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LED Resistor Calculator

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Find the correct current-limiting resistor for an LED circuit - essential for protecting an LED from drawing too much current and burning out.

LED Resistor Calculator

Live
Resistor needed
500 Ω
Nearest standard value
470 Ω
Resistor power rating
0.2 W (use 1/4W or higher)
Common LED forward voltages: red/yellow ≈ 2.0V, green/blue/white ≈ 3.0-3.4V - check your specific LED's datasheet.

The formula

Resistor (Ω) = (Supply voltage - Total LED forward voltage) / Desired current (A) Power rating needed = (Supply voltage - Total LED forward voltage) x Desired current (A)
Example

A 12V supply, single LED with 2.0V forward voltage, targeting 20mA: resistor = (12 - 2.0) / 0.020 = 500Ω - the nearest standard value is 470Ω, in a 1/4W or higher resistor.

Step-by-step guide

  1. Enter your supply voltage and the LED's forward voltage (check its datasheet, or use a typical value for its color).
  2. Enter your desired current - 20mA is a very common safe default for standard 5mm LEDs, but check your specific LED's maximum rating.
  3. Enter how many LEDs are wired in series, if more than one.
  4. Use the nearest standard resistor value at or above the calculated minimum, with adequate power rating.

Why LEDs need a current-limiting resistor at all

Unlike a standard resistor, an LED's voltage stays roughly constant (its forward voltage) once it's conducting - which means without something limiting the current, it will draw more and more current as temperature rises, in a runaway process that quickly destroys it. A correctly sized series resistor absorbs the extra voltage and limits current to a safe, stable level.

Common mistakes

Connecting an LED directly to a battery or supply with no resistor at all - this very often destroys the LED within seconds due to uncontrolled current.
Using a resistor rated for less power than the calculation shows - an underrated resistor can overheat, drift out of tolerance, or fail outright.

Frequently asked questions

What forward voltage should I use if I don't know my LED's exact value?

Red and yellow LEDs are commonly around 1.8-2.2V, while green, blue, and white LEDs are typically 3.0-3.4V, due to differences in the semiconductor materials used. Check the specific LED's datasheet for an exact figure when possible.

Why does this recommend the "nearest standard value" instead of the exact calculated resistance?

Resistors are manufactured only in a standard set of values (the E-series), not every possible number - rounding up to the next standard value at or above the calculated minimum keeps current safely at or below the target.

Can I wire multiple LEDs in parallel instead of series?

It's generally not recommended without a separate resistor for each LED, since even small manufacturing differences between LEDs can cause one to hog most of the current and burn out. Series wiring with one shared resistor (as this calculator assumes) or parallel branches each with their own resistor are both safer approaches.

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