Resistor Calculator

Calculate equivalent resistance for two resistors in series and parallel.

Check Resistor Calculator

Series equivalent
320 ohms
Parallel equivalent
68.75 ohms
Series tolerance range
304 ohms to 336 ohms
Parallel tolerance range
65.3125 ohms to 72.1875 ohms
Worst resistor power
0.25 W
Checks each resistor in both topology examples
Rating margin
1x
0.25 W selected per resistor

Reproducible network check

Series adds resistance; parallel adds conductance. Copy the same inputs, tolerance assumption, and power boundary.

Resistors: R1=100 ohms, R2=220 ohms.
Series equivalent: 320 ohms; 5% range 304 ohms to 336 ohms.
Parallel equivalent: 68.75 ohms; 5% range 65.3125 ohms to 72.1875 ohms.
At 5 V, worst individual-resistor dissipation across the two topology checks is 0.25 W; 0.25 W rating margin is 1x.
Tolerance bounds assume both resistors move to the same percentage extreme. Verify actual resistor values, topology, voltage, temperature, and derating before hardware use.

Formula result

Check before you use it

What the numbers show

Series resistance

The answer separates series and parallel equivalent resistance, shows the selected tolerance range, and reports the worst individual-resistor power against the chosen rating.

Parallel resistance

Use resistor calculator for series or parallel resistor reduction, unit conversion, tolerance comparison, and a first-pass resistor wattage check.

Two-resistor reference

Series resistance is always larger than either resistor alone.

Copy-ready formula handoff

Use this after solving the live calculator result, then paste the answer into a lab note, homework check, or engineering review.

Formula

Req(series) = R1 + R2, Req(parallel) = 1 / (1/R1 + 1/R2), P = V^2 / R

Inputs to check
  • First resistor: Resistance of the first branch or series element.
  • Second resistor: Resistance of the second branch or series element.
  • Equivalent resistance: Single resistance that would replace the two-resistor combination.
  • Resistor tolerance: Selected manufacturing tolerance used to show a possible equivalent-resistance range.
Before copying
  • This page covers a two-resistor reduction only.
  • The series and parallel results are separate topology checks; the calculator does not infer how the physical circuit is wired.
  • Using the parallel reciprocal formula for resistors that are actually in series along one current path.
  • Forgetting that a parallel equivalent must be smaller than the smallest branch resistance.
Resistor Calculator result: [paste the solved value from the calculator above].
Formula used: Req(series) = R1 + R2,  Req(parallel) = 1 / (1/R1 + 1/R2),  P = V^2 / R
Inputs checked: First resistor, Second resistor, Equivalent resistance, Resistor tolerance.
Assumptions: This page covers a two-resistor reduction only. The series and parallel results are separate topology checks; the calculator does not infer how the physical circuit is wired.
Worked example: 100 ohm and 220 ohm resistors at 12 V, 5% tolerance, and 0.25 W each. Series equivalent: 100 + 220 = 320 ohms; the 5% range is 304 to 336 ohms. Parallel equivalent: 1 / (1/100 + 1/220) = 68.75 ohms; the 5% range is about 65.31 to 72.19 ohms. At 12 V, the parallel check puts 1.44 W in the 100 ohm branch, so a 0.25 W part has only 0.17x rating margin and is not suitable for that condition.
Next check: Using the parallel reciprocal formula for resistors that are actually in series along one current path.

Equation context

Built for series or parallel resistor reduction, unit conversion, tolerance comparison, and a first-pass resistor wattage check. This page pairs the live calculator with the governing formula, variable glossary, and a worked example so the result is easier to trust and reuse.

Quick entry points

Homework CheckScenario ComparisonUnits And Assumptions

Use the calculator to verify arithmetic after you set up the formula yourself.

Change one input at a time to see which variable is driving the result.

Review the formula notes before using the answer in a lab or design check.

Variables to track

R1
First resistor

Resistance of the first branch or series element. Unit: ohms.

R2
Second resistor

Resistance of the second branch or series element. Unit: ohms.

Req
Equivalent resistance

Single resistance that would replace the two-resistor combination. Unit: ohms.

Tolerance
Resistor tolerance

Selected manufacturing tolerance used to show a possible equivalent-resistance range. Unit: %.

V
Applied voltage

Voltage used for the component power and rating-margin check. Unit: V.

Formula method and unit assumptions

Formula and example

Req(series) = R1 + R2, Req(parallel) = 1 / (1/R1 + 1/R2), P = V^2 / R

Worked example

100 ohm and 220 ohm resistors at 12 V, 5% tolerance, and 0.25 W each

  1. 1Series equivalent: 100 + 220 = 320 ohms; the 5% range is 304 to 336 ohms.
  2. 2Parallel equivalent: 1 / (1/100 + 1/220) = 68.75 ohms; the 5% range is about 65.31 to 72.19 ohms.
  3. 3At 12 V, the parallel check puts 1.44 W in the 100 ohm branch, so a 0.25 W part has only 0.17x rating margin and is not suitable for that condition.

Equivalent resistance alone is not a component-safety verdict; topology, applied voltage, tolerance, and resistor power rating all change the decision.

sourceload path

Assumptions

This page covers a two-resistor reduction only.
The series and parallel results are separate topology checks; the calculator does not infer how the physical circuit is wired.
Tolerance ranges assume both resistors can reach the selected percentage limit together.
The power check assumes a steady DC voltage and compares the worst individual resistor result with the selected per-resistor rating.
Temperature drift, pulse energy, enclosure heating, and manufacturer derating curves remain outside this simple model.

Common mistakes

Using the parallel reciprocal formula for resistors that are actually in series along one current path.
Forgetting that a parallel equivalent must be smaller than the smallest branch resistance.
Reading a 4.7 kohm value as 4.7 ohms instead of selecting or converting the unit.
Treating a calculated 1.0x wattage margin as a safe design margin without checking temperature and pulse derating.

Related formula checks

Ohm's Law CalculatorUse equivalent resistance to solve current or voltage.Power Dissipation CalculatorCheck heat after current through the equivalent branch is known.Voltage Divider CalculatorApply two-resistor behavior to output voltage calculations.Series Parallel Resistance CalculatorMove from a two-resistor pair to a simple mixed network.

Equation context and next checks

Formula and variable setup for Resistor Calculator

Calculate equivalent resistance for two resistors in series and parallel. The page is designed to help you move from the known values to the correct formula without rebuilding the derivation every time.

For resistor calculator, the safest workflow is to confirm the unit system first, then map each symbol to the physical quantity in your problem statement before solving.

  • R1: First resistor (ohms) - Resistance of the first branch or series element.
  • R2: Second resistor (ohms) - Resistance of the second branch or series element.
  • Req: Equivalent resistance (ohms) - Single resistance that would replace the two-resistor combination.
  • Tolerance: Resistor tolerance (%) - Selected manufacturing tolerance used to show a possible equivalent-resistance range.
  • V: Applied voltage (V) - Voltage used for the component power and rating-margin check.

How to read the result

The answer separates series and parallel equivalent resistance, shows the selected tolerance range, and reports the worst individual-resistor power against the chosen rating.

This tool is especially useful for series or parallel resistor reduction, unit conversion, tolerance comparison, and a first-pass resistor wattage check. The output becomes more trustworthy when you compare nearby cases instead of relying on one single run.

  • Series resistance
  • Parallel resistance
  • Two-resistor reference

Assumptions and limits

The calculator applies the standard textbook relation for this topic, which makes it a strong first-pass answer but not always a full real-world model.

Before you use the result in a lab, design review, or report, check whether the simplified assumptions still match the physical system you care about.

  • This page covers a two-resistor reduction only.
  • The series and parallel results are separate topology checks; the calculator does not infer how the physical circuit is wired.
  • Tolerance ranges assume both resistors can reach the selected percentage limit together.
  • The power check assumes a steady DC voltage and compares the worst individual resistor result with the selected per-resistor rating.
  • Temperature drift, pulse energy, enclosure heating, and manufacturer derating curves remain outside this simple model.

Quick glossary

First resistor

Resistance of the first branch or series element.

Second resistor

Resistance of the second branch or series element.

Equivalent resistance

Single resistance that would replace the two-resistor combination.

Ideal model

A simplified physics model that omits secondary effects so the first-order relationship is easier to inspect.

Formula checks before using the result

Formula questions

Checks before using the result

When should I use the resistor calculator?

Use resistor calculator for series or parallel resistor reduction, unit conversion, tolerance comparison, and a first-pass resistor wattage check, especially when the governing formula is already known and the main need is a fast, transparent calculation.

What is the main thing the resistor calculator tells me?

The answer separates series and parallel equivalent resistance, shows the selected tolerance range, and reports the worst individual-resistor power against the chosen rating.

What can make the resistor calculator answer inaccurate?

The answer is exact for the formula and assumptions on the page, but it can drift when the real system violates those assumptions. Common limits include This page covers a two-resistor reduction only. The series and parallel results are separate topology checks; the calculator does not infer how the physical circuit is wired. Tolerance ranges assume both resistors can reach the selected percentage limit together. The power check assumes a steady DC voltage and compares the worst individual resistor result with the selected per-resistor rating. Temperature drift, pulse energy, enclosure heating, and manufacturer derating curves remain outside this simple model.

Formula references and related examples

Formula Basis

Formula Notes And References

Series resistance is always larger than either resistor alone.
Parallel resistance is always lower than the smaller branch resistor.
In parallel, each branch sees the full applied voltage; in series, current is shared while voltage divides.
Use a manufacturer data sheet for temperature, pulse, surge, and long-term reliability limits.