LED Resistor Calculator

Enter your supply voltage, LED forward voltage and desired current to calculate the required series resistor value, the power it will dissipate, and the nearest E12 standard resistor to use.

Typical: red/yellow 1.8–2.2 V, green/blue/white 2.8–3.5 V.

Typical indicator LEDs: 10–20 mA. Check your LED datasheet.

How It Works

The series resistor formula

A current-limiting resistor drops the excess voltage between your supply and the LED's forward voltage. The relationship is Ohm's law applied to the remaining voltage:

  • Voltage across resistor = Vsupply − (Vf × n)
  • R = (Vsupply − Vf,total) / I
  • P = (Vsupply − Vf,total) × I

where Vf,total = Vf × number of LEDs in series, and I is the desired current in amps.

The calculator then finds the nearest value in the E12 preferred resistor series, which covers the most widely stocked resistors.

Worked example

Supply = 12 V, single red LED (Vf = 2 V), desired current = 20 mA:

  • Voltage across resistor = 12 − 2 = 10 V
  • R = 10 / 0.02 = 500 Ω
  • P = 10 × 0.02 = 200 mW
  • Nearest E12 value = 470 Ω

Use a resistor rated at least 400 mW (double the calculated 200 mW for safety). A standard 0.5 W (500 mW) resistor is the right choice here.

E12 Preferred Resistor Series

The E12 series contains 12 values per decade. These are the most commonly stocked resistors and are available in all standard wattages.

E12 Base Values Example Decade (×10) Example Decade (×100)
10 100 1000
12 120 1200
15 150 1500
18 180 1800
22 220 2200
27 270 2700
33 330 3300
39 390 3900
47 470 4700
56 560 5600
68 680 6800
82 820 8200

Frequently Asked Questions

How do I size an LED resistor?

Subtract the LED's forward voltage from your supply voltage to find the voltage drop across the resistor. Divide that by the desired LED current (in amps) to get the resistance using Ohm's law: R = (V_supply - V_f) / I. Choose the nearest standard value from the E12 or E24 series and verify the wattage rating.

What wattage resistor do I need for an LED?

The power dissipated in the resistor is P = (V_supply - V_f) x I. For a 12 V supply, a 2 V LED and 20 mA current that is 200 mW. Always choose a resistor rated at least double the calculated value (so at least 0.5 W in this example) to keep it running cool and extend its life.

What is the difference between LEDs in series and LEDs in parallel?

LEDs in series share the same current; their forward voltages add together, so a single resistor can control the whole string as long as the supply voltage is high enough. LEDs in parallel share the same voltage but each branch needs its own current-limiting resistor, because even small differences in forward voltage between LEDs cause unequal current sharing.

What is LED forward voltage?

Forward voltage (Vf) is the voltage drop across an LED when it is conducting at its rated current. It varies by colour and semiconductor material: red and yellow LEDs typically drop 1.8 to 2.2 V, while green, blue and white LEDs typically drop 2.8 to 3.5 V. Check the manufacturer datasheet for the exact value at your operating current.

Why does an LED need a series resistor?

An LED is a semiconductor diode, not a resistor. Its current increases exponentially with voltage, so connecting it directly to a supply without current limiting will very quickly destroy it. A series resistor limits the current to a safe value by absorbing the surplus voltage. Without one, even a small increase in supply voltage causes a large and damaging surge of current.

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