Parallel Inductors Calculator

What it is and when to use it

Inductors connected across the same two nodes form a parallel network. Each one carries part of the current, and because they share the same voltage the equivalent inductance is lower than any individual coil. Combining inductors this way is common when you need a smaller value than the standard series offers, or when you want to split current among several parts to reduce heating and saturation.

Use this calculator to find the equivalent inductance of two to four inductors, to check a parallel bank in a filter or switching converter, or to work out what single value would replace an existing combination. It assumes ideal, uncoupled inductors, so it is best for quick design estimates and homework rather than tightly coupled windings.

The formula and how it works

For n inductors in parallel the equivalent inductance L_eq satisfies:

1 / L_eq = 1 / L₁ + 1 / L₂ + ... + 1 / L_n
L₁ to L_n are the individual inductances, all in the same unit.
L_eq is the single inductance that would draw the same current for the same changing voltage.
For two inductors this simplifies to L_eq = (L₁ x L₂) / (L₁ + L₂).

The calculator ignores mutual inductance, the coupling between coils that share magnetic flux.

Worked example

Consider three inductors of 10 mH, 20 mH and 30 mH connected in parallel. The reciprocals are 1/10 = 0.1, 1/20 = 0.05 and 1/30 = 0.03333 per mH. Their sum is 0.18333 per mH.

Taking the inverse gives L_eq = 1 / 0.18333 = 5.4545 mH, which the calculator shows as 5.4545 mH, or 5.4545e-3 H. This is well below the smallest branch of 10 mH, as expected. For a quick check with only the first two, (10 x 20) / (10 + 20) = 6.667 mH, and adding the 30 mH branch pulls it down further.

Common mistakes and how to interpret the result

Adding inductances directly: that is the series rule. Parallel values must combine through reciprocals.
Mixing units: 10 mH and 20 uH in the same calculation without converting gives a result off by a factor of 1000.
Ignoring mutual coupling: if coils sit on a common core or very close together, the real equivalent differs from the ideal formula.
Expecting equal current sharing: current divides in inverse proportion to inductance for changing signals, so the smallest inductor carries the largest share.

Frequently Asked Questions

Why is the parallel inductance always smaller than the smallest inductor?

Each extra branch gives the current another path, and the reciprocal formula adds the reciprocals of the inductances. A sum of positive reciprocals is larger than any single reciprocal, so its inverse is smaller than any single inductance. Two equal inductors in parallel give exactly half of one.

Does this formula work if the inductors are close together?

Only if their magnetic fields do not interact. When two coils share flux, mutual inductance M changes the result: for two coils with aiding coupling the equivalent is (L1 L2 - M squared) divided by (L1 + L2 - 2M). This calculator assumes zero mutual coupling, which is typical when inductors are shielded or spaced apart.

Is it the same formula as parallel resistors?

Yes, the arithmetic is identical: reciprocal of the sum of reciprocals. Inductors in series add directly like resistors in series. The physical reason is different, since inductance relates voltage to the rate of change of current, but the equivalent value of ideal, uncoupled inductors follows the same pattern.

Which unit should I choose?

Pick the unit that matches your components, and enter all values in that same unit. The calculator converts internally, so the henry value shown is exact regardless of your choice. Mixing units, such as 10 mH and 20 uH, requires converting one of them first.