LED Resistor Calculator — KiloWattCalculator.com
Electrical Calculators & Unit Converters
Calculate the current-limiting resistor for a single LED or a series LED string. See the nearest standard resistor with color bands, the power dissipation, and a live circuit diagram.
Set your supply voltage, choose your LED (or enter its datasheet values), set how many LEDs are wired in series, and click Calculate.
E24 series, 5% tolerance (gold band) — the standard resistor range most widely stocked and sold in hobbyist kits.
Dot speed shows relative current. Diagram uses the nearest standard resistor value and its actual resulting current.
Results assume ideal components. Real LED forward voltage varies by manufacturer, batch, and temperature — always verify against your LED's datasheet.
An LED is a diode. Once the supply reaches its forward voltage, a small extra voltage produces a large extra current. Heat then lowers the forward voltage, which lets still more current flow. That loop is thermal runaway, and a fixed voltage source will not stop it. The series resistor is the part that sets the current.
Take a red LED with a 2.0 volt forward drop, run at 20 milliamps, on a 12 volt supply. The resistor must drop the other 10 volts. Resistance is that voltage divided by the current: 10 / 0.020 = 500 ohms. Five hundred ohms is not an E24 value. The nearest E24 value is 510 ohms. With 510 ohms the current is about 19.6 milliamps, and the resistor dissipates about 0.20 watt. A quarter-watt part is above that number and has little margin. A half-watt part is the one that sits comfortably above it. The calculator on this page does that nearest-value step and reports the dissipation. Check the LED datasheet before treating 2.0 volts and 20 milliamps as the part in your hand.
LEDs in series can share one resistor when they are the same type and the same current, and when the supply is still higher than the sum of their forward voltages. Two 2.0 volt LEDs on 12 volts leave 8 volts for the resistor, not 10. LEDs in parallel do not share current evenly from one resistor. Give each parallel string its own resistor. The resistor color code decoder is the page that turns the chosen value into bands. This page does not repeat that chart.
An LED does not regulate its own current. Without a resistor to limit current flow, an LED drawing power from a fixed voltage source will draw more and more current as it warms up, leading to thermal runaway and permanent damage. A series resistor sets a safe, stable operating current.
Subtract the total LED forward voltage from the supply voltage, then divide by the desired LED current: R = (Vsupply − n × Vf) ÷ I. This applies Ohm's Law to the voltage the resistor must drop.
Too small, and excess current flows through the LED, overheating and damaging it — sometimes immediately. Too large, and too little current flows, making the LED dim or preventing it from lighting at all. The calculated value balances safe current against full brightness.
Yes, as long as the combined forward voltage of every LED in the string stays below the supply voltage, with enough headroom left for the resistor to drop the remainder. Only wire same-color, same-current LEDs together in a series string.
Choose a resistor power rating at least twice the calculated power dissipation, rounded up to the next standard rating: 1/8W, 1/4W, 1/2W, 1W, or 2W. This safety margin keeps the resistor running cool and reliable.
Disclaimer: Results produced by this calculator are for reference and educational purposes only and are not a substitute for verifying LED forward voltage and current against the manufacturer's datasheet, or measuring actual circuit values with a properly rated meter. Calculated values assume ideal components and do not account for resistor tolerance, LED batch variation, temperature effects, wire or contact resistance, or other real-world circuit conditions. All values must be independently verified before use in any circuit design, repair, or component substitution, and any electrical work must comply with the National Electrical Code (NFPA 70) and the authority having jurisdiction and be performed or reviewed by a licensed electrician where required. KiloWattCalculator.com and EnterPlanet LLC accept no liability for decisions made on the basis of these calculations.