These are the only two main systems of measurements.

Most countries use the Metric System, which uses the measuring units such as meters and grams and adds prefixes like kilo, milli and centi to count orders of magnitude. The United States use the older Imperial System, where things are measured in feet, inches and pounds.   

Here are a few things to remember that will come in handy day to day:

  • 1 mile equals 1.6 Kilometers.
  • 1 inch is about 25 millimeters or 2.54 centimeters
  • A 3-foot measurement is almost exactly 1 meter
  • 1 Kilogram is just over 2 pounds
  • 1 pound is about 454 grams

Here are some examples:


Metric US or Imperial
1 millimeter [mm] arrow0.03937 in
1 centimeter [cm]10 mmarrow0.3937 in
1 meter [m]100 cmarrow1.0936 yd
1 kilometer [km]1000 marrow0.6214 mile


US or Imperial Metric
1 inch [in] arrow2.54 cm
1 foot [ft]12 inarrow0.3048 m
1 yard [yd]3 ftarrow0.9144 m
1 mile1760 ydarrow1.6093 km
1 int nautical mile2025.4 ydarrow1.853 km


Metric US or Imperial
1 sq cm [cm2]100 mm20.1550 in2
1 sq m [m2]10,000 cm21.1960 yd2
1 hectare [ha]10,000 m2arrow2.4711 acres
1 sq km [km2]100 haarrow0.3861 mile2


US or Imperial Metric
1 sq inch [in2] 6.4516 cm2
1 sq foot [ft2]144 in20.0929 m2
1 sq yd [yd2]9 ft2arrow0.8361 m2
1 acre4840 yd24046.9 m2
1 sq mile [mile2]640 acres2.59 km2


Metric US MeasureImperial
1 cu cm [cm3] arrow0.0610 in3
1 cu decimeter [dm3]1,000 cm3arrow0.0353 ft3
1 cu meter [m3]1,000 dm31.3080 yd3
1 liter [l]1 dm3arrow2.113 fluid pt1.7598 pt


US MeasureImperial Metric
1 cu inch [in3]16.387 cm3
1 cu foot [ft3]arrow0.02832 m3
1 fluid ounce1.0408 UK fl oz29.574 ml
1 pint (16 fl oz)0.8327 UK ptarrow0.4732 liters
1 gallon (231 in3)0.8327 UK gal3.7854 liters


Metric US or Imperial
1 milligram [mg] 0.0154 grain
1 gram [g]1,000 mgarrow0.0353 oz
1 kilogram [kg]1,000 g2.2046 lb
1 tonne [t]1,000 kgarrow1.1023 short ton
1 tonne [t]1,000 kgarrow0.9842 long ton


US or Imperial Metric
1 ounce [oz]437.5 grain28.35 g
1 pound [lb]16 ozarrow0.4536 kg
1 stone14 lb6.3503 kg
1 hundredweight [cwt]112 lbarrow50.802 kg
1 short ton (US) 0.9072 t
1 long ton (UK) arrow1.0160 t