LED Resistor Calculator
Enter your supply voltage, the LED forward voltage, and the target current to get the series resistor in ohms — plus the power it must dissipate.
Resistor and power at once
Enter the three values and this LED resistor calculator returns the series resistor (R = (Vs − Vf) / I) in ohms and the power it dissipates in watts.
Current in amps
Enter the LED current in amperes — a 20 mA LED is 0.02 A — and keep the supply voltage above the LED forward voltage.
What does an LED resistor calculator do?
Keeping the current safe
An LED resistor calculator finds the series resistor that limits the current through a light-emitting diode so it shines brightly without burning out. An LED has almost no resistance of its own, so connecting it straight to a supply lets a destructive current flow. A resistor in series drops the leftover voltage and pins the current to a safe value. This tool turns three measurements — the supply voltage, the LED forward voltage from its datasheet, and the current you want — into the resistor value in ohms, alongside the power the resistor must dissipate in watts so you can pick one with the right rating.
Enter a supply voltage, an LED forward voltage, and a current in amps to get the series resistor and its power dissipation instantly.
The resistor must drop the voltage left over after the LED, so its value is that voltage drop divided by the current, and the power it dissipates is the same drop times the current.
R = (Vs − Vf) / ISubtract the LED forward voltage (Vf) from the supply voltage (Vs) to find the voltage across the resistor, then divide by the current (I) in amps. The power the resistor must shed is P = (Vs − Vf) × I. Use volts and amps and the resistor comes back in ohms and the power in watts.
Suppose you are driving a red LED (forward voltage 2 V) from a 5 V supply and want 20 mA (0.02 A) through it.
Find the voltage across the resistor
5 − 2 = 3 V — what is left after the LED takes its share.
Divide by the current
3 / 0.02 = 150 Ω — the series resistor value.
Check the power
3 × 0.02 = 0.06 W, so a common 0.25 W resistor has plenty of margin.
The formula is exact, but a few practical points are worth keeping in mind.
Single LED, real components, and headroom
This calculator covers one LED in series with one resistor. For LEDs in series, add their forward voltages first; for parallel LEDs, give each its own resistor rather than sharing one. The supply must exceed the LED forward voltage or there is no voltage to drop and the result is invalid. Forward voltage also drifts a little with current and temperature, so treat the answer as a close starting point, round up to the nearest standard resistor value, and choose a power rating well above the calculated wattage.