${\displaystyle \frac{F}{l}=\frac{{\mu }_0}{2\pi }\cdot \frac{\left|I_1{I}_2\right|}{d}}$

where F/l is the magnitude of the lineic force between two straight parallel conductors of infinite length and negligible circular cross-section, placed at a distance d apart in vacuum and carrying electric currents I₁ and I₂

NOTE 1 – In vacuum, the product of the magnetic constant and the magnetic field strength H is equal to the magnetic flux density B:

B = μ₀H

NOTE 2 – The magnetic constant is related to the electric constant ε₀ and to the speed of light in vacuum c₀ by the relation
μ₀ε₀c₀² = 1

NOTE 3 – The value of the magnetic constant is exactly equal to

4π × 10⁻⁷ m kg s⁻² A⁻² = 1,256 637 061 4... μH/m