Science & Engineering

LED Resistor Calculator

Finds the series resistor for an LED using R = (supply voltage - forward voltage) / current, and the resistor's power dissipation, with current entered in milliamps.

What this calculator does

The LED Resistor Calculator estimates your result instantly from the values you enter, using standard science & engineering formulas. Finds the series resistor for an LED using R = (supply voltage - forward voltage) / current, and the resistor's power dissipation, with current entered in milliamps. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate rather than a certified or official figure.

LED Resistor Calculator

Result

—
Resistor (Ω)
—
Resistor Power (W)

About LED Resistor Calculator

This LED resistor calculator sizes the series resistor needed to drive an LED. It uses R = (Vsupply − Vforward) / I, converting the current from milliamps to amps first, and also reports the power the resistor must dissipate, P = (Vsupply − Vforward) × I.

The supply voltage must be greater than the LED's forward voltage, otherwise there is no voltage left for the resistor to drop.

Formula & Mathematical Calculation

Resistor (Ω) = f(Supply Voltage (V), LED Forward Voltage (V), LED Current (mA))

Calculated according to standard science formulas implemented in the calculator core.

Calculation Example

For a 5 V supply, an LED forward voltage of 2 V, and 20 mA of current: I = 0.02 A, so R = (5 − 2) / 0.02 = 150 Ω and P = (5 − 2) × 0.02 = 0.06 W. Choose a resistor rated at or above 0.06 W.

Key Benefits

  • Sizes the series resistor from R = (supply minus forward voltage) divided by current
  • Converts the LED current from milliamps to amps automatically
  • Also reports the resistor's power dissipation so you can pick a safe rating
  • Warns when the supply voltage does not exceed the LED forward voltage

How to Use this Calculator

  1. Enter the supply voltage
    Type the circuit's supply voltage in volts in the Supply Voltage (V) field.
  2. Enter the LED forward voltage
    Type the LED's forward voltage in volts in the LED Forward Voltage (V) field; it must be less than the supply.
  3. Enter the LED current
    Type the desired LED current in milliamps in the LED Current (mA) field, then press Calculate.

Assumptions & Limitations

What the calculation assumes

  • Every value you enter is treated as an exact, known amount.
  • Any rate or interval you enter is assumed to hold for the whole period unless the form asks for it separately.
  • Results use standard published formulas and account only for the values entered above.
  • Rounding is applied for display; the underlying calculation uses your full precision.
  • Results are returned in the same units you enter, unless the tool states the output unit.

What it does not cover

  • LED Resistor Calculator produces a calculated result for reference and learning, not a certified or official measurement.
  • Real-world outcomes differ when tolerances, rounding rules or local standards apply.
  • Definitions and standards vary by region and change over time; verify the convention that applies to your case.
  • Figures are only as accurate as the inputs you supply — check them before acting on the result.
  • For work where the result carries real consequences, confirm it against an authoritative source.

Frequently Asked Questions

It uses R = (supply voltage minus LED forward voltage) divided by the current in amps, after converting the entered milliamps to amps.

The resistor drops the difference between the supply and forward voltages. If the supply is not larger, there is no voltage for the resistor to drop and the calculator reports an error.

At least the dissipation the calculator reports, P = (supply minus forward voltage) times current in amps. Choosing a resistor rated above that value gives a safety margin.

Free to use, no account needed. Calculations run in your browser, so nothing you type is sent to a server. Results are estimates — how we build and check our calculators.