About the LED Resistor Calculator
This LED resistor calculator works out the current-limiting resistor you need to run an LED — or a string of LEDs in series — from a given supply voltage. Choose the LED color (or enter its forward voltage), the target current and the number of LEDs, and it returns the exact resistance, the next standard resistor value, the actual current with that resistor and the power rating the resistor needs.
It is written for Arduino and Raspberry Pi makers, hobby electronics, model railways, car and bike lighting, and anyone replacing indicator lamps. An LED behaves almost like a fixed voltage drop, so without a resistor even a small increase in supply voltage can push the current far past its rating and destroy it.
Forward voltages by color are typical values from datasheets; check your part, because blue, white and true-green LEDs vary between about 2.8 and 3.6 V. Standard indicator LEDs are usually rated 20 mA maximum, and many modern high-brightness parts look fine at 5–10 mA. The resistor is rounded up so the current never exceeds your target.
With the default inputs, the exact resistor value is 150 Ω. Change any value above to recalculate instantly.
How to use the led resistor calculator
- 1Enter your supply voltage (for example 5 V USB, 3.3 V logic or 12 V automotive).
- 2Pick the LED color, or choose Custom and enter the forward voltage from the datasheet.
- 3Enter the desired current in mA — 20 mA for standard LEDs, less for high-brightness parts.
- 4Set how many LEDs are wired in series in one string.
- 5Use the standard resistor value and wattage rating shown.
Formula and method
An LED drops a roughly constant forward voltage (Vf) once it is conducting. The resistor must absorb the rest of the supply voltage at the chosen current, so by Ohm’s law R = (Vs − N × Vf) ÷ I, where N is the number of LEDs in series and I is in amps.
The exact value is rounded up to the next standard E12 or E24 resistor so the current stays at or below your target; the actual current is then recalculated with that value. Resistor power is I² × R, and the recommended rating is the next standard wattage at least twice the dissipation, a common safety factor that keeps the part cool.
- R
- Series resistance in ohms
- Vs
- Supply voltage
- Vf
- LED forward voltage
- N
- Number of LEDs in series
- I
- LED current in amps (mA ÷ 1000)
Worked examples
Red LED on a 5 V supply at 20 mA
The resistor must drop 5 − 2.0 = 3 V at 0.02 A, so R = 3 ÷ 0.02 = 150 Ω, which is a standard value. It dissipates 0.02² × 150 = 0.06 W, so a 1/8 W or 1/4 W resistor is plenty.
Three white LEDs in series on 12 V
Three white LEDs drop 3 × 3.2 = 9.6 V, leaving 2.4 V for the resistor. At 20 mA that needs 2.4 ÷ 0.02 = 120 Ω, dissipating 0.048 W. The LEDs receive 0.192 W, so 80% of the power reaches them.
Blue LED on 3.3 V logic at 10 mA
A blue LED drops about 3.2 V, leaving only 0.1 V across the resistor: 0.1 ÷ 0.01 = 10 Ω. With so little headroom the current is very sensitive to the exact forward voltage, so a 5 V supply or a lower-Vf LED is a better choice.
Frequently asked questions
What resistor do I need for an LED on 5 V?+
For a red LED (about 2 V) at 20 mA use 150 Ω; for a blue or white LED (about 3.2 V) at 20 mA use about 90 Ω, rounded up to 100 Ω. Lower currents need proportionally larger resistors.
What resistor for a 12 V LED?+
A single red LED at 20 mA on 12 V needs (12 − 2) ÷ 0.02 = 500 Ω, so use 560 Ω (E12), which dissipates about 0.18 W. A single white LED needs (12 − 3.2) ÷ 0.02 = 440 Ω, so use 470 Ω (about 0.16 W). With a 2× safety factor, pick a 1/2 W resistor for either.
Can I use one resistor for several LEDs in parallel?+
It is not recommended. LEDs have slightly different forward voltages, so one LED hogs the current and can fail, then the next takes more. Give each parallel LED or series string its own resistor.
Why round the resistor up rather than down?+
Rounding up to the next standard value keeps the current at or slightly below your target. Rounding down would push more current than intended through the LED and shorten its life.
What happens if I use an LED without a resistor?+
Above its forward voltage an LED’s current rises very steeply, so without a resistor or a constant-current driver the current is limited only by the supply and wiring, which usually burns the LED out quickly.