Series & Parallel Resistor Calculator
Resistors in series add up; resistors in parallel combine to less than the smallest one. Enter up to six values (leave the rest blank) and this calculator returns both equivalents — the two calculations every electronics student and technician does weekly.
Formulas: Series: R = R₁ + R₂ + … · Parallel: 1/R = 1/R₁ + 1/R₂ + …
Worked example: 100 Ω and 200 Ω in series give 300 Ω; in parallel they give (100 × 200) ÷ (100 + 200) = 66.67 Ω — less than the smaller resistor, as parallel combinations always are. That rule is a quick sanity check: if your parallel answer is bigger than your smallest resistor, something is wrong.
For a two-resistor output voltage, use the voltage divider calculator.
Frequently asked questions
Why is the parallel result always smaller than the smallest resistor?
Parallel paths give current more routes, so total resistance must fall. Adding any parallel resistor, however large, lowers the total — it can never raise it.
What is the shortcut for two resistors in parallel?
Product over sum: R = (R₁ × R₂) ÷ (R₁ + R₂). For equal resistors it is even simpler: two equal resistors in parallel give half of one.
Does the same maths work for capacitors or inductors?
Reversed: capacitors in series combine like resistors in parallel, and vice versa. Inductors follow the resistor rules. Always check which component you are combining.