Series resistance calculator
When resistors are wired in series — end to end in a single line — the same current flows through each one, so their resistances add up. This calculator totals any number of resistor values for you. It is for electronics hobbyists, students and engineers working out an equivalent resistance.
How it works
The total (equivalent) series resistance is simply the sum of the individual values:
Rt = R1 + R2 + … + Rn
Enter your values in ohms, separated by commas or spaces. The result is always larger than the biggest single resistor, because every resistor adds to the path.
This is the opposite of a parallel connection, where the reciprocals add and the total is smaller than the smallest resistor.
Example
Three resistors of 100 Ω, 220 Ω and 470 Ω in series:
Rt = 100 + 220 + 470 = 790 Ω
| Resistors (Ω) | Series total (Ω) |
|---|---|
| 100, 220, 470 | 790 |
| 1000, 1000 | 2000 |
| 330, 220 | 550 |
Every calculation runs locally in your browser, with no network requests.
When to use series vs. parallel connections
Series is used when you want to:
- Raise resistance — add a series resistor to limit the current through a component (for example, a current-limiting resistor for an LED).
- Divide voltage — two resistors in series act as a voltage divider; the voltage across each is proportional to its resistance.
- Fuse a circuit with a known resistance — a sense resistor in series lets you measure current by measuring voltage across it (Ohm’s law: I = V / R).
Parallel is used when you want to:
- Lower resistance — parallel paths give current more routes, reducing total resistance.
- Increase current capacity — wiring two identical resistors in parallel halves the resistance and doubles the power handling.
Current and power through a series circuit
By Ohm’s law, current in a series circuit is:
I = V ÷ Rt
With the same current flowing through every resistor, the voltage drop across each is:
Vn = I × Rn
And power dissipated by each:
Pn = I² × Rn
This matters for component selection: a resistor carrying more than its rated power will overheat. For example, a 470 Ω resistor in a series circuit with 10 mA flowing through it dissipates 0.047 W — well within a standard 0.25 W rating. But at 100 mA it would dissipate 4.7 W and fail quickly.
Practical tips
- Use the series formula when designing LED current-limiting resistors: calculate the total resistance you need, then pick a standard E24 or E96 value that matches.
- For potentiometers or trimmers in series with fixed resistors, the total resistance sets the lower bound — turning the pot to zero only removes the variable portion.
- Check that no single resistor in the string handles more power than its wattage rating under worst-case voltage conditions.