Series Resistor Value Calculator

A resistor series value calculator determines the equivalent resistance of three fixed resistors connected end-to-end in one current path. Enter Resistance 1, Resistance 2, and Resistance 3 in ohms; the calculator returns the total opposition to current flow for the complete series network.

For a series circuit, the same circuit current passes through each resistor. Each resistor produces its own voltage drop, and those voltage drops add to the source voltage. The combined resistance is therefore used when reviewing a simple branch-circuit control circuit, voltage-divider arrangement, current-limiting network, sensor interface, electronic equipment repair, or component-level prototype.

This is fixed-resistance arithmetic. It does not select a resistor value, determine an approved resistor wattage, or establish electrical-code compliance.

Series Resistance Calculation

The calculator uses the entered values exactly as shown in the fields:

\(\displaystyle R_\text{equivalent} = R_1 + R_2 + R_3\)

Where:

• R_1 is Resistance 1
• R_2 is Resistance 2
• R_3 is Resistance 3
• R_\text{equivalent} is the Equivalent resistance

All input values must use the same resistance unit. With the displayed unit set to ohm, the resulting equivalent resistance is also shown in ohms.

Unlike a parallel resistor circuit, a series connection always produces a total resistance greater than each individual resistor. Adding resistance in series reduces circuit current for a fixed supply voltage, consistent with Ohm’s law:

\(\displaystyle I = \frac{V}{R_\text{equivalent}}\)

The series total can then be carried into a separate current, voltage-drop, voltage-divider, or resistor-power calculation.

Calculator Inputs and Results

Field or result	Electrical use
Resistance 1	First fixed resistor value in the series path
Resistance 2	Second fixed resistor value in the series path
Resistance 3	Third fixed resistor value in the series path
Equivalent resistance	Total resistance seen across all three series-connected resistors
Equivalent-to-smallest ratio	Equivalent resistance divided by the smallest entered resistor value

The Equivalent-to-smallest ratio is a quick comparison value, not a resistor rating. It shows how many times greater the total series resistance is than the smallest resistor in the entered set.

For example, a result of 6 x means the combined series resistance is six times the value of the smallest individual resistor.

Calculation Example

Enter the following fixed resistor values:

Input	Value
Resistance 1	10 ohm
Resistance 2	20 ohm
Resistance 3	30 ohm

The equivalent series resistance is:

\(\displaystyle R_\text{equivalent} = 10 + 20 + 30\)

\(\displaystyle \text{Equivalent resistance} = \text{60 ohm}\)

The smallest entered value is 10 ohm. The ratio calculation is:

\(\displaystyle \frac{60\text{ ohm}}{10\text{ ohm}} = 6\)

\(\displaystyle Equivalent-to-smallest ratio = \text{6 x}\)

If 12 V were applied across this ideal 60-ohm series network, the circuit current would be 0.2 A:

\(\displaystyle I = \frac{12\text{ V}}{60\text{ ohm}} = 0.2\text{ A}\)

That current calculation is not produced by this screen, but it illustrates how the equivalent resistance is used in downstream circuit review. Individual resistor voltage drops could then be calculated from V = IR.

Component and Field Limits

The result assumes ideal, fixed resistance values. It does not account for:

• Resistor tolerance or actual measured resistance
• Temperature coefficient and resistance drift
• Resistor wattage or thermal dissipation
• Maximum working voltage or voltage rating
• Lead temperature, enclosure temperature, or installation heat
• Frequency effects, inductance, capacitance, or other parasitics
• Manufacturer data, equipment listing requirements, or circuit protection characteristics

For energized equipment, calculate the power in each resistor separately:

\(\displaystyle P = I^2R\)

A resistor with the correct ohmic value can still fail if its wattage rating, voltage rating, temperature capability, or installation environment is unsuitable. Series resistors divide voltage in proportion to their resistance, so the voltage across each component must also remain within its manufacturer rating.

U.S. Installation Review

This worksheet supports component arithmetic, not conductor or installation design. It does not determine AWG or kcmil conductor size, ampacity, adjustment factor, correction factor, current-carrying conductor count, terminal rating, insulation temperature rating, voltage drop, raceway fill, branch-circuit protection, feeder sizing, or equipment suitability.

For U.S. project work, verify the resistor ratings, enclosure and thermal conditions, equipment listing, manufacturer instructions, and requirements adopted by the local AHJ. Where a resistor network is part of listed industrial control equipment, power supplies, electronic controls, or a field-installed assembly, the final installation decision depends on the complete equipment and wiring arrangement—not the equivalent resistance alone.