The tesla (symbol: T) is the unit of magnetic flux density (also called magnetic B-field strength) in the International System of Units (SI).

One tesla is equal to one weber per square metre. The unit was announced during the General Conference on Weights and Measures in 1960 and is named^[1] in honour of Serbian-American electrical and mechanical engineer Nikola Tesla, upon the proposal of the Slovenian electrical engineer France Avčin.

A particle, carrying a charge of one coulomb (C), and moving perpendicularly through a magnetic field of one tesla, at a speed of one metre per second (m/s), experiences a force with magnitude one newton (N), according to the Lorentz force law. That is,T=N⋅sC⋅m.

As an SI derived unit, the tesla can also be expressed in terms of other units. For example, a magnetic flux of 1 weber (Wb) through a surface of one square meter is equal to a magnetic flux density of 1 tesla.^[2] That is,T=Wbm2.

Expressed only in SI base units, 1 tesla is:T=kgA⋅s2,where A is ampere, kg is kilogram, and s is second.^[2]

Additional equivalences result from the derivation of coulombs from amperes (A), C=A⋅s:T=NA⋅m,the relationship between newtons and joules (J), J=N⋅m:T=JA⋅m2,and the derivation of the weber from volts (V), Wb=V⋅s:T=V⋅sm2.The tesla is named after Nikola Tesla. As with every SI unit named after a person, its symbol starts with an upper case letter (T), but when written in full, it follows the rules for capitalisation of a common noun; i.e., tesla becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

In the production of the Lorentz force, the difference between electric fields and magnetic fields is that a force from a magnetic field on a charged particle is generally due to the charged particle's movement,^[3] while the force imparted by an electric field on a charged particle is not due to the charged particle's movement. This may be appreciated by looking at the units for each. The unit of electric field in the MKS system of units is newtons per coulomb, N/C, while the magnetic field (in teslas) can be written as N/(C⋅m/s). The dividing factor between the two types of field is metres per second (m/s), which is velocity. This relationship immediately highlights the fact that whether a static electromagnetic field is seen as purely magnetic, or purely electric, or some combination of these, is dependent upon one's reference frame (that is, one's velocity relative to the field).^[4][5]

In ferromagnets, the movement creating the magnetic field is the electron spin^[6] (and to a lesser extent electron orbital angular momentum). In a current-carrying wire (electromagnets) the movement is due to electrons moving through the wire (whether the wire is straight or circular).

One tesla is equivalent to:^{[7][page needed]}

For the relation to the units of the magnetising field (ampere per metre or oersted), see the article on permeability.