LED Resistor Calculator
Enter supply voltage, LED forward voltage and current to get the resistor value, its power rating requirement and the nearest standard values to buy.
Last updated: 2026-09-15
How the calculation works
- The resistor drops the voltage the LEDs don't use: supply minus total forward voltage.
- Ohm's law sets the resistance for the target current; P = V_drop × I sets the resistor's minimum power rating.
- The nearest E12 standard values are suggested since resistors come in fixed series.
Formula
R = (V_supply − n × Vf) / I (series) R = (V_supply − Vf) / I (per LED, parallel)
| Symbol | Meaning | Unit |
|---|---|---|
R | Current-limiting resistor | Ω |
Vf | LED forward voltage | V |
I | LED forward current | A |
Worked example
Interpreting the result
The resistor converts excess voltage into heat to hold the current steady — that's why power LEDs need switching drivers instead (efficiency). If the calculated power exceeds ~100 mW, step up to a larger package or reconsider the design. Never connect parallel LEDs to one shared resistor: Vf mismatch sends most current through one LED.
Assumptions
- Constant Vf — it actually varies with current and temperature.
- Indicator-class currents (5–25 mA).
Limitations
- Not for power LEDs above ~100 mA — use a constant-current driver.
- Does not handle mixed colors in series (different Vf per color).
Frequently asked questions
What resistor for a 12V LED?
Depends on the LED: a 12 V 'LED with built-in resistor' needs none; a bare 2 V red LED on 12 V needs (12−2)/0.02 = 500 Ω.
Why not connect LEDs directly in parallel?
Forward voltage varies part-to-part; the lowest-Vf LED hogs current and burns out, then the next one follows. One resistor per LED, or proper series strings.