Parallel Resistor Calculator
Add the resistors you have, read the value they make together. The result is always smaller than the smallest one you typed.
How parallel resistance works
Each extra path lets more current through, so resistance drops. The math is one over the sum of the one-overs: 1/R = 1/R1 + 1/R2 + ... For exactly two resistors there's the shortcut R = (R1 × R2) ÷ (R1 + R2), and for N equal resistors it's simply the value divided by N. Two 1 kΩ resistors make 500 Ω; ten make 100 Ω.
Where you'll actually use this
Mostly when the value you need isn't in the drawer. No 2.5 kΩ in stock? Two 5.1 kΩ resistors in parallel get you to 2.55 kΩ, closer than the nearest E12 value. It also shows up when sharing current between resistors that would each run too hot alone: two equal resistors in parallel each carry half the current and dissipate half the power.
The other classic is measuring: an unknown resistor in parallel with your meter's input impedance reads lower than it really is. If a 1 MΩ resistor measures 900 kΩ, this calculator run backwards explains where the missing 100 kΩ went.