Air-Core Coil Calculator
Calculate inductance, estimated wire length, DC resistance, and optional resonant frequency.
What this coil calculator online does
This tool helps you quickly estimate key electrical values for a single-layer, air-core solenoid coil. If you are building RF circuits, DIY antennas, induction experiments, Tesla-style projects, or just learning electronics, you usually need a fast way to estimate inductance without opening a spreadsheet. That is exactly what this page is for.
With a few dimensions, you can instantly get an inductance estimate in microhenries (µH), predicted wire length, and resistance. If you also enter a capacitor value, the calculator estimates resonant frequency so you can tune tank circuits more easily.
Formulas used by the calculator
1) Inductance (Wheeler formula for single-layer air-core coils)
where r = coil radius in inches, l = coil length in inches, N = number of turns.
This is a popular practical formula for hobby and engineering estimation. It is accurate enough for many design steps, especially when the coil is a single-layer winding and dimensions are measured consistently.
2) Wire length and DC resistance
Wire length is estimated from the helix path per turn, then multiplied by turns. If wire diameter is provided, resistance is computed using copper resistivity at 20°C:
Real-world resistance increases with temperature and depends on actual conductor purity and insulation thickness.
3) Resonant frequency (optional)
If capacitance is entered, the calculator applies the classic LC relation:
This is useful for tuning filters, oscillators, and radio-frequency circuits.
How to use it correctly
- Measure coil diameter as the winding diameter (not radius).
- Measure coil length from first turn to last turn along the axis.
- Enter the real number of turns, including partial turns if needed.
- Use wire diameter only if you want resistance estimation.
- Add capacitance to estimate LC resonance in Hz, kHz, or MHz.
Worked example
Suppose you wind a coil with diameter 25 mm, length 30 mm, and 20 turns. The calculator will estimate inductance in µH. If your wire is 0.6 mm copper, it also estimates DC resistance. If your circuit uses a 100 pF capacitor, it shows an approximate resonant frequency. This lets you decide quickly whether to increase turns, change diameter, or use a different capacitor to hit your target frequency.
Design tips for better coils
Use stable geometry
Small spacing changes can shift inductance. Secure windings with a form or non-conductive support.
Mind resistance and Q factor
Thicker wire generally reduces resistance. Lower resistance often improves Q factor in RF designs.
Keep magnetic environment consistent
Nearby metal objects can alter effective inductance. Measure and test in the same enclosure or setup where the coil will operate.
Frequently asked questions
Is this valid for ferrite or iron-core inductors?
No. This calculator is for air-core, single-layer coils. Core materials need different equations and permeability data.
Why is my measured value different from calculated value?
Real coils include parasitic capacitance, lead effects, winding imperfections, and instrument tolerances. Use this calculator as a design estimate, then verify with an LCR meter.
Can I use decimals for turns?
Yes. If your winding includes partial turns, decimal input is accepted.
Final note
A good coil calculator online saves time in prototyping and troubleshooting. Use this page for fast early-stage sizing, then fine-tune with real measurements. For best results, pair calculations with bench testing and practical iteration.