Parallel Resistance Calculator

Resistors in parallel

When you connect resistors in parallel, the total resistance goes down. The current now has more than one path to follow, so overall it meets less resistance. Each resistor sees the same voltage across it but carries its own share of the current, depending on its value.

The formula

The total resistance R_total of resistors in parallel is the reciprocal of the sum of the reciprocals of each value:

Example

Take three resistors in parallel of 100 Ω, 200 Ω and 300 Ω:

Adding the reciprocals:

Then take the reciprocal of that to get the total:

Picture each resistor bridged across the same two points, giving the current a set of parallel paths. It splits between them according to their values, with the lower resistances taking more of it.

Two things to keep in mind

• The total is always smaller than the lowest single resistor in the group.
• The voltage is the same across every resistor, but the current through each one varies.

Wiring resistors in parallel is how you reach a value lower than any you have to hand, or spread current and therefore power across several components instead of one.