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LED Series Resistor CalculatorFree online other calculator — instant results, no sign-up

Calculate a current-limiting resistor for one LED.

Inputs

V
V
mA
Result
150 Ω

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About the LED Series Resistor Calculator

The LED Series Resistor Calculator finds the series resistance needed to limit current through one LED from a DC supply. It uses the supply voltage, LED forward voltage, and desired LED current to calculate resistance in ohms. With the default inputs of 5 V, 2 V, and 20 mA, the result is 150 Ω.

Supply voltage (V)
The DC voltage applied across the LED and series resistor together, measured in volts (V), with a default of 5 V.
LED forward voltage (V)
The expected voltage drop across the LED at the intended operating current, measured in volts (V), with a default of 2 V.
LED current (mA)
The desired current through the LED, measured in milliamperes (mA), with a default of 20 mA.

Formula and method

R = (V_supply − V_forward) / (I_LED / 1000). With the defaults: R = (5 − 2) / (20 / 1000) = 150 Ω.
  • RRequired series resistance in ohms (Ω).
  • V_supplyDC supply voltage in volts (V).
  • V_forwardLED forward voltage at the intended current, in volts (V).
  • I_LEDDesired LED current entered in milliamperes (mA); dividing by 1000 converts it to amperes.

Subtract the LED forward voltage from the supply voltage to find the voltage across the resistor. Convert the desired current from milliamperes to amperes, then apply Ohm’s law by dividing the resistor voltage by that current. For the default inputs, the resistor drops 5 − 2 = 3 V at 20 / 1000 = 0.020 A, requiring 3 / 0.020 = 150 Ω.

Worked example

These are the calculator's starting values and the results it actually produces for them.

Inputs

  • Supply voltage5 V
  • LED forward voltage2 V
  • LED current20 mA

Results

  • Result150 Ω

How to use the LED Series Resistor Calculator

  1. 1Enter the DC supply voltage; the default is 5 V.
  2. 2Enter the LED forward voltage from its datasheet at the intended operating current; the default is 2 V.
  3. 3Enter the desired LED current in mA, staying within its datasheet rating; the default is 20 mA.
  4. 4Confirm that the supply voltage exceeds the LED forward voltage and that the requested current is positive, then calculate the resistance.
  5. 5Read the resistance result and choose a suitable physical resistor, checking both its resistance tolerance and power rating before connecting it in series with the LED.

Reading your result

The result row gives the calculated series resistance in ohms (Ω). The default result of 150 Ω means that, under the assumed conditions, a 150 Ω resistor drops the remaining 3 V and limits LED current to 20 mA. This is a resistance value, not a resistor power rating; if the calculated value is unavailable, choosing the next higher standard resistance generally reduces current.

Limitations

This calculation assumes one forward-biased LED, a steady DC supply, and a fixed LED forward voltage. Actual current varies with supply accuracy, resistor tolerance, and the LED’s temperature-dependent forward voltage. The supply must exceed the LED forward voltage, and current must be positive; a zero or negative resistance result is not a usable current-limiting design. Check resistor dissipation separately using P = I²R with current in amperes: the default example dissipates 0.060 W, so select a power rating above that value with appropriate margin and derating. This tool does not model LED arrays, AC operation, transients, or constant-current drivers, and it does not establish a safe current for an unspecified LED.

Common questions

Why do the default inputs give 150 Ω?+

The 5 V supply leaves 3 V across the resistor after the LED drops 2 V. Dividing 3 V by the desired current of 20 mA, or 0.020 A, gives 150 Ω.

Is the default 20 mA safe for every LED?+

No. Use the LED datasheet to choose an operating current within its rating. Some LEDs require substantially less current, and an absolute maximum rating should not be treated as a routine operating target.

What if the supply voltage equals or is below the LED forward voltage?+

There is no positive voltage headroom for the resistor at the specified operating point. The target current cannot be reliably established with this simple series-resistor arrangement; a zero or negative calculated resistance is not a recommendation to omit the resistor.

Can the resistor go on either side of the LED?+

Yes. In a single series path, the same current flows through both components, so the resistor can go before or after the LED. The LED still needs the correct polarity, with its anode toward the positive supply and its cathode toward the return.

Does the calculator save my information?+

Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.

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Results are estimates for informational purposes and are not financial, medical or legal advice.