Inductance of a toroidal coil

Calculation formula (m is meters):
Inductance
= (permeability in a vacuum) × (relative permeability) × (square of the magnetic path cross-sectional radius [m]) × (square of the number of turns) ÷ (average diameter [m])
= (4 * π * 10^(-7)) * μr * (r^2) * (N^2) / Davg

* Average diameter (Davg)
= ((outer diameter of the annular magnetic path [m]) + (inner diameter of the annular magnetic path [m])) ÷ 2
= (outer diameter of the annular magnetic path [m]) – (diameter of the magnetic path cross-section [m])
= D – 2 * r

Magnetic Flux Density [T: Tesla]
= (Applied Voltage [V]) × (Application Time [seconds]) ÷ (Magnetic Circuit Cross-Sectional Area [square meters]) ÷ (Number of Windings)

◇ Examples of Relative Permeability (from Wikipedia)
Permalloy: 8000
Iron (99.8% pure iron): 5000
Ferrite (manganese-zinc-based): 640 (or higher)
Ferrite (nickel-zinc-based): 16 – 640
Wood: 1.00000043
Air: 1.00000037
Water: 0.999992