About the Resistor Calculator
This resistor calculator does the three jobs that come up at a workbench. Type a list of resistors and it gives the total when they are wired in series and when they are wired in parallel. Switch to LED mode to find the current-limiting resistor for a supply voltage, a forward voltage and a current, rounded up to a real value. Or type the value you need and it searches E12, E24 or E96 parts for the closest single resistor, series pair and parallel pair. Nothing is sent anywhere: the maths runs in your browser.
Updated September 2026
How to use the Resistor Calculator
- 1Choose Series / parallel, LED resistor or Make a value.
- 2For a combination, type the values separated by commas, spaces or new lines. 4.7k and 2M2 are fine.
- 3For an LED, enter the supply voltage, the LED forward voltage and the current you want.
- 4To make a value, type the resistance you need and choose the series of parts you have.
- 5Read the result and copy the value with the Copy button.
Series and parallel in one minute
Resistors in series add up: 100 Ω, 220 Ω and 330 Ω make 650 Ω. The same current flows through all of them and the voltage is shared between them.
Resistors in parallel give a total smaller than the smallest one. Add the reciprocals and take the reciprocal of the sum. Two equal resistors halve the value, three equal ones divide it by three. The calculator shows both totals for the same list so you can compare the two wirings.
A resistor of 0 Ω in a parallel group shorts it, so the total is 0 Ω. The tool says so instead of showing an error.
Choosing an LED resistor
An LED does not limit its own current, so a resistor must. Subtract the LED's forward voltage from the supply, then divide by the current you want. A 5 V supply, a 2 V red LED and 20 mA give 150 Ω. For 9 V and a 2.1 V LED at 20 mA the answer is 345 Ω, which is not a standard value.
The tool rounds up to the next value in the series you pick, here 360 Ω, so the current is slightly lower and the LED is safe. The real current and the heat in the resistor are shown. Many LEDs are bright at 5 to 10 mA, so lower currents are an easy way to save power.
- Forward voltage depends on colour: about 2 V for red, 2 to 3 V for green and yellow, and 3 to 3.3 V for blue and white.
- Use the datasheet when you have one. Values vary between parts.
- If the supply is barely above the forward voltage, the current will change a lot with small voltage changes. A regulated supply is better.
Building a value from standard parts
Shops sell resistors in preferred values, and the one you calculated may not exist. The Make a value mode lists the closest single part, the best pair in series and the best pair in parallel. Seventy-five ohms is not an E12 value, yet two 150 Ω resistors in parallel make exactly 75 Ω.
E12 fits 10% and 5% parts, E24 suits 5%, and E96 suits 1%. A higher series gives finer steps. Remember that tolerance adds its own error to whatever the combination gives.
Combining two resistors also spreads the heat between them, which helps when the power is close to the limit of one small part. Resistors come with colour bands, so the Resistor Color Code Calculator reads them for you. Check a circuit with the Ohm's Law Calculator, split a voltage with the Voltage Divider Calculator, and work out power in the Watts, Amps and Volts Calculator.
Features
- Series and parallel totals shown together
- Any number of resistors
- LED resistor rounded up to a standard value
- Real LED current, resistor heat and LED power
- Finds single, series and parallel combinations from E12, E24 or E96
- Understands k, M and 4k7 notation
Frequently asked questions
How do I calculate resistors in parallel?
Add 1 divided by each resistance, then take 1 divided by that sum. Two resistors can use the shortcut R1 × R2 ÷ (R1 + R2). Type the values in the list and read the parallel total.
Why is the parallel result smaller than my smallest resistor?
Each extra path gives the current another way through, so the overall resistance falls. The total is always lower than the smallest value in the group.
How do I find the resistor for an LED?
Take the supply voltage, subtract the LED's forward voltage and divide by the current in amps. Switch to LED resistor mode and enter the three numbers to get the value and the next standard size.
Why does the tool round the LED resistor up?
A higher resistance gives a slightly lower current, which is safe for the LED. Rounding down would push the current above the rating you chose.
What power rating should the resistor have?
Take the heat shown in milliwatts and pick a part rated at least twice as much. A 1/4 W (250 mW) resistor covers most small LED circuits.
Can I make an odd resistor value from two standard ones?
Yes. Open Make a value, type the resistance and choose E12, E24 or E96. You get the closest single part, the best series pair and the best parallel pair, with the error for each.
Does this work for several LEDs?
It treats one LED. For LEDs wired in series, add their forward voltages and use the sum as the LED voltage. For LEDs in parallel, give each its own resistor.