Resistors in Series and Parallel Calculator
Enter up to six resistor values, leave unused ones at 0, choose series or parallel and add a supply voltage to see the total and each resistor's share.
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Formula
- Series:
Rt = R1 + R2 + R3 + ... - Parallel:
1 / Rt = 1/R1 + 1/R2 + 1/R3 + ... - Total current
I = V ÷ Rt, total powerP = V² ÷ Rt - Series share:
Rn ÷ Rtof the supply voltage drops across resistor n. - Parallel share:
(1/Rn) ÷ (1/Rt)of the total current flows through resistor n.
A resistor left at 0 Ω is treated as an unused slot.
Worked example
100 Ω, 220 Ω and 330 Ω on 12 V.
- Series:
Rt = 650 Ω,I = 12 ÷ 650 = 18.46 mA, voltage shares 15.38%, 33.85% and 50.77%. - Parallel:
1/Rt = 0.01 + 0.004545 + 0.003030 = 0.017576, soRt = 56.90 Ω,I = 210.91 mA, current shares 56.90%, 25.86% and 17.24%.
Frequently asked questions
How do I add resistors in series?
Add the values: 100 Ω + 220 Ω + 330 Ω = 650 Ω. The same current flows through all of them.
How do I calculate resistors in parallel?
Add the reciprocals and invert the sum. For 100, 220 and 330 Ω: 1/100 + 1/220 + 1/330 = 0.017576 S, so the total is 56.90 Ω. It is always lower than the smallest resistor.
What does the share percentage mean?
In series it is the share of the supply voltage across that resistor (100 Ω of 650 Ω is 15.38%). In parallel it is the share of the total current through that branch (56.90% for the 100 Ω branch above).
How do I combine fewer than six resistors?
Leave the unused boxes at 0. A zero entry is treated as an empty slot, not a short circuit, and at least two resistors are needed.
Is this accurate for real circuits?
It gives ideal values for learning and design checks, not safety certification. Real resistors have a tolerance (often 1% to 5%) and a power rating you must also respect.