LED Resistor Calculator
Enter the supply voltage, the LED's forward voltage and the current you want, and get the series resistor value, its power and the nearest standard part.
How it works
An LED is not a resistor. Above its forward voltage the current rises very steeply for a tiny increase in voltage, so connecting one directly across a supply lets current run away until something fails. The series resistor is what actually sets the current.
The resistor has to drop whatever the LED does not: supply voltage minus forward voltage. Divide that by the current you want and you have the value. Everything else on the page follows from that one subtraction.
Forward voltage depends on the colour, because it depends on the semiconductor. Red is typically around 1.8–2.2 V, green and blue around 3.0–3.4 V, white similar to blue. Use the figure on the datasheet where you have one — guessing low means more current than you intended.
The formula
Resistor value
R = (V_supply − V_forward) ÷ I_forward
Power in the resistor
P = (V_supply − V_forward) × I_forward
Several LEDs in series
R = (V_supply − n × V_forward) ÷ I_forward
Worked examples
| Scenario | Working | Result |
|---|---|---|
| 5 V supply, 2 V red LED, 20 mA | (5 − 2) ÷ 0.02 | 150 Ω, 60 mW |
| 12 V supply, 3.2 V white LED, 20 mA | (12 − 3.2) ÷ 0.02 | 440 Ω → 430 Ω standard |
| 3.3 V supply, 3.2 V blue LED | Only 0.1 V left to drop | Not enough headroom — needs a driver |
When you'd use it
- Adding an indicator LED to a board
- Driving an LED from a microcontroller pin
- Working out why an LED is dim or too bright
- Checking the resistor in an existing design is right
Common questions
What happens without a series resistor?
The LED draws whatever current the supply can deliver. It may survive briefly, get very hot, dim permanently as the junction degrades, or fail outright. On a coin cell it often survives because the cell's internal resistance limits it — that is luck, not design.
The calculated value is not a resistor I can buy. What do I use?
Round up to the next standard value. A higher resistance means slightly less current, which is the safe direction — the LED is a little dimmer and well within its rating. Rounding down pushes current above your target.
Why does the tool refuse when the supply is close to the forward voltage?
Because there is nothing left for the resistor to drop, so no resistor value gives you a predictable current. Small changes in supply or temperature then swing the current wildly. Use a higher supply, or a constant-current LED driver.
Can I put several LEDs on one resistor?
In series, yes — subtract all their forward voltages, and the same current flows through every one. In parallel on a single resistor, no: their forward voltages are never identical, so one hogs the current and runs bright and hot while the others stay dim. Give each parallel branch its own resistor.
Is anything I enter sent to a server?
No. Every calculation runs in your browser, so component values, circuit figures and measurements are never uploaded, logged or stored. There is no account and no record of what you typed.