How do I calculate the resistor for an LED?

Enter the supply voltage, the LED forward voltage and the LED current in mA. The resistor value is (supply voltage minus LED voltage) divided by the current, and the tool suggests the nearest E12 standard value plus a safe power rating.

Should I pick the exact calculated value or a standard value?

Use a standard E12 value. The tool shows the nearest E12 resistor and also the next higher E12 value, which is the safer choice because it keeps the LED current slightly below the target.

What power rating should the resistor have?

The resistor must handle the power it dissipates, which is current squared times resistance. Choose a rating at least twice the dissipated power, for example a 1/4 W resistor for about 0.06 W of dissipation.

LED Resistor Calculator

Find the right series resistor for any LED: enter your supply voltage, the LED forward voltage and the target LED current.

Supply voltage (V)

LED forward voltage (V)

Typical: red 2.0 V, green 2.2 V, blue/white 3.0-3.4 V

LED current (mA)

Exact resistor value

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Nearest E12 standard

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Safer choice (next E12 up)

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Power dissipated

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Formula used

R = (Vs − Vf) ÷ I and P = I² × R

The resistor drops the extra voltage (supply minus LED voltage) at the target current. Always pick a power rating at least twice the dissipated power.

How to use

Enter your supply voltage, for example 9 V from a battery or 5 V from USB.
Enter the LED forward voltage (check the LED datasheet) and the target current in mA.
Pick the nearest E12 value, or the next higher value for a safer, slightly dimmer LED.

LED Resistor Calculator

How to use

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