Solenoid Inductance Calculator
Use this calculator for a cylindrical coil (solenoid). Enter your coil geometry and core material, then optionally calculate inductive reactance at a given frequency.
What this inductance calculator does
This page helps you estimate the inductance of a solenoid coil using the standard physics equation in SI units. It is useful for electronics projects, power supply design, filter circuits, audio crossovers, and educational work.
If you also enter a frequency, the calculator gives you inductive reactance, which tells you how much the inductor resists alternating current.
Formula used
Solenoid inductance
The calculator uses:
L = (μ0μrN2A) / ℓ
- L = inductance in henries (H)
- μ0 = vacuum permeability = 4π × 10-7 H/m
- μr = relative permeability of core material
- N = number of turns
- A = cross-sectional area of the coil = πr2
- ℓ = coil length (meters)
Inductive reactance (optional)
When frequency is provided, the calculator also computes:
XL = 2πfL
This value is given in ohms (Ω) and describes AC opposition from inductance.
How to use it correctly
- Enter the number of turns on your coil.
- Enter coil radius and coil length with proper units.
- Set relative permeability to match your core (1 for air).
- Optionally enter frequency to compute reactance.
- Click Calculate to see L in H, mH, and µH.
Worked example
Suppose your coil has 250 turns, radius 12 mm, length 80 mm, and an air core (μr = 1). The result is approximately 0.000444 H, or 0.444 mH.
At 10 kHz, that same inductor has reactance of about 27.9 Ω.
What changes inductance the most?
- Number of turns (N): inductance grows with N2, so turns have a strong effect.
- Core material (μr): high-permeability cores can increase inductance dramatically.
- Coil radius: larger radius means larger area A, which increases L.
- Coil length: increasing length lowers inductance (inversely proportional).
Common mistakes to avoid
- Mixing up diameter and radius (radius is half the diameter).
- Using incorrect units for geometry values.
- Forgetting that real inductors include parasitic resistance and capacitance.
- Assuming the formula is perfect for very short or unusual coil shapes.
Frequently asked questions
Is this accurate for every inductor?
It is a strong first-order estimate for ideal solenoids. Real components can differ due to winding spacing, core nonlinearity, and fringing fields.
What if I only know diameter?
Divide diameter by 2 and enter that value as radius.
Can I use this for RF design?
Yes, for early sizing. For final RF tuning, combine this with measurement or simulation because high-frequency parasitics become important.
Final note
This inductance calculator is designed for clarity and fast iteration. Use it to compare designs quickly, then validate with physical measurements for production-level accuracy.