Skip to content

LED Resistor Calculator: Series Resistor Value and Wattage

Updated

Enter the supply voltage, the LED’s forward voltage and the current you want through it. The calculator gives the exact resistor value, the nearest standard E24 value above it, the current you will actually get with that resistor and the power rating to buy. For a string of LEDs in series, enter how many there are.

How the value is worked out

An LED drops a roughly fixed voltage, its forward voltage Vf, and has almost no resistance of its own above that point. Connected straight to a supply higher than Vf it would draw as much current as the supply can give and burn out. The series resistor takes up the difference between the supply and the LED and so sets the current:

R = (Vs − n × Vf) / I

where Vs is the supply voltage, n the number of LEDs in series and I the current in amperes. The power dissipated in the resistor is:

P = I² × R

Example: one red LED on 12 V

  • Vf = 2.0 V, target current 15 mA.
  • R = (12 − 2.0) ÷ 0.015 = 667 Ω. The next E24 value up is 680 Ω.
  • Actual current: 10 V ÷ 680 Ω = 14.7 mA.
  • Power: 0.0147² × 680 = 0.147 W.

A 1/4 W resistor would run at 59 % of its rating here. That works, but it runs warm, and the calculator suggests the next rating that gives at least twice the dissipated power, here 1/2 W. In a sealed enclosure or a hot environment that margin is worth having.

Example: three white LEDs in series on 12 V

  • Vf = 3.0 V each, 20 mA.
  • R = (12 − 9.0) ÷ 0.020 = 150 Ω, already a standard value.
  • Power: 0.02² × 150 = 0.06 W, so a 1/8 W resistor is enough.

Putting LEDs in series uses the supply more efficiently, because less voltage is burned in the resistor. The catch is headroom: with only 3 V left across the resistor, a 10 % drop in the supply to 10.8 V reduces the current by 40 %, and the LEDs visibly dim. Leave at least 15 to 20 % of the supply voltage for the resistor if the supply is not regulated.

Things that go wrong

  • Wrong forward voltage. Use the datasheet value at the current you have chosen. As a rough guide, red and yellow LEDs sit around 1.8 to 2.2 V, green between 2 and 3.3 V depending on the chemistry, and blue and white around 2.8 to 3.4 V.
  • One resistor for several LEDs in parallel. The LEDs never share current evenly, because their forward voltages differ slightly. The one with the lowest Vf takes most of the current and fails first, then the next. Give every parallel branch its own resistor.
  • Power LEDs on a resistor. Above roughly 100 to 200 mA a resistor wastes a lot of power and the current drifts as the LED heats up and its forward voltage falls. High-power LEDs belong on a constant-current driver.
  • Automotive 12 V. A car’s “12 V” is about 14.4 V with the engine running and carries spikes. Calculate with 14.4 V so the current at full charging voltage does not exceed the LED’s rating.

Related: Ohm’s law calculator, voltage divider calculator, resistor symbols, diode and LED symbols.

Related converters