Skip to main content

Ferrite toroid permeability calculator

When you need this

There is an unmarked core in the box. Or it is marked, but the ferrite has aged and its permeability is no longer the datasheet figure. Wind a few turns, measure the inductance — and you have the real parameters of that particular core.

OD 22 mmID 11 mmh 5 mmsection
Wind a few turns evenly around the ring and measure the inductance
How many turns are wound, N
Measured inductance L (µH)
Toroid outer diameter OD (mm)
Toroid inner diameter ID (mm)
Toroid height h (mm)
Chamfer C (mm)
Result
Permeability μr
Inductance factor AL
Magnetic path length le

📐 Formulas​

This is the same AL formula, solved for the permeability.

he = h·(1 − 0.8584·(C/√2)² / (h·(OD−ID)/2))     effective height with the chamfer

AL [nH/N²] = L[nH] / N²                          straight from the measurement

μr = AL / (0.2 · he · ln(OD/ID))

le = 2π·ln(OD/ID) / (2/ID − 2/OD)                effective magnetic path
                                                  length (IEC 60205)

Notation:​

  • N — how many turns are wound
  • L — measured inductance [µH]
  • OD — outer diameter of the core [mm]
  • ID — inner diameter [mm]
  • h — height of the core [mm]
  • C — chamfer [mm]
Limits of the calculator:
  • Measure at a low frequency — a few kilohertz. Higher up the permeability of ferrite already depends on frequency, and you will get not the initial μi but μ′ at the measuring frequency
  • Wind evenly around the whole ring: an uneven winding gives a low reading
  • Use 15–30 turns: with two or three the error of the meter swallows the whole result
  • Allow for the stray inductance of the meter leads: short the probes and subtract the reading

🎯 Where it is used​

  • Identifying cores from the junk box.
  • Checking old ferrite before a project that has to work.
  • Input data for calculating a coil on that same core.
Practical notes:
  1. Ferrite ages and its permeability drops — sometimes by tens of per cent. Even for a marked core the check does no harm.
  2. Once you have μr, move on to the ferrite toroid coil calculator — the calculation will then rest on real figures rather than datasheet ones.
  3. If the result looks odd, check first not the formula but whether the meter's own current is saturating the core.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Ferrite toroid permeability calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/coils/toroid_permeability (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

Last updated on