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LED resistor calculator

Find the series resistor needed to drive an LED from a supply voltage.

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Enter Supply voltage (V), LED forward voltage (V), LED current (mA) and the LED resistor calculator works out Resistor (Ω), Resistor power (W) straight away. For instance, with Supply voltage (V) = 12, LED forward voltage (V) = 2 and LED current (mA) = 20 it returns Resistor (Ω) = 500 and Resistor power (W) = 0.2.

How to use it

  1. Enter your values: Supply voltage (V), LED forward voltage (V), LED current (mA).
  2. Read the result instantly: Resistor (Ω), Resistor power (W).

Frequently asked questions

How does the LED resistor calculator work?

It takes Supply voltage (V), LED forward voltage (V) and LED current (mA) and derives Resistor (Ω) and Resistor power (W) from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

3 values: Supply voltage (V), LED forward voltage (V) and LED current (mA). Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With Supply voltage (V) = 12, LED forward voltage (V) = 2 and LED current (mA) = 20, the calculator returns Resistor (Ω) = 500 and Resistor power (W) = 0.2. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

How much does the result change with different inputs?

It moves a lot. Using Supply voltage (V) = 24, LED forward voltage (V) = 4 and LED current (mA) = 40 instead, Resistor power (W) goes from 0.2 to 0.8 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Supply voltage (V) = 6, LED forward voltage (V) = 1 and LED current (mA) = 10, Resistor power (W) comes out at 0.05. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

At the bench: sizing a resistor for an LED, reading a colour band, working out a divider ratio, and checking a part will not cook.

What is the most common mistake?

Choosing a resistance and forgetting the power rating. A value that is electrically right will still burn if the package cannot dissipate what passes through it — check the watts as well as the ohms.

What is the difference between the LED resistor calculator and the LED savings calculator?

This one returns Resistor (Ω) and Resistor power (W); the LED savings calculator returns Old bulbs cost/year and LED cost/year. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Parallel resistor calculator is the closest one after this: Compute the total resistance of resistors connected in parallel.

What else is worth having open alongside it?

Series resistor calculator and Resistor Color Code Calculator — they come up in the same task often enough to be worth a second tab.

Further reading

All guides →
How-toHow to Calculate an LED Series ResistorSize the series resistor for any LED with one formula: R = (Vs − Vf) / I. Follow the steps, see a worked 5 V example, and pick the right resistor.ExplainerThe Voltage Divider: The Formula Is Trivial, the Assumption Behind It Is NotVout = Vin·R2/(R1+R2) is only true when nothing is connected to the output. Draw current and the voltage collapses, by an amount that follows one exact law. Here is that law, the ten-times rule checked against it, and why a lower-resistance divider costs you power forever.ExplainerWhat Is Ohm's Law? V = I × R Explained SimplyOhm's law links voltage, current, and resistance with one equation: V = I × R. Learn the triangle trick and see worked examples.ExplainerWhat Is Voltage Drop? Why Long Wires Lose VoltageVoltage drop is the voltage lost along a wire due to its resistance. Learn how length, gauge, and current affect it, and why it matters for your circuits.ExplainerResistors in Series and in Parallel: The Formulas, and the Three Things They Do Not Tell YouSeries adds, parallel adds reciprocals — that part is easy. The parts that catch people: which member dominates the answer, why two 5 % resistors in parallel are still 5 %, and why the smallest resistor in a parallel pair takes the largest share of the power.ExplainerLux and Lumens: The Difference, and How to Convert Between ThemLumens are what a lamp emits; lux is what lands on a surface. You cannot convert one to the other without an area, or without a distance and a beam angle. Anything that offers you a bare conversion factor is selling you a fiction.