LED Resistor Calculator
Calculate the correct current-limiting resistor and minimum wattage for LED circuits.
Inputs
Result
Example Inputs: Vs = 12V, Vf = 2V, LED count = 1, I = 20mA
Example Output: R = 500 Ohms, P = 0.2 W (recommend 0.25 W or higher)
Live Result: Enter values and click Calculate.
Formula
R = (Vs - Vf_total) / I, and P = I^2 * R
Worked Example
Use the example inputs and outputs above to walk through the calculation step-by-step.
FAQ
What resistor value does this return?
It computes the current-limiting resistor based on supply voltage, LED forward voltage, LED count, and target current.
Why is a higher power rating recommended?
The tool applies a practical derating guideline for thermal reliability.
What if total LED forward voltage exceeds supply?
The calculator rejects that case because no series resistor can correct insufficient headroom.
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Formula Notes
Subtract total forward voltage from supply voltage, then divide by target current.
Worked Example
A 12V source with one 2V LED at 20mA requires approximately 500 ohms.