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ENERGY CALCULATOR

Series Resistor Calculator

Calculate equivalent resistance, steady current, and dissipation for three series resistors.

Calculator

Ω
Ω
Ω
V
YOUR RESULTS
Equivalent series resistance600 Ω
Circuit current
0.02 A
First resistor voltage drop
2 V
Second resistor voltage drop
4 V
Third resistor voltage drop
6 V
Total resistor dissipation
0.24 W

Page guide

Equivalent resistance and series voltage drops

Equivalent resistance determines the steady current for the entered supply. Total power is the sum of resistor heating, not the rating required for any single component. Evaluate each resistor’s dissipation and design margin separately.

For branches connected across the same two nodes instead of one chain, the Parallel Resistor Calculator calculates the different equivalent resistance.

The formula

Series resistance R = R1 + R2 + R3. Current I = V/R. Individual drop Vi = I × Ri. Total dissipation P = V × I.

Ideal series elements share one current. Each voltage drop is proportional to its resistance, and their sum equals the supply voltage. A zero resistance represents an omitted or ideal wire segment; the total must remain positive.

Worked example

Resistors of 100, 200, and 300 Ω total 600 Ω. With 12 V applied, current is 0.02 A. Their voltage drops are 2, 4, and 6 V, and the complete string dissipates 0.24 W.

How to use this calculator

  1. Enter first resistance, second resistance using the units shown.
  2. Set third resistance, dc supply voltage.
  3. Calculate and review equivalent series resistance alongside the supporting quantities.

Series means one current path

A chain is series only when the current has no branching path between its resistors. A meter, sensor, or connected load can change that arrangement. The input voltage here is applied across the complete three-element string. For two resistors, enter zero for the third rather than combining a parallel network as if it were a series chain.

Nominal values and heating

The calculation treats resistance as fixed. Real tolerances and temperature coefficients can shift both current and voltage division. Heating can also alter component behavior, and the supply may have a current limit or internal resistance. Compare individual power with component specifications and operating conditions; the arithmetic does not select a resistor technology or thermal rating.

Assumptions & limitations

What this calculation assumes

  • Resistors are ideal, constant, and connected in a single unbranched DC path.

What to keep in mind

  • Excludes tolerances, supply impedance, temperature rise, and reactive components.

Common questions

Can one resistor be zero?

Yes, provided at least one resistor is positive. Zero represents an ideal wire or an omitted element.

Why does the largest resistor drop the most voltage?

All series resistors carry the same current. Multiplying that current by a larger resistance produces a larger voltage drop.

Do the individual drops add to the supply voltage?

Yes. For this ideal series string, the voltage drops sum to the entered source voltage.

Sources & further reading

Reviewed on