LED Resistor Calculator
Estimate a series resistor and its dissipation from supply, forward voltage and current.
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Calculate voltage headroom before resistance
Enter supply voltage, forward voltage for one LED, the number of LEDs in series and target current in milliamperes. The calculator subtracts combined LED forward voltage from the supply and divides the remaining voltage by current. It also reports calculated resistor dissipation and applies the user-entered power margin so a real component rating can be reviewed.
Use the LED and supply data sheets
Forward voltage is not a fixed constant and varies by device, current and temperature. A supply without positive headroom cannot reach the requested current through a positive resistor, and large strings or high-power LEDs often require a regulated driver instead. Confirm polarity, current limits, thermal conditions, resistor series values and component ratings before wiring.
Frequently asked questions
What if supply voltage is below total LED forward voltage?
The tool reports missing headroom because no positive resistor can create the target current from those assumptions.
How is resistor power found?
Voltage across the resistor is multiplied by target current.
Is LED forward voltage exact?
No. It varies with device, current and temperature, so use the relevant data sheet.