Informal conversion of common units

Informal conversion of common measuring units often needs to be done without calculator or computer. This article is an informal listing of some quick and dirty conversions between the system used in the United States and metric or SI units. When people say "a foot long" or "ten kilometers from here", they rarely mean ".3048 m" or "6.21571 miles".

These are all rough approximations; for more accurate information, see Conversion of units.

Volume:

 1 liter          2 pints
 half liter       1 pint
 4 liters         1 gallon

Length:

 1 inch           2.5 cm
 4 inch           10 cm
 3 cm             just over 1 inch (just over, round down conversion)
 10 cm            4 inch (just under, round up conversion)
 1 meter          1 yard (just over)
 1 meter          3 feet
 30 cm            1 foot
 1 km             half a mile (just over, round down conversions)
 3 miles          5 km (just under, round up conversions)

Mass:

 1 kg             2 pounds
 1 lb             half a kilogram

Temperatures:

• Fahrenheit to Celsius -- subtract 30 and divide by 2
• Celsius to Fahrenheit -- multiply by 2 add 25