Common engineering constants
Frequently used physical constants and unit conversions for engineering calculations, with both SI and imperial values where they differ. Keep them handy to avoid the unit-conversion slips that cause most calculation errors.
Physical constants
| Constant | Symbol | Value | Units |
|---|---|---|---|
| Standard gravity | g | 9.807 (32.17) | m/s² (ft/s²) |
| Pi | π | 3.14159 | — |
| Euler number | e | 2.71828 | — |
| Standard atmosphere | atm | 101.325 (14.696) | kPa (psi) |
| Universal gas constant | R | 8.314 | J/mol·K |
| Avogadro number | NA | 6.022 × 10²³ | /mol |
| Boltzmann constant | k | 1.381 × 10⁻²³ | J/K |
| Speed of light | c | 299,792,458 | m/s |
| Speed of sound (air, 20°C) | — | 343 (1125) | m/s (ft/s) |
| Water density (4°C) | ρ | 1000 (62.4) | kg/m³ (lb/ft³) |
| Air density (15°C) | ρ | 1.225 (0.0765) | kg/m³ (lb/ft³) |
| Water specific heat | Cp | 4186 (1.00) | J/kg·K (BTU/lb·°F) |
| Absolute zero | — | -273.15 (-459.67) | °C (°F) |
Handy unit conversions
| Quantity | Equivalent |
|---|---|
| 1 inch | 25.4 mm |
| 1 foot | 0.3048 m |
| 1 pound-force | 4.448 N |
| 1 psi | 6.895 kPa |
| 1 horsepower | 745.7 W |
| 1 BTU | 1055 J |
| 1 US gallon | 3.785 L |
| 1 foot-pound | 1.356 J |
Values in parentheses are the imperial equivalent. These are common reference figures at standard conditions; precise work should use values for the actual temperature, pressure, and material.
Converting units for a calculation?
Browse the engineering calculators for unit conversions and physics tools.
Constants every engineer reaches for
A handful of constants show up across nearly all engineering math: g for weight and dynamics, the gas constant R for thermodynamics, standard atmospheric pressure as a reference, and the densities of water and air. Keeping their values and units straight prevents the unit-conversion errors that cause most calculation mistakes.
Mind the unit system
Most errors come from mixing unit systems. The same constant has very different numbers in SI and imperial, like g at 9.81 m/s squared versus 32.2 ft/s squared. Decide on one consistent system for a calculation, convert all inputs into it, and the constants will line up.
FAQ
What is the value of standard gravity?
9.807 m/s squared, or about 32.17 ft/s squared. It is the standard acceleration used to convert mass to weight at the surface of the Earth.
What is the universal gas constant?
8.314 joules per mole per kelvin in SI units. It links pressure, volume, temperature, and amount of gas in the ideal gas law.
What is standard atmospheric pressure?
101.325 kilopascals, equal to 14.696 psi or one atmosphere, the average air pressure at sea level.