Series and parallel resistors
Ideal resistors.
Method
Worked case: 10 Ω and 15 Ω are 25 Ω in series and 6 Ω in parallel.
What to write beside the result
A review can only check this figure if the inputs sit next to it. Write the value, the unit and the assumption the form used. A diameter in millimetres and a diameter in metres are not the same reading. A force in kilonewtons divided as if it were newtons is a thousand-fold error.
The result is the arithmetic on this page. It is not a utilisation, not a selected product, and not a clause from a standard. If the number is going into a shared container, name the page, the date and the inputs. The file-name checker does not read the calculation.
Logic: how this figure is built.
How to read two resistors
Series resistance is the sum. Parallel resistance is the product over the sum for two resistors. 10 Ω and 15 Ω are 25 Ω in series and 6 Ω in parallel. The parallel result is always smaller than the smaller resistor.
The parallel formula on this page is two resistors. A third resistor needs another step. Equal resistors in parallel share the current. Unequal resistors do not.
Tolerance is not applied. A 10 Ω 5% resistor is not exactly 10 Ω. The page is the nominal pair.
Use it to check a bias network or a load. It does not give the power in each resistor.
- Related: voltage divider
- electrical logic
Last checked 3 October 2026. See the common-calculations note and the disclaimer.