LED Resistor Calculator
LED Resistor Calculator works out Resistance needed and Power in the resistor from Supply voltage, LED forward voltage (per LED) and Forward current. The default figures on this page give resistance needed of 150.000 ohm; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- 1 LED(s) at 2 V each drop 2 V.
- Voltage left for the resistor = 5 - 2 = 3 V.
- R = 3 / 0.02 = 150 ohm.
- P = 3 x 0.02 = 0.06 W.
- Nearest E24 value at or above that: 150 ohm, giving 0.02 A.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the LED Resistor Calculator calculates
Sizes the series current-limiting resistor for one or more LEDs from the supply voltage, the LED forward voltage and the wanted forward current, and reports the power the resistor must dissipate.
How to use this LED Resistor Calculator
Enter Supply voltage, LED forward voltage (per LED), Forward current and LEDs in series on this resistor. Resistance needed updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Resistance needed, Power in the resistor and Voltage across the resistor.
The method follows Ohm's law applied to the series resistor: R = (Vs - Vf) / I, P = (Vs - Vf) x I, IEC 60063, Preferred number series for resistors and capacitors and Researched engineering contract R947 (LED-R).
5 V supply, 2 V red LED at 20 mA example
With Supply voltage: 5, LED forward voltage (per LED): 2, Forward current: 0.02, the calculator returns Resistance needed: 150, Power in the resistor: 0.06, Nearest preferred value (rounded up): 150. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Resistance needed, Power in the resistor and Voltage across the resistor. The inputs you can vary are Supply voltage, LED forward voltage (per LED), Forward current, LEDs in series on this resistor and Preferred value series to round to.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do I calculate the resistor needed for an LED?
Sizes the series current-limiting resistor for one or more LEDs from the supply voltage, the LED forward voltage and the wanted forward current, and reports the power the resistor must dissipate. Open “How this is worked out” beside the result to see each step for your own figures.
What size resistor should I use with an LED?
Sizes the series current-limiting resistor for one or more LEDs from the supply voltage, the LED forward voltage and the wanted forward current, and reports the power the resistor must dissipate. On this page it is computed from Supply voltage, LED forward voltage (per LED) and Forward current and shown as Resistance needed, Power in the resistor and Voltage across the resistor.
How to match a resistor to an LED?
It needs Supply voltage, LED forward voltage (per LED), Forward current, LEDs in series on this resistor and Preferred value series to round to. Nothing else is required, and the figures stay in your browser.
Can I use a 100k resistor for LED?
Sizes the series current-limiting resistor for one or more LEDs from the supply voltage, the LED forward voltage and the wanted forward current, and reports the power the resistor must dissipate. On the figures this page opens with — supply voltage 5 V, LED forward voltage (per LED) 2 V and forward current 0.02 A — the resistance needed is 150 ohm. Put your own figures in their place and the working updates beside the result.
What information do I need for the LED Resistor Calculator?
You need Supply voltage, LED forward voltage (per LED), Forward current, LEDs in series on this resistor and Preferred value series to round to. The page opens with default figures that give resistance needed of 150.000 ohm, so you can see the scale before entering your own.
What does this LED Resistor Calculator show?
It shows Resistance needed, Power in the resistor, Voltage across the resistor, Nearest preferred value (rounded up), Current with that preferred value, Power in the LEDs, Total power drawn and Suggested resistor rating, with the working and any warnings for your entries.