Five Unit Conversion Shortcuts Worth Memorizing

Memorise the conversions you use repeatedly

You do not need to carry an entire conversion table in your head. A few reliable anchors are enough for quick estimates, while exact work can stay with a converter.

Miles and kilometres

One mile is approximately 1.6 kilometres.

• Convert miles to kilometres by multiplying by 1.6.
• Convert kilometres to miles by dividing by 1.6.

Five miles is about eight kilometres. Ten kilometres is about 6.2 miles.

Pounds and kilograms

One kilogram is approximately 2.2 pounds.

For a quick conversion from kilograms to pounds, double the kilograms and add 10%. A 20 kg suitcase is roughly 40 + 4 = 44 lb.

For pounds to kilograms, divide by 2.2. A 22 lb item is approximately 10 kg.

Celsius and Fahrenheit

For an exact temperature conversion:

• °F = °C × 9/5 + 32
• °C = (°F − 32) × 5/9

For a quick weather estimate from Celsius to Fahrenheit, double the Celsius value and add 30. At 20°C, this shortcut gives 70°F; the exact result is 68°F.

Litres and US gallons

One US gallon is approximately 3.785 litres. Four litres is therefore a useful rough mental substitute for one gallon.

This estimate is good for planning capacity. Use the exact factor for fuel cost, recipes, or technical work.

Cups and millilitres

A US cup is approximately 237 millilitres, though many recipes round it to 240 ml. Half a cup is about 120 ml, and a quarter cup is about 60 ml.

Recipe conversions can still be imprecise because ingredients have different densities. A cup of flour and a cup of honey occupy similar volume but have very different weights.

Keep units attached to every value

The safest conversion habit is to write the unit beside every number. A value without a unit invites errors, especially when US and imperial measures share names but not exact sizes.

Use mental shortcuts to check whether an answer is plausible, then use current and exact conversion factors when accuracy matters.