[1] If a current of 1 ampere flowing through a coil produces flux linkage of 1 weber turn, that coil has a self-inductance of 1 henry.‌ The unit is named after Joseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England.

Expressed in combinations of SI units, the henry is:[3] where: H = henry, kg = kilogram, m = metre, s = second, A = ampere, N = newton, C = coulomb, J = joule, T = tesla, Wb = weber, V = volt, F = farad, Ω = ohm, Hz = hertz, rad = radian The International System of Units (SI) specifies that the symbol of a unit named for a person is written with an initial capital letter, while the name is not capitalized in sentence text, except when any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case.

The United States National Institute of Standards and Technology recommends users writing in English to use the plural as henries.

Formulae can be used to calculate the inductance of many common arrangements of conductors, such as parallel wires, or a solenoid.

A large motor winding with many turns around an iron core may have an inductance of hundreds of henries.