LED Resistor Calculator: Series Resistor for Any LED
Enter the supply voltage, pick the LED colour (or its forward voltage), the current and how many LEDs in series. You get the resistor value, the nearest standard resistor and its power rating.
- Nothing you type is stored
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Resistor needed
440 Ω
- Nearest standard (E12, higher)
- 470 Ω
- Current with standard resistor
- 18.7 mA
- Power in resistor
- 0.165 W
- Suggested resistor rating
- 0.5 W
Forward voltages are typical values; check your LED’s datasheet.
Show working
- R = (12 − 1 × 3.2) ÷ 0.02 = 440 Ω → E12 470 Ω
- I = 8.8 ÷ 470 = 18.7 mA
- P = 0.0187² × 470 = 0.165 W
How to use
- Enter the supply voltage (5 V, 12 V and so on).
- Pick the LED colour or enter its forward voltage, and the current (20 mA is typical).
- Enter how many LEDs are in series to see the resistor.
Good to know
R = (supply − number of LEDs × forward voltage) ÷ current. For 12 V, one white LED at 3.2 V and 20 mA: (12 − 3.2) ÷ 0.02 = 440 Ω; the next standard E12 value is 470 Ω, giving about 18.7 mA.
The tool rounds up to the next E12 value so the current stays at or below your target. Power in the resistor = I² × R; choose a resistor rated at least twice that (¼ W is enough for most single LEDs on 5–12 V).
Typical forward voltage: red about 2.0 V, yellow 2.1 V, green 2.2 V (true green and blue/white LEDs about 3.0–3.4 V). Check your LED’s datasheet. For many LEDs or high-power LEDs, a constant-current driver is better than a resistor.
Questions
What resistor for an LED on 12 V?
For a white or blue LED at 20 mA, 440 Ω → use 470 Ω. For a red LED, (12 − 2) ÷ 0.02 = 500 Ω → use 560 Ω.
What resistor for an LED on 5 V?
Red LED at 20 mA: (5 − 2) ÷ 0.02 = 150 Ω. White LED: (5 − 3.2) ÷ 0.02 = 90 Ω → use 100 Ω.
Can I put LEDs in series?
Yes, if the supply is higher than the sum of their forward voltages. Then one resistor serves the string.
Why round the resistor up?
A larger resistor gives slightly less current, which is safer for the LED.
What wattage resistor do I need?
At least twice I² × R. Most indicator LEDs need only ¼ W.