Electronics · Free Calculator
Air Core Coil Inductance Calculator
Find the inductance of a single layer air core coil from its diameter, length and turns, or the turns needed for a target value.
Wheeler formulaSingle layer coilTurns for target LRF coils
Inductance1.607 µH
Turns per mm1
How this result was calculated
- d = 0.3937 in, l = 0.7874 in
- L = d²N² ÷ (18d + 40l) = 1.607 µH
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Wheeler Formula
L (µH) = d² N² ÷ (18 d + 40 l)
d is the coil diameter and l is the coil length, both in inches (the calculator converts from mm), N is the number of turns. Measure d to the centre of the wire.
Worked example
A coil 10 mm in diameter and 20 mm long with 20 turns: d = 0.394 in, l = 0.787 in, so L = 0.155 × 400 ÷ (7.09 + 31.5) = 1.61 µH.
Key insight: inductance rises with the square of the turns, so doubling the turns on the same former gives about four times the inductance.
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Frequently Asked Questions
How accurate is Wheeler formula?
About 1 % for single layer coils longer than 0.4 times their diameter.
Does wire thickness matter?
Only slightly. Use the diameter measured to the wire centre for best accuracy.
Can I use this for ferrite cores?
No. A core multiplies the inductance by its effective permeability; use the core AL value instead.
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