Calculator KB-49
Inductor Calculator
Coils for radio receivers, filters, and induction projects.
Result
In brief: Calculate a single-layer air-core inductor using Wheeler’s formula: inductance from dimensions and turns, turns for a specified inductance, and wire length. Example: Unknown value: 6.788 µH.
Calculation example
| Winding diameter | 25 mm |
|---|---|
| Winding length | 25 mm |
| What to calculate | Inductance from turns |
| Number of turns | 20 pcs |
| Unknown value | 6.788 µH |
|---|---|
| Inductance | 6.788 µH |
| Turns | 20 |
| Wire length | 1.57 m |
| Winding pitch | 1.25 mm |
How it is calculated
Wheeler’s formula is accurate to about 1% for single-layer coils longer than 0.4 times their diameter. Inductance increases with the square of the number of turns: twice as many turns gives four times the inductance.
What to look for
- Measure the diameter at the wire’s center, especially if the wire is thick relative to the former.
- Nearby metal reduces inductance. Fine-tune the coil by compressing or stretching the turns.
Frequently asked questions
How do you calculate the inductance of an air-core coil?
Use Wheeler's formula: L = r² · N² / (9 · r + 10 · l), with dimensions in inches and the result in µH. 20 turns with a diameter and length of 25 mm each give about 6.8 µH.
How many turns do you need for 10 µH?
About 24 turns with a diameter and length of 25 mm each. Inductance increases with the square of the number of turns: twice as many turns gives four times the inductance.
Why does the actual inductance differ from the calculated value?
Wire thickness, winding pitch, and nearby metal all have an effect. The formula is accurate to about 1% when the length is more than 0.4 times the diameter. Fine-tune the rest by spreading the turns and checking with an LC meter.
