Coil Inductance Calculator
Calculate inductance of a single-layer solenoid (air-core) with Wheeler's or estimate rod-core coils (experimental). Solve for L or for turns, get instant unit conversions, and share a permalink.
Results
Air-core uses Wheeler (single-layer). Rod-core is an experimental estimate using a simplified magnetic path and leakage factor.
Theory, Validity & Tips
Air-core single-layer solenoids are estimated with Wheeler's formula L(µH) = (r²*N²) / (9r + 10l) where r and l are in inches. Accuracy is usually within a few percent for practical aspect ratios.
- Best accuracy when
l >= 0.8*D. For very short "pancake" coils or extremely long coils, error increases. - Use consistent units. The tool converts mm↔in automatically.
- Wire gauge affects resistance, Q and SRF, not L directly in the ideal model.
Rod-core (beta) uses L ~ k_eff * µ0 * µr * N² * A / l_eff, with k_eff between 0 and 1 capturing end effects/leakage. Treat as order-of-magnitude.
FAQ
How accurate is Wheeler for air-core?
Typically within a few percent for single-layer coils with reasonable aspect ratio. Proximity to metal or nearby coils reduces accuracy.
Can I use ferrite with Wheeler?
Not directly. Use the Rod-core tab (beta) with ĂŽÂĽr and k_eff. For toroids/spirals, use a dedicated model.
How do I get the number of turns for a target L?
Select "Solve for turns" and enter target L; we compute N and round to the nearest integer.
Which units should I enter?
Use mm or inches—toggle at the top. Results show multiple output units simultaneously.
What about multilayer or PCB spiral coils?
Use a multilayer/spiral calculator (coming soon on NodeLoop). Different geometry, different model.
References
- Harold A. Wheeler, empirical formula for single-layer solenoids.
- Basic magnetic circuit model for rod cores (order-of-magnitude).