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LED Resistor Calculator
An LED needs a series resistor to limit its current. Enter the supply voltage, the LED’s forward voltage and current, and how many LEDs are in series to get the exact resistance, the next standard value to buy and the power rating it needs.
- Next standard value (E12)
- 150 Ω
- LED current with that resistor
- 20 mA
- Power in the resistor
- 60 mW
- Resistor power rating to buy
- 1/8 W
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Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Enter the supply voltage.
- Enter the LED’s forward voltage and its current in mA, from the datasheet.
- Enter how many identical LEDs are wired in series.
- Buy the standard value shown, at the power rating shown or higher.
Worked examples
A red LED (2 V, 20 mA) on 5 V: (5 − 2) ÷ 0.02 = 150 Ω
- Resistor needed
- 150 Ω
- Next standard value (E12)
- 150 Ω
- LED current with that resistor
- 20 mA
- Power in the resistor
- 60 mW
- Resistor power rating to buy
- 1/8 W
A white LED (3.2 V, 20 mA) on 9 V: 290 Ω, so use 330 Ω
- Resistor needed
- 290 Ω
- Next standard value (E12)
- 330 Ω
- LED current with that resistor
- 17.58 mA
- Power in the resistor
- 101.9 mW
- Resistor power rating to buy
- 1/4 W
Three white LEDs in series on 12 V: (12 − 9.6) ÷ 0.02 = 120 Ω
- Resistor needed
- 120 Ω
- Next standard value (E12)
- 120 Ω
- LED current with that resistor
- 20 mA
- Power in the resistor
- 48 mW
- Resistor power rating to buy
- 1/8 W
Typical forward voltages
Red, orange and yellow LEDs drop roughly 1.8–2.2 V. Green varies from about 2 V for older types to 3 V or more for bright “pure green” ones. Blue and white drop about 3.0–3.4 V. Standard 5 mm indicator LEDs are usually rated for 20 mA maximum and are plenty bright at 5–10 mA. Always check the datasheet for your part.
Why round up
Resistors come in standard values; the E12 series runs 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their multiples of ten. Choosing the next value above the calculated one gives slightly less current, which is the safe direction. The result shows the current you will actually get.
Series and parallel
LEDs in series share one resistor and the same current, but their forward voltages add and must total less than the supply. If the result is blank, the supply is too low for that many LEDs. Do not put LEDs in parallel on a single resistor — small differences between them make the current divide unevenly; give each parallel branch its own resistor.
Questions people ask
What resistor do I need for an LED on 5 V?
For a red LED at 2 V and 20 mA: (5 − 2) ÷ 0.02 = 150 Ω. For a white LED at 3.2 V: 90 Ω, so use 100 Ω.
What resistor do I need for an LED on 12 V?
For one white LED at 3.2 V and 20 mA: (12 − 3.2) ÷ 0.02 = 440 Ω, so use 470 Ω at ½ W. Three in series need only 120 Ω.
What happens without a resistor?
An LED’s current rises very steeply with voltage, so connecting one straight to a supply above its forward voltage lets far too much current flow and usually destroys it.