal = 0.2200 UK gal | 1 US gal = 3.785 L; 1 UK gal = 4.546 L |
| 1 L = 1.057 US quarts | 1 US cup = 236.6 mL (a metric cup is 250 mL) |
| 1 m³ = 35.31 ft³ | 1 ft³ = 28.32 L |
The US and UK gallon are different: a UK gallon is about 20% larger. Recipes and fuel figures from the two countries are not interchangeable.
Temperature
| °C | °F | What it is |
|---|---|---|
| −40 | −40 | where the two scales meet |
| 0 | 32 | water freezes |
| 20 | 68 | room temperature |
| 37 | 98.6 | normal body temperature |
| 100 | 212 | water boils (at 1 atm) |
| 180 | 356 | common oven setting |
Formulas: °F = °C × 9/5 + 32, and °C = (°F − 32) × 5/9. Quick mental estimate: double the Celsius value and add 30 (20 °C → 70 °F, true value 68 °F).
Area and speed
| Conversion | Value |
|---|---|
| 1 m² | 10.764 ft² |
| 1 hectare | 10,000 m² = 2.471 acres |
| 1 acre | 4046.86 m² = 0.4047 ha |
| 1 km² | 100 ha = 0.3861 mi² |
| 100 km/h | 62.14 mph = 27.78 m/s |
| 1 mph | 1.609 km/h |
The seven SI base units and how they are defined
The modern metric system is the International System of Units (SI). Since 20 May 2019, all seven base units are defined by fixing the exact value of a constant of nature, so they no longer depend on any physical object. The kilogram, for example, used to be the mass of a platinum-iridium cylinder kept near Paris; it is now defined through the Planck constant.
| Quantity | Unit (symbol) | Defined by fixing |
|---|---|---|
| Time | second (s) | caesium-133 hyperfine frequency = 9,192,631,770 Hz |
| Length | metre (m) | speed of light c = 299,792,458 m/s |
| Mass | kilogram (kg) | Planck constant h = 6.62607015 × 10−34 J·s |
| Electric current | ampere (A) | elementary charge e = 1.602176634 × 10−19 C |
| Temperature | kelvin (K) | Boltzmann constant k = 1.380649 × 10−23 J/K |
| Amount of substance | mole (mol) | Avogadro constant NA = 6.02214076 × 1023 mol−1 |
| Luminous intensity | candela (cd) | luminous efficacy Kcd = 683 lm/W at 540 THz |
Everything else, such as the newton (kg·m/s²), pascal (N/m²) and watt (J/s), is a derived unit built from these seven.
Writing metric units correctly
Examiners and reviewers notice these. The rules come from the SI Brochure published by the BIPM and are repeated in national guides such as NIST SP 811.
| Wrong | Right | Rule |
|---|---|---|
| 5Km, 5 KM, 5 kms | 5 km | kilo is lowercase k; symbols take no plural s |
| 20kg | 20 kg | leave a space between number and unit (except °, ′, ″ for angles) |
| 10 mtr, 10 mts | 10 m | use the official symbol, not an abbreviation |
| 3 kg. | 3 kg | symbols are not abbreviations, so no full stop |
| 25 °K | 25 K | kelvin takes no degree sign |
| 5 mcg | 5 µg | the SI symbol for micro is µ (in medicine “mcg” is used on purpose to avoid misreading µ as m) |
| 1 ton (meaning 1000 kg) | 1 t or 1 tonne | “ton” is a US or UK unit; the metric one is the tonne |
Which countries use the metric system?
Almost every country uses the metric system officially. India adopted it through the Standards of Weights and Measures Act passed in December 1956, which took effect from 1 October 1958; old units were phased out by 1962. Today the Legal Metrology Act, 2009 governs weights and measures in trade, and the CSIR-National Physical Laboratory in New Delhi maintains India’s national measurement standards and Indian Standard Time.
The United States still uses US customary units in daily life (miles, pounds, gallons, °F), although science, medicine, the military and much of industry work in metric, and every US customary unit is legally defined in terms of SI units. The United Kingdom uses metric for most trade but keeps miles on road signs and pints for draught beer and milk. Myanmar and Liberia are often listed alongside the US, but both use metric units widely in practice.
Frequently asked questions
What is the easiest way to do metric conversions?
Within the metric system, count the prefix steps and move the decimal point: right when going to a smaller unit, left when going to a larger one. Between metric and imperial, multiply by a fixed factor such as 2.54 cm per inch or 0.4536 kg per pound.
How many centimetres are in an inch?
Exactly 2.54 cm. The inch has been defined as 25.4 mm since 1959, so this factor has no rounding error.
How do I convert kg to pounds?
Multiply by 2.2046. For example, 70 kg × 2.2046 = 154.3 lb. To go the other way, multiply pounds by 0.4536.
Is a millilitre the same as a cubic centimetre?
Yes. 1 mL = 1 cm³ = 1 cc, because 1 litre is defined as 1 cubic decimetre (1000 cm³).
What is the difference between metric and SI units?
SI is the modern, formally defined version of the metric system, with seven base units fixed by constants of nature. “Metric” is the everyday name and also covers accepted non-SI units such as the litre, tonne, hectare and bar.
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