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Field guide

Metric, imperial, and the math between them.

Two measurement systems dominate everyday life. The metric system (formally the SI, Système International d'unités) is decimal: every unit is a power of ten of every other unit in the same family. The imperial / US customary system uses historical ratios: 12 inches per foot, 16 ounces per pound, 16 cups per gallon. That have to be memorised individually.

Conversion between the two systems is exact and fixed by international agreement. The numbers below are the values this calculator uses internally; they're NIST-standard and accurate to the last published digit.

Length

All length conversions pivot through the meter. The relationships between metric sub-units are powers of ten:

And the imperial side uses fixed integer ratios:

The bridge: 1 inch ≡ 2.54 cm, exactly. From that one definition every other metric ↔ imperial length conversion follows by simple multiplication. So 1 mile = 5,280 × 12 × 2.54 cm = 160,934.4 cm = 1.609344 km, exactly.

Weight (mass)

Strictly speaking these are mass conversions; “weight” in physics is force = mass × gravity, but everyday English uses “weight” for the kilogram and pound, and this calculator follows that convention.

Useful rule of thumb: 1 kg ≈ 2.2 lb. The exact value is 2.20462262...; for most purposes multiplying by 2.2 is fine. Going the other way, 1 lb ≈ 0.4536 kg.

Temperature

Temperature is the one category where conversion requires an offset, not just a multiplier. Celsius and Fahrenheit cross at −40°, the only point where the two scales agree.

Three reference points worth memorising: water freezes at 0 °C = 32 °F, body temperature is 37 °C ≈ 98.6 °F, and water boils at 100 °C = 212 °F (at 1 atmosphere). Kelvin is the absolute scale used in physics, same step size as Celsius, but offset so that 0 K is absolute zero (the coldest possible temperature, equal to −273.15 °C).

Volume

Volume conversions are the messiest because the US and UK disagree on what a “gallon” means. This calculator uses US gallons and US cups , by far the most common context for English-language search.

For UK conversions, divide a US gallon by 0.832674 to get the imperial gallon equivalent. For cooking, a UK cup is often interpreted as 250 ml, slightly larger than the US cup at 236.59 ml.

Why this calculator updates everything at once

Most converters use a “from / to” pair: pick a source unit, pick a target unit, see one answer. That forces an extra click for every new comparison. This page instead keeps a single source-of-truth value (in the category's base unit) and re-derives every other unit from that value on every keystroke. Type into the kilometre field, every other length field updates the same instant.

Worked example

You're cooking a US recipe that calls for 2 cups of flour and you only have a kitchen scale. Convert to ml or L? You can't directly; you need flour density (~0.6 g/ml). But to volume in metric:

For the weight: 473 ml × 0.6 g/ml ≈ 284 g. The unit converter handles the volume side; substance density is on you (or a kitchen scale).

Disclaimer

All conversion factors used here are NIST / SI-standard values, accurate to the last published digit. For measurements where rounding error matters (legal, engineering, medical dosing), use the closed-form factor with full precision rather than a calculator's 8-digit display rounding.