🔗Series & Parallel Resistor Calculator

About this tool

Type your resistor values separated by commas, using the usual shorthand such as 220, 4.7k, 4k7, 1M or 10R. The calculator shows the total resistance in series and in parallel. Below, enter a target value and choose E12 or E24 to find the single standard resistor or the pair, in series or parallel, that comes closest, with the error.

How to combine resistors

Resistors in series carry the same current, so their resistances simply add: 100 Ω + 220 Ω + 470 Ω = 790 Ω. In parallel, each resistor gives the current another path, so the reciprocals add: 1/R = 1/R₁ + 1/R₂ + …, and the total is always less than the smallest resistor. Those same three in parallel make about 59.98 Ω. Two equal resistors in parallel give exactly half of one, which is a handy trick: two 10 kΩ resistors in parallel make 5 kΩ, a value that isn't in the E12 series. Standard resistor values come in E-series, where E12 has 12 values per decade (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2) and E24 has 24. When the value you need isn't in your parts drawer, combining two standard parts in series or parallel can usually get within a percent or two. Honest limits: real resistors have tolerances, typically 1% or 5%, so the actual total varies, and combinations change the power each resistor dissipates, so check power ratings. The resistor colour code tool reads bands, the Ohm's law calculator relates voltage, current and resistance, and the voltage divider calculator designs dividers.

Frequently asked questions