LED Resistor Calculator
Calculate the correct current-limiting resistor for your LED circuit
Learn about E24/E96 standard resistor values and how to pick the right one for your LED circuit.
Frequently Asked Questions
How do I calculate the resistor value for an LED?
Use Ohm's Law: R = (Vs - Vf) / If, where Vs is the supply voltage, Vf is the LED forward voltage, and If is the desired LED current in amps. For example, with a 5V supply, 2V red LED, and 20mA current: R = (5 - 2) / 0.020 = 150 Ξ©.
What happens if I use no resistor with an LED?
Without a current-limiting resistor, the LED will draw excessive current and burn out almost instantly. LEDs are current-driven devices β the resistor limits current to a safe level defined by Ohm's Law. Even a brief overcurrent can permanently damage the LED junction.
Can I connect multiple LEDs in series?
Yes, but the supply voltage must exceed the sum of all LED forward voltages. The resistor value is calculated as R = (Vs - Vf Γ n) / If, where n is the number of LEDs in series. If the total forward voltage equals or exceeds the supply voltage, no resistor value will work.
Do I need a separate resistor for each parallel LED?
Yes, each parallel LED (or series string) should have its own resistor. Sharing a single resistor among parallel LEDs can cause current imbalance due to variations in forward voltage between individual LEDs, leading to uneven brightness or premature failure.
What power rating should the resistor have?
The resistor's power rating should exceed the actual power dissipation (P = V_drop Γ I) by at least 2Γ for safety. Common ratings are ΒΌW (0.25W) for standard indicator LEDs and Β½W or 1W for high-brightness LEDs. This calculator shows the recommended rating automatically.