When you do not have the exact resistor value you need, two or three standard values combined often get you there. And when you measure a resistor in circuit, the meter sees it in parallel with everything else on the same nets. Both situations need the same arithmetic, which this calculator does for any number of resistors.
How to use it
- Choose Series or Parallel.
- Type the values in one list, separated by commas or spaces:
10k, 4.7k, 2k2, 330. - Read the total and, in the table, each resistor's share of the voltage (series) or current (parallel).
The formulas
- Series: the total is the sum — R = R1 + R2 + R3…
- Parallel: the reciprocals add — 1/R = 1/R1 + 1/R2 + R3… For two resistors this is R1 × R2 ÷ (R1 + R2).
The parallel total is always smaller than the smallest resistor in the group. That fact explains a common surprise at the bench.
Worked example: an in-circuit reading
You measure a 10 kΩ pull-up in circuit and read about 3.2 kΩ. Looking at the schematic, the same two nets are also joined through a 4.7 kΩ resistor elsewhere. In parallel, 10 kΩ and 4.7 kΩ give 3.2 kΩ — so the reading is exactly what a healthy board should show. Without that check you might replace a perfectly good resistor. The reasoning is covered in How to Test a Resistor with a Multimeter (In and Out of Circuit).
Building an odd value
A repair calls for 6.8 kΩ in a feedback position and the drawer has only 10 kΩ and 22 kΩ. Two 10 kΩ in series with nothing else gives 20 kΩ; 10 kΩ in parallel with 22 kΩ gives 6.875 kΩ — about 1 % high. For a feedback divider that may shift the rail noticeably, so a correct single part is still the better repair; for a pull-up it is fine. The nearest standard values are listed by Standard Resistor Value Calculator (E6–E96).
Where it matters on a laptop board
- Interpreting in-circuit resistance readings.
- Understanding why a rail's resistance to ground is so low — dozens of chips in parallel.
- Combining parts for a temporary or test repair.
The circuit rules behind it are summarised in series and parallel circuits.
FAQ
Why is my parallel total lower than every resistor?
Each extra parallel path gives current another way through, so the combination always conducts better than any single branch.
Can I combine resistors to replace a current-sense resistor?
Avoid it. Sense resistors need precise, low values with good thermal behaviour; use the specified part.
Does the power rating add up too?
In series, each resistor carries the same current; in parallel, each sees the same voltage. Check each resistor's share against its own rating.