Metric vs imperial: the thirty-second version
The metric system (SI) scales by tens — 1 km = 1,000 m = 100,000 cm — and is the official standard nearly everywhere. Imperial/US customary units survive in daily life in the United States (and partially the UK), with feet, pounds, gallons, and Fahrenheit. Conversions between the systems are fixed by definition: the inch has been exactly 2.54 cm since 1959, the pound exactly 0.45359237 kg — so every conversion here is exact math, not approximation.
The anchors worth memorizing
| Conversion | Exact / precise value | Rough rule of thumb |
|---|---|---|
| 1 inch → cm | 2.54 cm (exact) | ≈ 2.5 cm |
| 1 foot → m | 0.3048 m (exact) | ≈ 30 cm |
| 1 mile → km | 1.609344 km (exact) | ≈ 1.6 km |
| 1 kg → lb | 2.20462 lb | ≈ 2.2 lb |
| 1 US gallon → L | 3.78541 L | ≈ 3.8 L |
| 1 US fl oz → ml | 29.5735 ml | ≈ 30 ml |
| °C → °F | ×9/5 + 32 | double it, add 30 |
| 1 hectare → acres | 2.47105 acres | ≈ 2.5 acres |
Temperature is the odd one out
Every other conversion is a simple multiplication, but temperature scales have different zero points, so the formula needs an offset: °F = °C × 9/5 + 32. The two scales cross at −40°, the one temperature that's the same in both. Kelvin is Celsius shifted so 0 K is absolute zero: K = °C + 273.15 — no degree symbol, by convention.
US vs imperial: same names, different sizes
A US gallon is 3.785 L but an imperial (UK) gallon is 4.546 L — a 20% difference wearing the same name. Pints and fluid ounces differ the same way (a US pint is 473 ml, a UK pint 568 ml). When a recipe or fuel-economy figure crosses the Atlantic, check which gallon it means; this converter includes both.
Frequently asked questions
Why does the US still use imperial units?
Momentum and cost: the Metric Conversion Act of 1975 made metrication voluntary, and industries that were expensive to convert never did. US science, medicine, and manufacturing are largely metric; road signs and kitchens are not.
Are these conversions exact or rounded?
The factors are exact where definitions are exact (inch, foot, mile, pound are all defined in metric terms) and carried to 10+ significant digits elsewhere. Displayed results are rounded to six decimals; the math underneath is not.
What's the difference between weight and mass?
Mass (kg, lb) measures the amount of matter; weight is the force gravity exerts on it. On Earth they move together, so everyday usage merges them — this converter converts mass units, which is what scales, recipes, and shipping labels mean.
How do I convert units in my head quickly?
Memorize the anchors: 2.5 cm to an inch, 30 cm to a foot, 1.6 km to a mile, 2.2 lb to a kg, and 'double plus 30' for °C→°F. These get you within a few percent — close enough for shopping, travel, and cooking.
Does this converter work offline?
Yes — all conversion factors live in the page's JavaScript, so once loaded it needs no connection and sends nothing anywhere.