Series Resistor Calculator
Total series resistance plus current and per-resistor voltage drops.
What it does
Resistors in series carry the same current, and their resistances simply add. The calculator also shows how the applied voltage divides across each resistor.
Where it's used
Making a non-standard value from standard parts, voltage dividers and current-sense chains.
How to use it
- Type the resistor values separated by commas, e.g.
330, 470, 1k. - Enter the applied voltage.
- Read the total resistance, the current and the voltage and power of each resistor.
The formula explained
- R_total
- R1 + R2 + … + Rn
- I
- V / R_total — the same through every resistor
- V_n
- I × R_n, the voltage across resistor n
Worked example
Three resistors in series on a 9 V battery.
Inputs:
- Resistor values (comma separated): 330, 470, 1k Ω
- Applied voltage: 9 V
The calculator gives:
- Total resistance: 1.8 kΩ
- Resistor count: 3
- Current (same in all): 5 mA
Tips and common mistakes
- The total is always larger than the biggest single resistor.
- The largest resistor takes the largest share of the voltage and the power.
- Tolerances add up too: three ±5 % parts give a total that can be off by 5 %.
Questions people ask
How do I find the voltage across each resistor?
All of them carry the same current, I = V / R_total, so each one drops V_n = I × R_n. With 330 Ω, 470 Ω and 1 kΩ on 9 V the current is 5 mA, giving 1.65 V, 2.35 V and 5 V — which add back up to 9 V.
Why put resistors in series?
To make a value you don’t have, to share a high voltage (each resistor has a maximum working voltage, a few hundred volts for common through-hole parts) or to spread the heat over several parts.
What happens if one resistor in a series chain fails?
If it fails open, the whole chain stops conducting and the full supply voltage appears across the broken resistor. If it fails short, the total drops and the others take more voltage and power.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.