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] | |  | 0.03937 in |
| 1 centimeter [cm] | 10 mm |  | 0.3937 in |
| 1 meter [m] | 100 cm |  | 1.0936 yd |
| 1 kilometer [km] | 1000 m |  | 0.6214 mile |
|
| US or Imperial | | Metric |
|---|
| 1 inch [in] | |  | 2.54 cm |
| 1 foot [ft] | 12 in |  | 0.3048 m |
| 1 yard [yd] | 3 ft |  | 0.9144 m |
| 1 mile | 1760 yd |  | 1.6093 km |
| 1 int nautical mile | 2025.4 yd |  | 1.853 km |
|
| Metric | | US or Imperial |
|---|
| 1 sq cm [cm2] | 100 mm2 | | 0.1550 in2 |
| 1 sq m [m2] | 10,000 cm2 | | 1.1960 yd2 |
| 1 hectare [ha] | 10,000 m2 |  | 2.4711 acres |
| 1 sq km [km2] | 100 ha |  | 0.3861 mile2 |
|
| US or Imperial | | Metric |
|---|
| 1 sq inch [in2] | | | 6.4516 cm2 |
| 1 sq foot [ft2] | 144 in2 | | 0.0929 m2 |
| 1 sq yd [yd2] | 9 ft2 |  | 0.8361 m2 |
| 1 acre | 4840 yd2 | | 4046.9 m2 |
| 1 sq mile [mile2] | 640 acres | | 2.59 km2 |
|
| Metric | | US Measure | Imperial |
|---|
| 1 cu cm [cm3] | |  | 0.0610 in3 |
| 1 cu decimeter [dm3] | 1,000 cm3 |  | 0.0353 ft3 |
| 1 cu meter [m3] | 1,000 dm3 | | 1.3080 yd3 |
| 1 liter [l] | 1 dm3 |  | 2.113 fluid pt | 1.7598 pt |
| US Measure | Imperial | | Metric |
|---|
| 1 cu inch [in3] | | 16.387 cm3 |
| 1 cu foot [ft3] |  | 0.02832 m3 |
| 1 fluid ounce | 1.0408 UK fl oz | | 29.574 ml |
| 1 pint (16 fl oz) | 0.8327 UK pt |  | 0.4732 liters |
| 1 gallon (231 in3) | 0.8327 UK gal | | 3.7854 liters |
| Metric | | US or Imperial |
|---|
| 1 milligram [mg] | | | 0.0154 grain |
| 1 gram [g] | 1,000 mg |  | 0.0353 oz |
| 1 kilogram [kg] | 1,000 g | | 2.2046 lb |
| 1 tonne [t] | 1,000 kg |  | 1.1023 short ton |
| 1 tonne [t] | 1,000 kg |  | 0.9842 long ton |
|
| US or Imperial | | Metric |
|---|
| 1 ounce [oz] | 437.5 grain | | 28.35 g |
| 1 pound [lb] | 16 oz |  | 0.4536 kg |
| 1 stone | 14 lb | | 6.3503 kg |
| 1 hundredweight [cwt] | 112 lb |  | 50.802 kg |
| 1 short ton (US) | | | 0.9072 t |
| 1 long ton (UK) | |  | 1.0160 t |
