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.

Supply voltage (V)

LED forward voltage (V)

LEDs in series

LED current (A)

Series resistor = 300 Ω

Resistor power0.12 W

Voltage across resistor6 V

The formula

The resistor drops the leftover voltage: R = (supply voltage minus number of LEDs times forward voltage) / current. Its power dissipation is that leftover voltage times the current. The supply must be greater than the LEDs’ total forward voltage.

Worked example

For a 12 V supply driving three 2 V LEDs in series at 20 mA: the resistor drops 12 minus 6 = 6 V, so R = 6 / 0.02 = 300 ohms, dissipating 0.12 W. Enter 12, 2, 3, 0.02 to confirm.

Frequently asked questions

How do I size an LED resistor?

Subtract the total LED forward voltage from the supply, then divide by the current: R = (Vs minus N times Vf) / I.

How many LEDs can I put in series?

As many as fit under the supply voltage with some headroom for the resistor. Their forward voltages add up.

What power rating should the resistor have?

At least the dissipation shown (leftover voltage times current), with margin, so a quarter or half watt part is often used.

Who this calculator is for

This calculator is for electronics students, hobbyists and engineers.

What this calculator assumes

You enter values in the units shown.
Ideal components are assumed.
Results are rounded for display.

Formula and sources

Related calculators

Ohm’s Law Calculator: V, I, R, P.
LED Resistor Calculator: series resistor for an LED.
Resistor Colour Code Calculator: decode resistor bands.

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