SI Unit Conversions
SI (Système International) is the modern metric system of units used across science and engineering worldwide, built from seven base units — metre, kilogram, second, ampere, kelvin, mole, and candela — from which every other SI unit is derived. This page covers the SI base and derived units relevant to engineering, along with the standard prefixes (kilo, milli, mega, micro, etc.) used to scale them up or down.
For converting between SI, imperial, and other unit systems for a specific quantity, use our universal unit converter.
Use this chart to convert between various SI Defined Units
| SI DEFINED UNITS | |||
|---|---|---|---|
| Quantity | Name | Symbol | |
| length (distance) | meter | m | |
| mass | kilogram | kg | |
| time | second | s | |
| temperature | Kelvin | K | |
| current | Ampere | A | |
| quantity | mole | mol | |
| luminosity | candela | cd | |
| SI DERIVED UNITS | |||
| Quantity | Name | Symbol | Units |
| frequency | Hertz | Hz | m-1 |
| velocity | - | - | m s-1 |
| acceleration | - | - | m s-2 |
| force | Newton | N | kg m s-2 |
| pressure (stress) | Pascal | Pa | N m-2 = kg m-1 s-2 |
| energy (work, heat) | Joule | J | N m = kg m2 s-2 |
| momentum (impulse) | - | - | N s = kg m s-1 |
| power | Watt | W | J s -1 = kg m2 s-3 |
| electric charge | Coulomb | C | A s |
| electric potential (emf) | Volt | V | J C-1 = W A -1 = kg m2 s-3 A-1 |
| resistance | Ohm | W | V A-1 = kg m2 s-3 A-2 |
| conductance | Siemens | S | A V-1 = W-1 = kg-1 m-2 s3 A2 |
| magnetic flux | Weber | Wb | V s = kg m2 s-2 A -1 |
| inductance | Henry | H | Wb A-1 = kg m2 s-2 A-2 |
| capacitance | Farad | F | C V-1 = kg-1 m-2 s4 A2 |
| electric field strength | - | - | V m-1 = N C-1 |
| electric displacement | - | - | C m-2 |
| magnetic field strength | - | - | A m-1 |
| magnetic flux density | Tesla | T | Wb m-2 = N A-1 m-1 = kg s-2 A-1 |
| plane angle | radian | rad | 2p (circle) |
| solid angle | steradian | sr | 4p (sphere) |
| luminous flux | lumen | lm | cd sr |
| illuminance | lux | lx | lm m² |
| SI ACCEPTED UNITS | |||
| Quantity | Name | Symbol | Units |
| length | Ångstrom | Å,å | 1 a = 10-10 m |
| volume | Liter | L, l | 1 L = 10-3 m³ |
| mass | metric ton | t | 1 t = 1000 kg |
| pressure | bar | bar | 1 bar = 105 Pa = 105 N m-2 |
| time | minute | min | 1 min = 60 sec |
| time | hour | h | 1 h = 60 min = 3600 sec |
| time | day | d | 1 d = 24 h = 86 400 sec |
| plane angle | degree | o | 1o = (p/180) rad |
| plane angle | minute | ' | 1' = (1/60)o = (p/10 800) rad |
| plane angle | second | " | 1" = (1/60)' = (p /648 000) rad |
| GEOMETRIC CONSTANTS | |||
| EARTH CONSTANTS | |||
| Constant | Symbol | Value | |
| Standard Atmosphere | atm | 101 325 Pa (exact) | |
| Standard Acceleration of Gravity | ga | 9.806 65 m s-2 (exact) | |
| Astronomical Unit (Earth-Sun) | au | 1.495 978 706 91 (30) 1011 m | |
| PLANCK CONSTANTS | |||
| Constant | Symbol | Value | |
| Planck Constant | h | 6.626 075 5 10 (40) 10-34 J s | |
| Planck Constant = h/{e} (in electron Volts) | h/{e } | 4.135 669 2 (12) 10-15 eV s | |
| Planck Constant = h/2p | h | 1.054 572 66 (63) 10-34 J s | |
| Planck Constant = h/2p{e} (in electron Volts) | h/{e} | 6.582 122 0 (20) 10-16 eV s | |
| Planck Mass = (hc/G)1/2 | mP | 2.176 71 (14) 10-8 kg | |
| Planck Length = h/mPc = (hG/c3)1/2 | l P | 1.616 05 (10) 10-35 m | |
| Planck Time = lP/c = (hG/c5)1/2 | tP | 5.390 56 (34) 10 -44 s | |
| RYDBERG CONSTANTS | |||
| Constant | Symbol | Value | |
| Rydberg Constant = meca²/2h | RɎ | 10 973 731.534 (13) m-1 | |
| Rydberg Constant = RɎ c (in Hertz) | RɎc | 3.289 841 949 9 (40) 1015 Hz | |
| Rydberg Constant = RɎhc (in Joules) | RɎhc | 2.179 874 1 (13) 10-18 J | |
| Rydberg Constant = RɎ hc/{e} (in electron Volts) | RɎhc/{e} | 13.605 698 1 (40) eV | |
| PHYSICAL CONSTANTS | |||
| Constant | Symbol | Value | |
| Bohr Radius = a/4pR Ɏ | ao | 0.529 177 249 (24) 10-10 m | |
| Hartree Energy = e2/4peoao = 2RɎhc | Eh | 4.359 748 2 (26) 10-18 J | |
| Hartree Energy = Eh/{e} (in electron Volts) | Eh/{e} | 27.211 396 1 (81) eV | |
| Quantum of Circulation = h/2me | h/2me | 3.636 948 07 (33) 10-4 m2 s-1 | |
| Quanta of Circulation = h/m e | h/me | 7.273 896 14 (65) 10-4 m² s-1 | |
| CHEMICAL CONSTANTS | |||
| Constant | Symbol | Value | |
| Avogadro's number | N A, L | 6.022 136 7 (36) 1023 mol-1 | |
| Atomic Mass Constant = m(12C)/12 = 1 Dalton (Da) | mu | 1.660 540 2 (10) 10-27 kg | |
| Atomic Mass Constant = muc²/{e} (in electron Volts) | mu | 931.494 32 (28) MeV | |
| Faraday Constant | F | 96 485.309 (29) C mol -1 | |
| Molar Planck Constant = NA h | NAh | 3.990 313 23 (36) 10-10 J s mol-1 | |
| Molar Planck Constant = NAhc | NAhc | 0.119 626 58 (11) J m mol-1 | |
| Molar Gas Constant | R | 8.314 510 (70) J mol-1 K-1 | |
| Molar Volume = RT/p (ideal gas at 1 atm/273.15 K) | Vm | 22 414.10 (19) cm³ mol-1 | |
| Loschmidt Constant = NA/Vm | no | 2.686 763 (23) 1025 m³ | |
The 20 SI prefixes used to form decimal multiples and submultiples of SI units are given below:-
| FACTOR | NAME | SYMBOL |
|---|---|---|
| 1024 | yotta | Y |
| 1021 | zetta | Z |
| 1018 | exa | E |
| 1015 | peta | P |
| 1012 | tera | T |
| 109 | giga | G |
| 106 | mega | M |
| 103 | kilo | k |
| 102 | hecto | h |
| 101 | deka | da |
| 10-1 | deci | d |
| 10-2 | centi | c |
| 10-3 | milli | m |
| 10-6 | micro | ? |
| 10-9 | nano | n |
| 10-12 | pico | p |
| 10-15 | femto | f |
| 10-18 | atto | a |
| 10-21 | zepto | z |
| 10-24 | yocto | y |
It is important to note that the kilogram is the only SI unit with a prefix as part of its name and symbol. Because multiple prefixes may not be used, in the case of the kilogram the prefix names of Table?5 are used with the unit name "gram" and the prefix symbols are used with the unit symbol "g." With this exception, any SI prefix may be used with any SI unit, including the degree Celsius and its symbol ?C.
Example 1:10-6 kg = 1 mg (one milligram), but not 10-6 kg = 1 ?kg (one microkilogram)
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FREQUENTLY ASKED QUESTIONS
What's the difference between a base unit and a derived unit?
SI base units (metre, kilogram, second, etc.) are the fundamental, independently defined units. Derived units (such as the newton, pascal, or joule) are built from combinations of base units according to the physical relationship they represent — a newton, for example, is defined as one kilogram-metre per second squared.
Why do some engineering fields still use non-SI units?
Legacy standards, industry convention, and international trade with countries that haven't fully adopted SI (notably the US) all contribute to non-SI units (psi, inches, feet, pounds) persisting in some sectors, particularly aerospace, oil and gas, and some manufacturing supply chains. Always check which unit system a drawing, specification, or standard is using before assuming SI throughout.
How do I convert between an SI unit and its imperial equivalent?
Use our universal unit converter, which covers acceleration, angles, area, length, force, mass, power, tensile strength, temperature, and velocity conversions directly.
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