ConverterHub

LED Resistor Calculator

Size the series resistor for your LED in seconds

Result

350 Ω
390 Ω
0.140 W

How it is calculated

The resistor must drop the voltage the LED does not use, at the current you want. With the supply voltage Vs in volts, the LED forward voltage Vf in volts and the current I in milliamps: R = (Vs − Vf) ÷ (I ÷ 1000) ohms, and the heat in the resistor is P = (Vs − Vf) × I ÷ 1000 watts. The third result is the smallest E12 standard value that is equal to or larger than R, never a smaller one, so the real current is a little below your target and the LED is safe. The exact resistance is shown rounded to whole ohms and the power to three decimals. The supply must be higher than the forward voltage and the current above zero, otherwise there is no result. For values under 10 Ω the tool suggests 10 Ω.

Worked example

A 9 V supply, a red LED with a 2 V forward voltage, and a target of 20 mA. Voltage across the resistor: 9 − 2 = 7 V. Current in amps: 20 ÷ 1000 = 0.02 A. Resistance: 7 ÷ 0.02 = 350 Ω. Power: 7 × 0.02 = 0.140 W. The E12 series has 330 and 390, and 330 is below 350, so the tool picks 390 Ω. With 390 Ω the actual current is 7 ÷ 390 = about 18 mA.

Try this example →

Common mistakes

  • Typing the current in amps (0.02) instead of milliamps (20); the field is in mA, so the resistor comes out a thousand times too large.
  • Using the wrong forward voltage. It differs by LED colour, so check the data sheet of your LED, not a generic value.
  • Choosing the exact resistance as a part number. Resistors come in standard steps, so use the E12 value shown, which rounds up to protect the LED.
  • Ignoring the power rating: 0.140 W is above a 1/8 W resistor, so choose a 1/4 W or larger part, or a lower current.
  • Entering the supply of a battery that is not fresh. Voltage drops as the battery drains, so the LED current changes as well.

About this calculator

This calculator sizes the series resistor an LED needs using R = (Vsupply − Vf) / I — supply voltage minus the LED's forward voltage, divided by the target current — then shows the power the resistor dissipates and the nearest standard E12 resistor value you can actually buy.

Electronics hobbyists, Arduino and Raspberry Pi builders, and students use it every time they wire an LED to a circuit, since running an LED without the right resistor either burns it out (too little resistance) or leaves it dim (too much).

Was this helpful?

You can pick up to 3 reactions.

Comments (0)

  • Be the first to comment.

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators