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LED Resistor Calculator

Calculate a series resistor for an LED, select a preferred value, and check its power margin.

Input Parameters

Nearest preferred value: 150 Ω

Circuit diagram
Supply voltage (Vs)Recommended resistor (R)LED forward voltage (VF)LED current (I)5 V150 Ω2 V20 mA

Equations

R=VS−VFILEDR=\dfrac{V_S-V_F}{I_{LED}}
PR=I2RP_R=I^2R
PLED=VFIP_{LED}=V_FI
Calculated Values
Ideal resistance150 Ω
Recommended resistor150 Ω
Resistor power dissipation60 mW
LED power40 mW
Total circuit power100 mW
Total circuit current20 mA
Recommended resistor rating125 mW

Rating includes a 2x derating margin. For continuous operation in a warm enclosure, choose the next higher rating.

LED guide

Limit LED current safely

An LED is not a resistor: its forward voltage varies with current, temperature, and colour. A series resistor or constant-current driver is needed to control current.

Method and assumptions

The starting resistance is R = (Vs − Vf)/I. The tool also calculates dissipation in the resistor and LED to help choose a reasonable power rating.

Inputs to verify

Forward voltage Vf

Use minimum, typical, and maximum values from the datasheet at the intended current and temperature.

Target current

Do not confuse absolute maximum current with the recommended continuous operating current.

Recommended workflow

  1. 1Choose operating current from the LED datasheet and desired brightness.
  2. 2Evaluate R with low and high supply voltage and low and high Vf.
  3. 3Select a preferred value and check power with thermal margin.

Checkable example

With 5 V, a 2 V red LED, and 20 mA, R is 150 Ω and dissipates 60 mW. A 0.125 W resistor can provide margin, subject to worst-case Vs and Vf.

Limits to keep in mind

For LED strings, a variable supply, high brightness, or a power LED, prefer a constant-current driver and verify package thermal dissipation.

Frequently asked questions

Why does a blue LED need another resistor?

Its forward voltage is generally higher. The Vs − Vf voltage available to the resistor therefore changes with colour and technology.

Can multiple LEDs be placed in parallel?

Avoid connecting them directly in parallel with one resistor: Vf variation can concentrate current. One resistor per branch or a suitable driver is safer.

References to consult

  • Method references: LED datasheet, Vf/If curve, recommended current, and package thermal resistance.

Original educational content, reviewed for technical clarity on 20 August 2026. Always verify datasheets, applicable standards, and your design before power-up or manufacture.