Toroid winding calculator

How it works

Winding a choke against RFI rather than an inductor? See the ferrite common-mode choke chart: impedance per turn for Fair-Rite mixes 31, 43, 52 and 61 on each band.

A toroid's inductance grows with the square of the number of turns: twice the turns gives four times the inductance. The core's AL value (the "inductance index") is the inductance one turn would give. Datasheets give it in different units: current tables use nH per turn², older catalogues give iron powder cores in µH per 100 turns and ferrite cores in mH per 1000 turns. The calculator converts between them, which is where most mistakes happen: the same T50-2 is 4.9 nH/N² and 49 µH/100 turns.

Iron powder cores have a low permeability and are used for tuned circuits and filters; the mix number sets the frequency range (the colour of the paint tells the mix). Ferrite cores have a much higher permeability, so they need far fewer turns, and their AL varies much more from core to core.

Worked example: 1.0 µH on a T50-2 core (AL = 4.9 nH/N²). N = √(1000 nH / 4.9 nH) = √204.08 = 14.29, rounded up to 15 turns. 15 turns give L = 4.9 × 15² = 4.9 × 225 = 1102.5 nH = 1.10 µH (+10 %). 14 turns give 4.9 × 196 = 960.4 nH = 0.96 µH (−4 %). With the old units: 100 × √(1 / 49) = 14.29 turns, the same answer.

Second example (ferrite): 100 µH on an FT-37-43 (AL = 350 nH/N²): N = √(100 000 / 350) = 16.90, so 17 turns give 350 × 289 = 101 150 nH = 101.2 µH.

Wire length per turn is estimated as (OD − ID) + 2 × H + 2 × wire diameter, plus 25 mm for each lead. One-layer capacity is estimated as π × (ID − wire diameter) / wire diameter. Both are approximations of our own, not from a datasheet. Turns on a toroid count each pass of the wire through the centre hole.

Indicative calculation; verify before use. AL values are the manufacturers' nominal values for new cores at low signal levels and room temperature. Real cores vary within the manufacturer's tolerance, and ferrite varies much more than iron powder. Check the current datasheet and measure the finished coil with an LCR meter or antenna analyzer.

Amidon, Micrometals and Fair-Rite are trademarks of their respective owners. Ohmsums is not affiliated with or endorsed by them.

Sources (checked 2026-11-03):

• AL in nH/turn² for iron powder and ferrite toroids (Amidon): https://www.amidoncorp.com/2ipt/
• L = AL × (N/100)² with AL in µH/100 turns (coil32): https://coil32.net/online-calculators/amidon-iron-powder-cores-calculator.html
• Background on toroidal inductors: https://en.wikipedia.org/wiki/Toroidal_inductors_and_transformers