LED resistor calculator
Series resistor for one LED or a string of LEDs. Results update as you type.
Indicative calculation; verify before use.
Circuit
Forward voltage varies by part and current. Use the value from your LED's datasheet at the current you choose.
Resistor
| Buy (next higher) | Real current | Power | Rating |
|---|---|---|---|
| – | |||
| – |
- Voltage across resistor
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- Power in LEDs
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- Efficiency (LED / total)
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How it works
The resistor takes up the voltage the LEDs do not use, and Ohm's law gives the resistance that sets the current. We round up to the next standard value, so the real current is at or slightly below what you asked for; the table shows the real current and power for that value.
The rating column applies a 2× power margin. That is our rule of thumb, not a standard: resistors run hot near their rated power, and the allowed power drops at higher ambient temperature. Check the derating curve in your resistor's datasheet.
For several LEDs, prefer a series string with one resistor. LEDs in parallel on a shared resistor do not share current evenly; give each parallel branch its own resistor.
Worked example: a 12 V supply with three blue or white LEDs in series (3.2 V each) at 20 mA. V_R = 12 − 3 × 3.2 = 2.4 V and R = 2.4 V / 0.020 A = 120 Ω. That is already an E12 value, so the current stays at 20 mA. P_R = 2.4 V × 0.020 A = 0.048 W; with the 2× margin that is 0.096 W, so a ⅛ W (0.125 W) resistor is enough.
Sources (checked 2026-10-30):
- V_R and R = V_R / I (Ohm's law): https://en.wikipedia.org/wiki/Ohm's_law
- P_R = V_R × I = I² × R (electric power): https://en.wikipedia.org/wiki/Electric_power
- E12/E24 standard values (IEC 60063, as tabulated): https://en.wikipedia.org/wiki/E_series_of_preferred_numbers
- Typical LED forward voltages by colour (vary by manufacturer; use your datasheet): https://en.wikipedia.org/wiki/Light-emitting_diode
- Rating ≥ 2 × P_R is our rule of thumb, not a standard.