Case 1
- First coil L1
- 0.1H
- Second coil L2
- 0.2H
0.066667H
Open with these valuesParallel Inductance Calculator
0.066667H
Result: 0.066667 HTwo coils in parallel behave like one of L1 × L2 ÷ (L1 + L2), which is always smaller than the smaller coil. Two equal coils give exactly half: 10 mH beside 10 mH is 5 mH. Enter both values in henries — 100 mH is 0.1 — and keep the coils apart, because the formula assumes no magnetic coupling.
Worked examples
Case 2
- First coil L1
- 0.001H
- Second coil L2
- 0.001H
0.000500H
Open with these valuesCase 3
- First coil L1
- 10H
- Second coil L2
- 40H
8.000000H
Open with these valuesHow it's calculated
L = L1 × L2 ÷ (L1 + L2)
- StepEnter the first inductance in henries — 100 mH is 0.1.
- StepEnter the second the same way; millihenries ÷ 1000, microhenries ÷ 1000000.
- ResultRead the equivalent inductance, always below the smaller coil.
Reference table
| L1, L2 | What it is | Equivalent |
|---|---|---|
| 0.001, 0.001 | Two 1 mH coils — half of one | 0.000500 |
| 0.1, 0.2 | 100 mH beside 200 mH | 0.066667 |
| 0.5, 0.5 | Two equal half-henry coils | 0.250000 |
| 1, 1 | Equal coils always halve | 0.500000 |
| 2, 3 | 2 H beside 3 H | 1.200000 |
| 10, 40 | A big coil barely moved by a bigger one | 8.000000 |
Questions
How do I calculate inductance in parallel?
For two coils, multiply the two inductances and divide by their sum: L = L1 × L2 ÷ (L1 + L2). Henries in, henries out — 0.1 H beside 0.2 H gives 0.066667 H.
Why is the result smaller than either coil?
A second parallel path gives the current another route, so the pair opposes a change in current less than either coil would alone. The equivalent inductance is therefore always below the smaller of the two, exactly as with resistors in parallel.
How is parallel different from series?
Coils in series simply add: L = L1 + L2. Coils in parallel combine by the reciprocal rule, which for two of them is the product over the sum, so series raises the total and parallel lowers it.
What do two equal coils give?
Exactly half the value of one. Two 10 mH coils in parallel act as a single 5 mH coil, because L × L ÷ (L + L) is L ÷ 2.
Does this account for mutual coupling?
No. The product-over-sum form assumes the coils are magnetically independent. If they sit close together or share a core, mutual inductance changes the real value and the fuller formula with the coupling term is needed.
Sources and last check
Information, not professional advice.