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Electrical Calculators

LED Resistor Calculator

Verified formula Updated Jun 2026 Private — runs on your device

Enter details
V
V
mA
Verified formula Private

Resistor value

350.0

Resistor power dissipated
0.1400

For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.

How to use the LED Resistor Calculator

The LED Resistor Calculator works out your resistor value, along with 1 related figure in an instant. Enter supply voltage, led forward voltage and led current and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.

  1. Enter the supply voltage.
  2. Enter the led forward voltage.
  3. Enter the led current.
  4. Read off your resistor value, together with resistor power dissipated — the calculator updates automatically, with no button to press.

Formula

The LED Resistor Calculator uses the formula:

Resistor value = (Supply voltage - LED forward voltage) ÷ (LED current ÷ 1000)

Worked example

For example, with supply voltage of 9 V, led forward voltage of 2 V and led current of 20 mA, the resistor value is 350.0.

Inputs used
Supply voltage 9 V
LED forward voltage 2 V
LED current 20 mA
Results
Resistor value 350.0
Resistor power dissipated 0.1400

Results are estimates for educational use, not professional advice.

Frequently asked questions

Subtract the LED's forward voltage from the supply, then divide by the LED current in amps. For 9 V, a 2 V LED at 20 mA, the resistor is (9 − 2) ÷ 0.02 = 350 ohms.

An LED draws more current as voltage rises and can burn out. A series resistor limits the current to a safe value.

Use a resistor rated above the power it dissipates. The calculator shows the wattage so you can pick a part with margin to spare.

Pick the next higher standard value. A slightly larger resistor gives a little less current, which is safe for the LED.

Enter the supply voltage. Enter the led forward voltage. Enter the led current. Read off your resistor value, together with resistor power dissipated — the calculator updates automatically, with no button to press.

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