Understanding AL Values in Magnetic Cores
If you’ve ever looked at a ferrite core datasheet, you’ve probably seen a specification called the AL value.
For many engineers, especially those new to magnetic design, this number can be confusing.
Is it an inductance?
Does it change with current?
Why does it decrease when an air gap is added?
How is it actually used?
Understanding the AL value is one of the keys to designing inductors and transformers efficiently.
This guide explains what the AL value represents, how it is measured, and how engineers use it during magnetic component design.
What Is the AL Value?
The AL value is the inductance produced by one turn squared on a particular magnetic core.
Manufacturers usually express AL in:
- nH/turn²
The relationship is:
L = AL × N²
Where:
- L = Inductance
- AL = Core inductance factor
- N = Number of turns
This simple relationship allows engineers to estimate the number of turns needed for a desired inductance.
👉 Related Guide: How to Calculate Inductor Energy Storage
Why Is It Called “Turns Squared”?
Inductance does not increase linearly with the number of turns.
If you double the turns:
You do not double the inductance.
You increase it by approximately four times.
For example:
- 10 turns
- AL = 100 nH/turn²
Inductance:
100 × 10²
= 10,000 nH
= 10 µH
Now double the turns:
20 turns
100 × 20²
= 40 µH
Notice the inductance increased by four.
This quadratic relationship is one of the most important concepts in magnetic design.
Where Does the AL Value Come From?
Core manufacturers determine the AL value by winding a known number of turns on a standard core and measuring the resulting inductance under controlled conditions.
The published AL value assumes:
- A specific core set
- A defined test frequency
- A small test signal
- No DC bias
This means the AL value is a starting point—not a guarantee of real-world performance.
How Air Gaps Affect AL
One of the biggest reasons AL changes is the introduction of an air gap.
Adding an air gap:
- Reduces permeability
- Lowers the AL value
- Increases energy storage capability
- Improves saturation performance
👉 Related Guide: Air Gap Design in Power Inductors
This is why a gapped ETD core may have a dramatically lower AL value than the same ungapped core.
AL Value and Saturation
A higher AL value often means fewer turns are needed to reach a target inductance.
However, fewer turns can also result in higher flux density for a given current.
Engineers must verify that the core will not saturate under operating conditions.
👉 Related Guide: Understanding Magnetic Saturation
Check Saturation Margin
Inductor Saturation Risk Checker
Estimate flux density from inductance, peak current, turns, and effective core area.
Estimated Flux Density: T
Risk Level:
Approximation: B ≈ L × Ipk / (N × Ae). Final design should use actual core data, gap, material Bsat, and temperature limits.
Need a full saturation and gap-checked design?
Start Design AnalysisAL Value Is Only Part of the Story
Two cores may have identical AL values but perform very differently.
Other factors include:
- Core material
- Window area
- Temperature
- Core losses
- Air gap
- Frequency
👉 Related Guide: How to Select the Right Magnetic Core Size
Selecting Wire Size
Once the number of turns has been estimated, engineers verify that the winding fits inside the available window area.
👉 Related Guide: Choosing Wire Gauge for Power Inductors
Evaluate Wire Options
Wire Current Density Calculator
Estimate required copper area and approximate AWG size from RMS current and target current density.
Total Copper Area Required: mm²
Area Per Conductor: mm²
Approximate Suggested AWG:
This is a first-pass estimate. Real winding design also requires insulation diameter, window fill, AC loss, bend radius, and thermal checks.
Want optimized winding and CAD output?
Start Design AnalysisEstimating Losses
After selecting a core and winding, engineers estimate:
- Copper losses
- Core losses
- Thermal performance
Estimate Losses
Inductor Loss Estimator
Estimate copper loss, core loss, and total loss for a preliminary inductor design.
Copper Loss: W
Core Loss: W
Total Loss: W
Thermal Concern:
Need a thermal-checked design package?
Start Design Analysis👉 Related Guide: How Core Losses Are Calculated in Magnetic Components
Quick Design Evaluation
Modern magnetic design tools automate much of this process by comparing multiple candidate cores.
Inductor Quick Feasibility Checker
Use this quick estimator to check peak current, stored energy, and preliminary design difficulty.
Peak Current: A
Ripple Current: A p-p
Stored Energy: mJ
Preliminary Difficulty:
Likely Core Direction:
This is a quick educational estimate only. Final design requires core geometry, gap, winding, loss, fill factor, and thermal checks.
Need a manufacturable design package?
Run the full SolidMagnetics designer to generate optimized candidates, CAD files, BOM data, and design deliverables.
Start Design AnalysisPractical Design Checklist
When using AL values:
✔ Verify the datasheet conditions.
✔ Account for any air gap.
✔ Calculate the required turns.
✔ Verify window area.
✔ Estimate losses.
✔ Check saturation margin.
✔ Validate thermal performance.
Conclusion
The AL value is one of the most useful specifications in a magnetic core datasheet, but it should never be used in isolation.
Successful magnetic design requires combining AL values with core size, air gap, wire selection, saturation analysis, thermal evaluation, and manufacturability considerations.
Understanding how these factors work together allows engineers to create efficient and reliable magnetic components.
Ready to Generate Your Custom Magnetic Design?
Upload your electrical requirements and receive:
- 3D CAD model
- Manufacturing drawings
- BOM
- Build-ready geometry