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Approximate conversion factors

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The table below lists quick "rules of thumb" that may jog your memory. They are not accurate conversion factors, but rather approximate values for doing quick calculations that don't require strict accuracy.

Customary Unit Approximate Metric Values

(Do not use as conversion factors)

acre 4000 square meters or

0.4 hectare

barrel 0.16 cubic meter
British thermal unit 1000 joules
British thermal unit per pound-mass 2300 joules per kilogram or

2.3 kilojoules per kilogram

calorie 4 joules
centipoise 1* millipascal-second
centistokes 1* square millimeter per second
darcy 1 square micrometer
degree Fahrenheit (temperature difference) 0.5 Kelvin
dyne per centimeter 1* millinewton per meter
foot 30 centimeters or

0.3 meter

cubic foot (cu ft) 0.03 cubic meter
cubic foot per pound-mass (ft3/lbm) 0.06 cubic meter per kilogram
square foot (sq ft) 0.1 square meter
foot per minute 0.3 meter per minute or

5 millimeters per second

foot-pound-force 1.4 joules
foot-pound-force per minute 0.02 watt
foot-pound-force per second 1.4 watts
horsepower 750 watts (¾ kilowatt)
horsepower, boiler 10 kilowatts
inch 2.5 centimeters
kilowatt-hour 3.6* megajoules
mile 1.6 kilometers
ounce (avoirdupois) 28 grams
ounce (fluid) 30 cubic centimeters
pound-force 4.5 newtons
pound-force per square inch (pressure, psi) 7 kilopascals
pound-mass 0.5 kilogram
pound-mass per cubic foot 16 kilograms per cubic meter
section 260 hectares or

2.6 million square meters or
2.6 square kilometers

ton, long (2240 pounds-mass) 1000 kilograms
ton, metric (tonne) 1000* kilograms
ton, short 900 kilograms

*Exact equivalents