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.
Frequently Asked Questions
How accurate are the conversions?
What does the Data category convert?
Can I convert Celsius to Fahrenheit here?
What units are included?
What's the difference between metric and imperial?
Why don't all countries just use the same units?
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