The capacity of an inductor is controlled by four factors:
  • The number of coils - More coils means more inductance.
  • The material that the coils are wrapped around (the core)
  • The cross-sectional area of the coil - More area means more inductance.
  • The length of the coil - A short coil means narrower (or overlapping) coils, which means more inductance.
Putting iron in the core of an inductor gives it much more inductance than air or any non-magnetic core would.

The standard unit of inductance is the henry. The equation for calculating the number of henries in an inductor is:

H = (4 * Pi * #Turns * #Turns * coil Area * mu) / (coil Length * 10,000,000)

The area and length of the coil are in meters. The term mu is the permeability of the core. Air has a permeability of 1, while steel might have a permeability of 2,000.