How to use this calculator

Enter the supply voltage, pick the LED color to fill in a typical forward voltage (or type the value from the datasheet), and set the current you want. For several LEDs wired end to end in one string, enter how many. The calculator gives the exact resistance, rounds it up to the next standard E12 value, and recalculates the current and power with that real part.

Formulas

Kirchhoff’s voltage law around the loopV_s = n V_f + I R

Resistor value and powerR = V_s − n V_fI P_R = I² R

An LED drops a nearly fixed forward voltage no matter how much current flows, so it can’t limit its own current. The resistor takes up the rest of the supply voltage and, by Ohm’s law, sets the current.

Worked example

One red LED (2.0 V) at 20 mA from 5 V (the default)

Voltage across the resistor: 5 − 2.0 = 3.0 V.
R = 3.0 / 0.020 = 150 Ω, which is a standard E12 value.
P = 0.020² × 150 = 60 mW, so a 1/8 W resistor works and a common 1/4 W one has plenty of margin.

Three white LEDs (3.2 V each) in series from 12 V at 20 mA

V_R = 12 − 3 × 3.2 = 2.4 V, so R = 2.4/0.020 = 120 Ω.
80% of the power reaches the LEDs. Only 2.4 V is left for regulation, so if the supply sags the brightness will drop noticeably.

Typical forward voltages

Red and infrared LEDs drop about 1.3 to 2.0 V, yellow and orange about 2.0 to 2.2 V, older standard green about 2.2 V, and bright green, blue and white (InGaN) LEDs about 2.9 to 3.4 V. Forward voltage rises a little with current and falls as the LED warms up. Standard 3 mm and 5 mm indicator LEDs are usually rated for 20 mA continuous, but they are often bright enough at 5 to 10 mA, which also extends their life.

Assumptions and limits

The forward voltage is constant at the chosen current. Use the datasheet value at your current for the most accurate result.
Standard values use the E12 series (10% steps), the range found in most parts kits, rounded up so the current never exceeds the target.
The suggested power rating is at least twice the dissipation, a common rule of thumb for resistor life.
High-power LEDs (350 mA and up) should use a constant-current driver; a resistor wastes too much power and can’t compensate for heating.

Frequently asked questions

Can several LEDs share one resistor in parallel?

Not reliably. LEDs have slightly different forward voltages, and the one with the lowest voltage takes most of the current. Give each parallel string its own resistor.

What if the exact resistor isn’t available?

Use the next higher value. The current drops a little and the LED is slightly dimmer, which is safe. A lower value raises the current and can overheat the LED.

Do I need a resistor with a 3.3 V supply and a white LED?

Some resistance is still needed, but with only about 0.1 V to spare the current depends heavily on each LED’s exact forward voltage, which varies between parts and with temperature. A small resistor works with uneven brightness; for consistent output use a higher supply or a constant-current driver.