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.
Open calculatorLast 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.
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).
Series resistor for one LED at 20 mA
Typical forward voltages are used; check your LED's datasheet for the real figure.
| Supply | LED | Calculated resistor | Next standard E12 value | Resistor power |
|---|---|---|---|---|
| 5 V | red (≈ 2.0 V) | 150 Ω | 150 Ω | 60 mW |
| 5 V | white or blue (≈ 3.2 V) | 90 Ω | 100 Ω | 36 mW |
| 9 V | red (≈ 2.0 V) | 350 Ω | 390 Ω | 140 mW |
| 12 V | red (≈ 2.0 V) | 500 Ω | 560 Ω | 200 mW |
| 12 V | white or blue (≈ 3.2 V) | 440 Ω | 470 Ω | 176 mW |
| 24 V | white or blue (≈ 3.2 V) | 1.04 kΩ | 1.2 kΩ | 416 mW |
R = (supply voltage − LED forward voltage) ÷ LED current.
Worked scenario: a red LED on a 12 V supply
With 12 V, a 2 V LED and 20 mA, the resistor must drop 10 V and works out to 500 Ω. It dissipates 200 mW, so a ¼ W resistor is comfortable.
Why you round up to a standard value
Resistors come in preferred values, so the exact number is rarely available. Choose the next value up: it lowers the current slightly and the LED simply glows a little less, whereas a lower value would push the LED above its rated current.
Mistakes to avoid
- Using a typical forward voltage that does not match the actual LED. Blue and white LEDs differ from red and yellow ones.
- Forgetting the resistor's power rating when the supply is high.
- Wiring several LEDs in parallel on one resistor. Small differences between LEDs then make the current share unevenly; give each its own resistor or wire them in series.
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.