Système International d'Unités: the international system of units now recommended for all scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units, SI units have now replaced c.g.s. units and Imperial units. The system has seven base units and two dimensionless units (formerly called supplementary units), all other units being derived from these nine units. There are 18 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters). Decimal multiples of the units are indicated by a set of prefixes; whenever possible a prefix representing 10 raised to a power that is a multiple of three should be used.
| Physical quantity | Name | Symbol |
| length | metre | m |
| mass | kilogram | kg |
| time | second | s |
| electric current | ampere | A |
| thermodynamic temperature | kelvin | K |
| luminous intensity | candela | cd |
| amount of substance | mole | mol |
| *plane angle | radian | rad |
| *solid angle | steradian | sr |
| *dimensionless units |
| Physical quantity | Name of SI unit | Symbol of SI unit |
| frequency | hertz | Hz |
| energy | joule | J |
| force | newton | N |
| power | watt | W |
| pressure | pascal | Pa |
| electric charge | coulomb | C |
| electric potential difference | volt | V |
| electric resistance | ohm | Ω |
| electric conductance | siemens | S |
| electric capacitance | farad | F |
| magnetic flux | weber | Wb |
| inductance | henry | H |
| magnetic flux density (magnetic induction) | tesla | T |
| luminous flux | lumen | lm |
| illuminance | lux | lx |
| absorbed dose | gray | Gy |
| activity | becquerel | Bq |
| dose equivalent | sievert | Sv |
| Submultiple | Prefix | Symbol | Multiple | Prefix | Symbol |
| 10−1 | deci | d | 10 | deca | da |
| 10−2 | centi | c | 102 | hecto | h |
| 10−3 | milli | m | 103 | kilo | k |
| 10−6 | micro | μ | 106 | mega | M |
| 10−9 | nano | n | 109 | giga | G |
| 10−12 | pico | p | 1012 | tera | T |
| 10−15 | femto | f | 1015 | peta | P |
| 10−18 | atto | a | 1018 | exa | E |
| 10−21 | zepto | z | 1021 | zetta | Z |
| 10−24 | yocto | y | 1024 | yotta | Y |
| From | To | Multiply by |
| in | m | 2.54 × 10−2 |
| ft | m | 0.3048 |
| sq. in | m2 | 6.4516 × 10−4 |
| sq. ft | m2 | 9.2903 × 10−2 |
| cu. in | m3 | 1.63871 × 10−5 |
| cu. ft | m3 | 2.83168 × 10−2 |
| l(itre) | m3 | 10−3 |
| gal(lon) | l(itre) | 4.546 09 |
| miles/ hr | m s−1 | 0.477 04 |
| km/ hr | m s−1 | 0.277 78 |
| lb | kg | 0.453 592 |
| g cm−3 | kg m−3 | 103 |
| lb/ in3 | kg m−3 | 2.767 99 × 104 |
| dyne | N | 10−5 |
| poundal | N | 0.138 255 |
| lbf | N | 4.448 22 |
| mmHg | Pa | 133.322 |
| atmosphere | Pa | 1.013 25 × 105 |
| hp | W | 745.7 |
| erg | J | 10−7 |
| eV | J | 1.602 10 × 10−19 |
| kW h | J | 3.6 × 106 |
| cal | J | 4.1868 |
