Free Universal Unit Converter: Length, Weight, Volume, Temperature & More
Convert any value across seven categories — length, weight, volume, temperature, area, speed, time — with NIST-exact conversion factors and an all-units snapshot for every input. 100% client-side.
Seven Categories
Length, weight, volume, temperature, area, speed, and time. Switch with one tab — no dropdown digging, no separate calculator per category.
NIST-Exact Factors
We use the exact internationally-agreed multipliers (1 mile = 1609.344 m, NOT 1609 or 1600). Temperature uses true offset transforms, not linear approximations.
All-Units Snapshot
Every input shows the result in EVERY other unit of that category in one grid — perfect for cross-referencing without re-typing for each target unit.
100% Client-Side
Conversion math runs in your browser. Measurements stay on your device — useful for cooking recipes, lab data, and any value you do not want logged.
The Unit Converter That Uses the Exact Numbers
Unit conversion is the second-most-Googled arithmetic operation after percent calculations (and the first if you count cooking-recipe sub-queries). Yet most online converters cut corners on the factors: 1 mile becomes 1609 (correct: 1609.344), 1 inch becomes 2.5 cm (correct: 2.54), and temperature gets a linear approximation that fails at 0°C. Our Free Online Universal Unit Converter uses the exact NIST / BIPM internationally-agreed factors across seven categories, handles temperature with proper offset transforms, and shows you the result in every other unit of the category at once.
Pair this converter with our Percentage Calculator (for unit-tax and discount math), the Age Calculator (for date-based unit math), the Tip Calculator (everyday-utility sibling), and the Number Base Converter (for non-decimal numerical bases).
The Seven Categories & Their Base Units
| Category | SI Base Unit | Units We Support |
|---|---|---|
| Length | metre (m) | mm, cm, m, km, in, ft, yd, mi, nautical mile |
| Weight / Mass | kilogram (kg) | mg, g, kg, t, oz, lb, stone, US ton |
| Volume | litre (L) | mL, L, m³, tsp, tbsp, fl oz, cup, pt, qt, gal (US/UK) |
| Temperature | Celsius (°C) | °C, °F, K (offset transforms) |
| Area | square metre (m²) | mm², cm², m², ha, km², in², ft², yd², acre, mi² |
| Speed | m/s | m/s, km/h, mph, ft/s, knot, Mach |
| Time | second (s) | ms, s, min, h, day, week, month (avg), year (avg) |
Where Unit Conversion Has Famously Gone Wrong
Mars Climate Orbiter (1999)
$327M spacecraft crashed into Mars because one team used pound-force-seconds and another used newton-seconds. Different units, same number, no conversion done. NASA report: "the root cause was the failed translation of English units to metric."
Gimli Glider (1983)
Air Canada Boeing 767 ran out of fuel at 41,000 ft because the fueller used pounds where the crew expected kilograms. Pilot glided to an unused runway. No fatalities, but a perfect "always specify your units" case study.
Recipe Disasters
A US recipe calling for "1 cup" in the UK is 16.5% smaller than expected (US cup = 237 mL, UK cup = 284 mL). Enough to ruin batter ratios on the first attempt.
Temperature Linear-Approximation Bugs
Some calculators forget the +32 offset for °C-to-°F conversion, getting answers ~32° too low. We always apply the offset transform.
Conversions Worth Memorising
| If you know | Multiply by | To get |
|---|---|---|
| Inches | 2.54 | Centimetres |
| Feet | 0.3048 | Metres |
| Miles | 1.609 | Kilometres |
| Pounds | 0.4536 | Kilograms |
| Kilograms | 2.2046 | Pounds |
| US gallons | 3.785 | Litres |
| Square feet | 0.0929 | Square metres |
| Acres | 4047 | Square metres |
| mph | 1.609 | km/h |
| Knots | 1.852 | km/h |
Notice that mph → km/h is the same multiplier as miles → km (1.609) — because dividing both sides by "per hour" cancels the unit. That symmetry is a useful sanity check when you need to convert quickly without a calculator.