Coil Inductance Calculator
Calculate inductor value using both the solenoid equation and the Wheeler single-layer air-core formula.
Units: diameter and length in mm, frequency in kHz. Wheeler result is intended for single-layer air-core coils.
What this coil inductance calculator does
This tool helps you quickly estimate inductance for common coil designs. It’s useful when designing RF inductors, tuning LC circuits, prototyping filters, and checking whether a hand-wound coil is in the right range. You get two estimates:
- Solenoid formula: physics-based, includes relative permeability (μr).
- Wheeler formula: practical shortcut for single-layer air-core coils.
Inductance formulas used
1) Solenoid equation (SI units)
Where L is inductance in henries, μ0 is vacuum permeability (4π × 10-7 H/m), μr is relative permeability, N is turn count, A is cross-sectional area, and l is coil length.
2) Wheeler single-layer air-core equation
Wheeler’s equation is fast and popular for practical air-core inductor calculations. It assumes a single-layer coil and works best when geometry is reasonable (not an ultra-short or ultra-long winding).
How to enter values correctly
Number of turns (N)
Enter full turns as an integer. Fractional turns can be used if your design includes tapped or partial turns.
Coil diameter
Use the average diameter of the winding. For best accuracy, measure from centerline-to-centerline of the wire bundle.
Coil length
This is the axial winding length, not wire length. Closely packed turns usually produce shorter coil length.
Relative permeability (μr)
Use 1 for air-core coils. For ferrite/powdered-iron cores, use manufacturer data if available. Real cores are nonlinear and frequency-dependent, so calculated values are still estimates.
Worked example
Suppose you wind a coil with 100 turns, 25 mm diameter, 30 mm length, and μr = 1. The solenoid method gives roughly a few hundred microhenries, while Wheeler may differ somewhat. That difference is normal because each model makes different assumptions.
How to increase or decrease inductance
- Increase turns: strongest effect (inductance rises with N²).
- Increase diameter: larger area usually increases inductance.
- Shorten coil length: typically increases inductance.
- Use higher μr core: can greatly increase inductance.
- For lower inductance: reverse the above choices.
Common design mistakes
- Mixing units (mm vs inches vs meters).
- Using air-core formulas for magnetic-core inductors without caution.
- Ignoring parasitic capacitance and self-resonant frequency in RF circuits.
- Expecting perfect agreement between calculator and bench measurements.
FAQ
Is this an air core coil calculator?
Yes. It includes a dedicated air-core Wheeler estimate and a broader solenoid inductance formula.
Can I use it as an inductor reactance calculator?
Yes. If you enter frequency, it also reports inductive reactance using XL = 2πfL.
Why are my measured values different?
Real-world effects matter: wire insulation spacing, core tolerance, temperature, nearby metal, and measurement method. Use this as a strong starting point, then verify with an LCR meter.
Final note
This coil inductance calculator is ideal for quick design iteration in electronics projects. For precision work, follow up with simulation and lab measurements, especially at high frequency.