Unit Converter
One interface for the six conversion categories you actually reach for: length (meters, feet, miles), weight (kilograms, pounds, stones), volume (liters, gallons, cups), time, speed and area. Pick a category, enter a value and swap source and target units in a click — all factors are hard-coded from the BIPM SI definitions so round-trips do not drift.
How to convert units
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1
Choose a category
Length, weight, volume, time, speed or area — each loads its own set of units.
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2
Enter the value
Decimals and scientific notation both work: 12.5, 1e6, 0.0254.
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3
Pick source and target
The dropdowns default to the two most common units; hit swap to invert them.
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4
Read the converted value
The result updates on every keystroke and trims trailing zeros for readability.
Conversion factors used by this tool
Every converter stores units against one base per category. Your value is multiplied into base units and then divided into the target, which keeps round-trips accurate to the factor’s precision.
Length (base: meter)
| Unit | Factor to meter |
|---|---|
| kilometer | 1000 |
| centimeter | 0.01 |
| millimeter | 0.001 |
| inch | 0.0254 (exact) |
| foot | 0.3048 (exact) |
| yard | 0.9144 (exact) |
| mile | 1609.344 (exact) |
Weight / mass (base: gram)
| Unit | Factor to gram |
|---|---|
| kilogram | 1000 |
| milligram | 0.001 |
| tonne (metric) | 1,000,000 |
| ounce (avdp) | 28.3495 |
| pound (avdp) | 453.592 |
| stone | 6350.29 |
Volume (base: liter)
| Unit | Factor to liter |
|---|---|
| milliliter | 0.001 |
| cubic meter | 1000 |
| US gallon | 3.78541 |
| US quart | 0.946353 |
| US pint | 0.473176 |
| US cup | 0.236588 |
| US fluid ounce | 0.0295735 |
Notes on gotchas
- US vs UK gallons differ by ~20%. This tool uses US customary units; pick the length converter alone if your recipe or spec uses imperial gallons.
- Temperature is not a linear unit. It needs an offset, not just a factor, so it lives in a dedicated temperature tool rather than this converter.
- Year length. Used as 365.25 days (Julian year) for stable astronomical math — not 365 and not 365.2425.
Frequently Asked Questions
Volume units default to the US system (gallon, quart, pint, cup, fl oz). Length and weight units like foot, inch and pound are identical in both systems, so no ambiguity there.
Because the international foot has been defined as exactly 0.3048 m since 1959. Many unit conversions are defined rather than measured, which is why round-trips like foot → meter → foot come back as the same value.
Do it in one step. Pick the speed category, set the source to km/h and the target to m/s — the tool uses a single base unit so you avoid rounding errors that accumulate in multi-step conversions.
Internally the math uses 64-bit floats, which give about 15–17 significant decimal digits. The display trims trailing zeros but you can rely on results to around 1 part in 10^14.