Physical unit converter: 79 quantities and over 900 units, from SI to traditional

What this converter does

It converts any value between all the units of one quantity and shows the full table at once, so you see the number you were after and, beside it, every other equivalent. It covers 79 quantities and more than 900 units: the International System with its prefixes, imperial and US customary units, the CGS units that survive in electromagnetism, the technical ones (kilopond, metric horsepower, technical atmosphere), the historical ones that still turn up in old documentation, and the traditional Castilian units that remain alive in farming and building in Spain.

The search box at the top finds a unit by name or symbol across every quantity, and the page address stores the conversion (quantity, units and value), so it can be linked or sent to a colleague.

The quantities covered

Electrical and magnetic, the ones most asked for in a test laboratory: current, voltage, resistance, conductance, capacitance, inductance, charge, magnetic flux, flux density, field strength, magnetomotive force, permeability, permittivity, electric field, resistivity, conductivity (including % IACS for copper), current density, magnetic moment and dipole moment. It includes the electromagnetic and electrostatic CGS units (abampere, statvolt, gauss, oersted, maxwell, gilbert), because core catalogues and older manuals still use them.

Mechanical and spatial: length, area (with circular mil and kcmil), volume, mass, time, speed, acceleration, force, pressure and stress, energy, power (including apparent and reactive power), torque, impulse, mass and area moments of inertia, stiffness, surface tension, density, specific volume, linear and areal density, plane and solid angle, angular velocity and acceleration, frequency and wavenumber.

Thermal: temperature and temperature difference, specific heat, heat capacity, thermal conductivity, heat transfer coefficient, absolute and areal thermal resistance (R-value in European and American units, clo and tog), heat flux density and irradiance, temperature coefficient (ppm/K versus ppm/°F), specific energy, energy density and power density.

Fluids: dynamic and kinematic viscosity, volumetric and mass flow rate and fuel consumption, where the units are reciprocals of one another (L/100 km versus km/L or mpg) and the converter handles it without you having to think about it.

Light and radiation: luminous intensity, luminous flux, illuminance, luminance, radioactivity, absorbed dose, equivalent dose, dose rate and exposure.

Others: amount of substance, molar concentration, molar mass, fractions (%, ‰, ppm, ppb, basis points), information in decimal and binary bits and bytes, and data transfer rate.

Where the factors come from

SI units and prefixes follow the SI Brochure (BIPM, 9th edition) and the ISO 80000 series. Non-SI units that are accepted or in common use follow NIST Special Publication 811, the usual reference for conversions to imperial and US customary units. Physical constants (elementary charge, speed of light, electron mass, Bohr magneton, vacuum permeability) are the CODATA 2018 values in force since the 2019 redefinition of the SI.

A few details worth knowing. The calorie has three values in use: thermochemical (4.184 J), International Table or IT (4.1868 J) and the nineteenth-century 15 °C calorie; the converter distinguishes the first two. The BTU has the same problem and is handled the same way. The gallon, pint and fluid ounce exist in US and imperial versions with different values (an imperial gallon is 20% larger). The ton may be metric (1000 kg), short (907 kg) or long (1016 kg). The year may be Julian (365.25 days), common or tropical. And the month used here is the mean month, 30.44 days.

Traditional Castilian units (vara, legua, arroba, fanega, cántara, celemín) had different values by region: the Castile values are given, which were the legal reference from the nineteenth century until the metric system took over.

Worked example

A North American motor nameplate reads 25 hp and it has to be compared with a European 18.5 kW motor. Under “power”, 25 hp (mechanical) = 18.64 kW: they are equivalent. Had the plate said 25 PS or CV, it would be 18.39 kW, slightly below.

A magnetic core catalogue gives saturation as 12 000 G and coercive force as 0.8 Oe. In SI: 12 000 G = 1.2 T; 0.8 Oe = 63.7 A/m.

An American gauge reads 145 psi. Under “pressure and stress”: 145 psi = 10.0 bar = 1.00 MPa = 9.87 atm = 10.2 kgf/cm².

An IEEE table gives a thermal expansion coefficient of 9.4 ppm/°F. Under “temperature coefficient”: 9.4 ppm/°F = 16.9 ppm/K, which is the coefficient of copper.

What this converter does not do

It does not convert between different quantities: it will not go from kilowatts to amperes or from frequency to wavelength, because that needs one more datum (the voltage, the propagation velocity) and is no longer a unit conversion. It does not handle logarithmic scales — decibels, dBm, pH, astronomical magnitudes — nor hardness scales or the Beaufort scale, which are neither linear nor affine. Currencies are not physical units and are not included. Nor are local units from outside Castile or the historical variants of each unit: where an old unit had several values, only the one adopted as reference appears.

What it relates to

The equivalence between kcmil, AWG and mm² is the one applied by the cable size calculator in this same section. The tesla, gauss and oersted conversions are the ones needed to read the datasheets of current transformers and cores tested with transformer test equipment, and the resistivity and conductivity conversions help interpret the reading of a micro-ohmmeter or an earth tester.