Electrical & Electronics
LED Resistor Calculator
Find a standard resistor for one LED or a series string, with estimated current and power dissipation.
Your inputs
Your results
- Recommended resistor
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- Estimated current
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- Ideal resistance
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- Resistor dissipation
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- Suggested resistor rating
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- Power from supply
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R = (Vs − n × Vf) / IThe rating is at least twice the calculated dissipation. Apply the manufacturer’s temperature derating.
Choose a series resistor for an LED string
Estimate a current-limiting resistor for identical LEDs connected in one series string to a fixed DC supply. The tool calculates the ideal resistance, rounds upward to the selected preferred-value series, and estimates current and dissipation. It assumes a fixed forward voltage per LED; it does not model temperature changes, tolerance or a regulated current driver.
Step by step
- Enter the DC supply voltage and the forward voltage of one LED at the intended operating current. Enter the number of identical LEDs connected in series.
- Enter the target current in mA and choose E12, E24 or E96 for the resistor value. Check that the supply exceeds the combined LED forward voltage.
- Review recommended resistance, estimated actual current, resistor dissipation and suggested power rating. Compare those values with the actual LED and resistor specifications before printing the calculation.
Settings and limits
- Series count and headroom
- The resistor voltage is Vs − n × Vf, where n is the LED count. Headroom must be positive; otherwise this model cannot reach the requested current with a positive series resistor. The count describes one series string, not parallel branches.
- Preferred resistance
- Ideal resistance is R = (Vs − n × Vf) / I. The tool selects the first value in the chosen series that is at least this large. Estimated current is then recalculated using that recommended value, so it can be lower than the target.
- Power estimate
- Resistor dissipation is PR = I² × R at the estimated current. The suggested rating is selected from a preset list to be at least twice the calculated dissipation, or uses the doubled value above that list. This margin is a calculation rule; verify the part's thermal limits and derating separately.
Worked example
Enter a 12 V supply, three series LEDs at 2 V forward voltage each, a 20 mA target and E12 resistance values. The resistor has 6 V across it, giving an ideal value of 300 Ω.
| R | R (E12) | I | PR | Pmax |
|---|---|---|---|---|
| 300 Ω | 330 Ω | 18.1818 mA | 109.091 mW | 0.25 W |
The recommended E12 value is 330 Ω. Estimated current is 18.1818 mA and resistor dissipation is about 109.091 mW. The suggested power rating is 0.25 W, which exceeds twice the calculated dissipation.
Questions and troubleshooting
Can I use one resistor for parallel LED strings?
The calculation covers one series path and does not model how current divides among parallel strings. Calculate each series branch separately; the tool cannot predict current sharing from the series count alone.
Why is the actual current below the target?
The preferred-value selection rounds resistance upward. With the same supply and assumed forward voltage, a larger resistor produces a smaller current. Change the preferred series to compare available resistance steps.
Why are the results blank or a headroom error shown?
Check that supply, forward voltage and target current are positive and that the LED count is a valid whole number. A supply at or below the sum of forward voltages has no positive headroom. Correct the assumptions rather than entering a negative resistor value.