Mental math shortcuts for everyday unit conversions

Quick, good-enough tricks for temperature, distance, weight and speed — and how far off each shortcut is.

By Unit Converter Tool Editorial Team. Updated 2 November 2026. 2 min read.

When you are reading a weather forecast abroad or a recipe on your phone, an approximate answer in two seconds beats an exact one in thirty. These shortcuts are deliberately rough; each one notes how far off it can be.

Celsius to Fahrenheit: double it and add 30

The exact formula is °F = °C × 1.8 + 32. Doubling and adding 30 is close for everyday temperatures: 20 °C → 70 °F (exact: 68 °F). It drifts at the extremes: 35 °C → 100 °F (exact: 95 °F). Going back: subtract 30 and halve.

Handy anchors: 10 °C = 50 °F, 16 °C ≈ 61 °F, 28 °C ≈ 82 °F — the digits roughly swap.

Kilometres to miles: multiply by 0.6 (or use Fibonacci)

1 km = 0.621371 mi, so multiply by 6 and divide by 10: 50 km ≈ 30 mi (exact 31.07). Consecutive Fibonacci numbers also work because their ratio approaches 1.618: 5 mi ≈ 8 km, 8 mi ≈ 13 km, 13 mi ≈ 21 km.

Kilograms to pounds: double it and add 10%

80 kg → 160 + 16 = 176 lb (exact 176.37). Pounds to kilograms: halve it and subtract 10%: 200 lb → 100 − 10 = 90 kg (exact 90.72).

Speed

km/h to mph uses the same ×0.6 trick: 100 km/h ≈ 60 mph (exact 62.14). For m/s to km/h multiply by 3.6 — that one is exact.

Volume

A litre is a little more than a US quart (1.057 qt). A US gallon is just under 4 litres (3.785).

Length

An inch is 2.5 cm, near enough (exact 2.54). A metre is about 3 ft 3 in (3.2808 ft). A foot is 30 cm (exact 30.48).

When not to estimate. Medication doses, engineering tolerances, aviation and anything involving money or safety deserve the exact figure. Use the converter.