LED Array Resistor Calculator

This LED array calculator works out the series resistor you need to safely drive a string of LEDs wired one after another, and shows how much power that resistor must dissipate. LEDs need their current limited or they draw more and more until they fail, and a single resistor placed in series with the string does that job by dropping whatever voltage the LEDs do not use. Enter your supply voltage, the forward voltage of one LED (typically around 2V for red or amber, higher for white or blue), how many LEDs sit in series, and the current you want each LED to run at. The calculator returns the resistor value in ohms, the power it must dissipate in watts, and the voltage dropped across it, using R equals supply voltage minus the LEDs' combined forward voltage, divided by current. Check the resistor power result against resistor ratings, quarter watt and half watt parts are common, and choose one rated above the figure shown. If you are running several strings in parallel, size each string separately with its own resistor rather than sharing one across the whole array. Your supply voltage must be higher than the LEDs' total forward voltage or the calculator will flag that the string cannot work as wired. Treat the result as an ideal-component estimate for planning your circuit, not a substitute for checking manufacturer datasheets.

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