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Unit Converter

Convert length, weight, volume, area, speed, and data — with two-way input and instant results.

Millimeter (mm)
MM
Centimeter (cm)
CM
Meter (m)
M
Kilometer (km)
KM
Inch (in)
IN
Foot (ft)
FT
Yard (yd)
YD
Mile (mi)
MI

How the Unit Converter Works

Select a category (Length, Weight, Volume, Area, Speed, or Data), then choose your source and target units from the dropdowns. Enter a value in either field — changing the top value updates the bottom, and vice versa. Click the swap button (↔) to flip the conversion direction.

All conversions are based on exact conversion factors. Results are displayed with up to 10 significant figures, using scientific notation for very large or very small values.

A Brief History of Measurement Systems

The metric system was created during the French Revolution in 1795 to replace a chaotic patchwork of local units that varied from town to town across Europe. Before metrication, a 'foot' in Paris wasn't the same length as a 'foot' in London or Amsterdam. The beauty of the metric system is its simplicity: everything is based on powers of 10, so conversions are just a matter of moving a decimal point rather than memorising arbitrary ratios like 12 inches to a foot or 5,280 feet to a mile.

The United States is one of only three countries — alongside Myanmar and Liberia, that haven't officially adopted metric for everyday use, though metric is standard in US science, medicine, the military, and international trade. Canada officially adopted metric in 1970, but in practice it's a fascinating hybrid: Canadians typically know their height in feet and inches and their weight in pounds, but their temperature in Celsius and their speed in km/h. That cultural mix is exactly why unit converters stay useful even in countries that have 'gone metric.'

When Precision Matters

Unit conversion errors can range from mildly annoying to catastrophic. The most famous example is NASA's Mars Climate Orbiter, which crashed in 1999 because one engineering team used metric units (newton-seconds) while another used imperial units (pound-force-seconds) for the same thruster data. The $125 million spacecraft burned up in Mars's atmosphere. Turns out the single root cause was a failure to convert between unit systems — a mistake that a simple converter would've prevented.

In everyday life, precision matters most in cooking (baking is basically applied chemistry — too much or too little of an ingredient changes the whole reaction), construction (building codes specify tolerances in millimetres), and medicine (drug dosages are calculated in mg/kg of body weight, and a factor-of-2 error can be dangerous). For casual use — estimating drive distances or checking if furniture fits — rough mental shortcuts work fine: 1 inch is roughly 2.5 cm, 1 kg is roughly 2.2 lbs, 1 mile is roughly 1.6 km.

All units at once, two new categories, and pinned favorites

Editing any field now updates every other unit in the category simultaneously — not just the pair you selected. Type a value in kilometres and centimetres, miles, inches, and all the rest fill in at once. Two categories are new: Energy (J, kJ, cal, kcal, Wh, kWh, BTU, eV) and Time (ms, s, min, h, day, week, month, year). Time is approximate for month and year — a month is treated as 30.4375 days and a year as 365.25 days, which covers the common use cases. Starred categories go to the top of the selector. Pin the ones you use daily and stop scrolling past Length to get to Energy.

Recent conversions — last 5 — are stored in localStorage and shown as a history list you can tap to restore. Clear the history with one button. Pinned favorites also persist in localStorage between visits. What isn't here: temperature (that's its own tool), currency rates, or custom unit definitions.

When to Use a Unit Converter

Convert units for international recipes (US cups vs. metric grams), DIY projects (imperial lumber dimensions vs. metric plans), scientific calculations (SI units), travel (miles to km, Fahrenheit to Celsius, gallons to litres), fitness (lbs to kg, inches to cm), and medicine (mg/kg dosing). Canada is officially metric but uses a mix in daily life: height in feet/inches, weight in pounds, temperature in Celsius, distance in km, and lumber in inches. Try our Cooking Converter, Temperature Converter, Square Footage Calculator, or File Size Converter for specialized needs.

Metric vs. Imperial: Common Conversions

Key reference conversions:
Length: 1 inch = 2.54 cm. 1 foot = 30.48 cm. 1 mile = 1.609 km.
Weight: 1 lb = 453.6 g. 1 oz = 28.35 g. 1 kg = 2.205 lbs.
Volume: 1 US cup = 236.6 mL. 1 US gallon = 3.785 L. 1 fl oz = 29.57 mL.
Area: 1 sq ft = 0.0929 m². 1 acre = 4047 m² = 0.405 hectares.

The US is one of only three countries (with Myanmar and Liberia) that hasn't officially adopted the metric system as its primary measurement standard.

Frequently Asked Questions

How accurate are the conversions?
The conversions use precise SI-defined factors. For most everyday uses, results are accurate to at least 6 significant figures. Some units like the pound and mile have exact defined values (1 lb = 453.592 g exactly).
What does the Data category convert?
The Data category converts between bits, bytes, and decimal prefixes (KB = 1,000 bytes, MB = 1,000,000 bytes, etc.). Note: this uses SI decimal prefixes, not binary (1 KB = 1,000 B, not 1,024 B).
Can I convert Celsius to Fahrenheit here?
Temperature conversion requires a formula (not just a multiplication factor), so it's handled by a dedicated Temperature Converter tool. Check the related tools below.
What units are included?
Length: mm, cm, m, km, inch, foot, yard, mile. Weight: mg, g, kg, oz, lb, stone. Volume: ml, L, fl oz (US), cup, pint, gallon. Area: mm², cm², m², km², in², ft², yd², acre, hectare. Speed: m/s, km/h, mph, knots. Data: bit, byte, KB, MB, GB, TB, PB.
What's the difference between metric and imperial?
The metric system (SI) uses base units — metre, kilogram, litre — with decimal prefixes (kilo = ×1,000, centi = ×0.01, milli = ×0.001). This makes scaling between units trivial. The imperial system uses historically derived units with irregular ratios: 12 inches per foot, 3 feet per yard, 1,760 yards per mile, 16 ounces per pound. Imperial units are still commonly used in the United States, and partially in the UK and Canada for everyday measurements like body weight and height.
Why don't all countries just use the same units?
Unit systems reflect historical trade routes, colonial heritage, and the difficulty of coordinating change across millions of people. The UK officially converted to metric in the 1960s–1970s but still uses miles on road signs, pints for beer, and stone for body weight. Canada metricated in the 1970s but kept a cultural mix. The US tried metrication in the 1970s but met strong public resistance, leaving the country in a hybrid state — science uses metric, everyday life uses customary. Globalization has pushed slow convergence though — most packaging now shows both.
Why does Canada use both metric and imperial units?
Canada officially adopted the metric system in the 1970s, but the transition was incomplete. Proximity to the US means Canadians still encounter imperial units in construction (2×4 lumber), body measurements (height in feet, weight in pounds), and some food packaging. Young Canadians tend to use metric exclusively; older generations often switch between both systems depending on context.
What is the difference between a US gallon and an imperial gallon?
A US gallon = 3.785 litres. An imperial gallon (used in the UK and historically in Canada) = 4.546 litres — about 20% larger. Canada switched to litres for fuel in 1979. When a US recipe calls for '1 gallon', use 3.785 L, not 4.546 L. This difference matters significantly for large-volume recipes or fuel consumption comparisons.

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