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MagSim Knowledge · Toroid Core Basics
Toroid Core AL Value Calculation
The AL value is the core constant that links inductance and turn count for a toroid core. With L = AL · N2 you can work out the required number of turns in seconds – how that works, and where the simple formula reaches its limits, is what this article covers.
The basic formula
For a coil wound on a toroid core, inductance L, turn count N and the core's AL value are related by:
L = AL · N2
Rearranged for the number of turns needed to reach a target inductance:
N = √(L / AL)
Target: an inductance of 7.2 µH on a core with AL = 4.5 nH.
N = √(7200 nH / 4.5 nH) = √1600 = 40 turns
Where a core's AL value actually comes from
The AL value isn't an arbitrary datasheet figure – it follows directly from the core's geometry and material:
AL = (µ₀ · µᵣ · Ae) / le
How MagSim handles this for you
Reading Ae and le off a datasheet by hand, or deriving them yourself for round, rectangular and triangular cores, is error-prone. MagSim calculates the effective cross-section and path length automatically from your geometry input (including the filling factor) and links them directly to the stored permeability data of the chosen core material – no manual steps in between.
Why a single number often isn't enough in practice
The formula above gives a static approximation. In reality the AL value isn't constant – it changes with frequency, and core manufacturers themselves state tolerances of up to 40 %. Anyone designing around the datasheet figure alone easily underestimates that spread.
That's why MagSim simulates the AL and impedance curve directly across the relevant frequency range instead of a single value, with a tolerance band at the five practically relevant check points:
- AL · 10 kHz
- AL · 100 kHz
- |Z| · 600 kHz
- |Z| · 2 MHz
- |Z| · 60 MHz
Design your own toroid core now
Enter geometry, pick a core material – MagSim calculates the AL value and impedance curve automatically and generates the matching CAD drawing right away.
Register for freeFrequently asked questions
What exactly does the AL value mean?
It states the inductance a single turn produces on a given core, and serves as the conversion factor between a target inductance and the required number of turns.
How do I calculate the number of turns for a target inductance?
With N = √(L / AL) – plug in the target inductance and the chosen core's AL value in matching units, then take the square root.
Why does the real value deviate from the calculated AL value?
Material manufacturing tolerances, the frequency dependence of permeability, and winding effects (e.g. leakage inductance from uneven winding) all cause deviations. Datasheet tolerances of up to 40 % are normal.