LED Resistor Calculator

Find the correct resistor value and wattage to safely drive an LED from any supply voltage.

LED Resistor Calculator tool

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Example

Input: 12 V supply, 1 white LED (3.2 V forward voltage), 20 mA

Result: Exact 440.00 Ω, standard E24 470.00 Ω (actual 18.72 mA), 500 mW (1/2 W) rating recommended

R = (12 - 3.2) / 0.020 = 440 ohms; the next standard value, 470 ohms, gives 8.8 / 470 = 18.72 mA and dissipates 164.77 mW.

LED Resistor Values at 20 mA

LED color (Vf)5 V supply9 V supply12 V supply
Red (2.0 V)150 Ω360 Ω510 Ω
Orange (2.0 V)150 Ω360 Ω510 Ω
Yellow (2.1 V)150 Ω360 Ω510 Ω
Green (2.2 V)150 Ω360 Ω510 Ω
Blue (3.2 V)91 Ω300 Ω470 Ω
White (3.2 V)91 Ω300 Ω470 Ω
UV (3.3 V)91 Ω300 Ω470 Ω

Next higher E24 value for one LED, using the calculator presets.

  1. Select the LED color from the presets (red, green, blue, white, etc.) or enter a custom forward voltage.
  2. Enter your power supply voltage in the Vsupply field.
  3. Set the desired LED current (typically 20 mA for standard LEDs).
  4. Enter the number of LEDs wired in series (leave it at 1 for a single LED).
  5. View the exact resistor value, the next higher standard E24 resistor, the actual current, and the recommended wattage rating.
  6. If the tool reports that the supply voltage is too low, reduce the number of series LEDs or use a higher supply voltage.

Choosing a Current-Limiting Resistor for an LED

An LED needs a resistor in series to limit its current. The value is R = (supply voltage - LED forward voltage) / current. For a white LED (3.2 V) on a 12 V supply at 20 mA, that is (12 - 3.2) / 0.020 = 440 ohms. The next standard size up is 470 ohms.

Forward voltage is the voltage an LED drops when it is lit. It depends mostly on color, so the tool has presets: red and orange 2.0 V, yellow 2.1 V, green 2.2 V, blue and white 3.2 V, and UV 3.3 V. For a specific part, choose Custom and type the value from its datasheet. The current starts at 20 mA, a common rating for small 5 mm LEDs. Many modern LEDs are bright at 5 to 10 mA.

The result shows the exact resistance and the next higher value from the E24 series, the standard set of resistor sizes sold in stores. Rounding up keeps the current at or below your target. You also see the real current with that resistor and the power it uses. A suggested wattage rating gives at least twice that power as a safety margin.

For several LEDs in series, up to 20, the tool adds up their forward voltages. It warns you if the supply is too low to light them. For LEDs in parallel, give each branch its own resistor. LEDs sharing one resistor split the current unevenly, and one can burn out. To check the resistor you have on hand, use the Resistor Color Code Calculator.

Formula / How It Works

R = (Vsupply - Vforward) / I | For series LEDs: R = (Vsupply - n x Vforward) / I | Resistor power = (Vsupply - n x Vforward)^2 / R (using the chosen E24 value)

Frequently Asked Questions

For a red LED (2.0 V) at 20 mA, R = (5 - 2.0) / 0.020 = 150 ohms. For a blue or white LED (3.2 V), R = 1.8 / 0.020 = 90 ohms, so use 91 or 100 ohms. For a dimmer, longer-lasting LED, use 10 mA and double the resistance.
Not from a plain voltage source. An LED's current rises very sharply once the voltage passes its forward voltage. Without something to limit it, the LED can overheat and fail in seconds. The exceptions are LEDs with a built-in resistor, and LED drivers that control current directly.
Up. A larger resistor gives slightly less current, which is safe. A smaller one gives more current than planned and shortens the LED's life. That is why the tool picks the next higher E24 value, such as 470 ohms for a calculated 440 ohms.
In series, the forward voltages add up, and one resistor sets the current for the whole string. Three red LEDs need 6 V plus some headroom, so a 9 V or 12 V supply works well. In parallel, give each LED or string its own resistor, so each one gets its fair share of current.

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