💡 LED Resistor Calculator
Enter your supply voltage, the LED's forward voltage and its current to get the exact series resistor, plus the nearest standard value and its power.
Resistor needed
150 Ω
Nearest standard (E12)
150 Ω
Resistor power
60 mW
R = (supply − LED forward voltage) ÷ current (Ohm's law). Round up to the nearest standard value so the current stays at or below the LED's rating. Pick a resistor rated well above the power shown, a ¼ W part is usually plenty. 🔒 In your browser.
How the led resistor calculator works
An LED needs a resistor in series to limit its current, or it burns out. The value is Ohm's law on the voltage the resistor must drop: R = (supply voltage − LED forward voltage) ÷ LED current. The tool computes that, rounds up to the nearest standard (E12) value so the current stays safe, and gives the resistor's power dissipation so you can pick a part rated above it.
Round the resistor up, never down, a larger resistor means slightly less brightness but a safe current, while a smaller one risks overdriving the LED. Typical forward voltages are about 1.8 V for red, 2.0 V for yellow/green and 3.0-3.4 V for blue/white; check your LED's datasheet.
Frequently asked questions
How do I calculate the resistor for an LED?
R = (supply voltage − LED forward voltage) ÷ LED current. For a 5 V supply, a 2 V red LED at 20 mA: (5 − 2) ÷ 0.02 = 150 Ω. Round up to the nearest standard value.
What resistor do I need for an LED on 5V?
It depends on the LED, but for a common red LED (2 V, 20 mA) on 5 V it's about 150 Ω; for a blue/white LED (3.2 V) it's around 90-100 Ω. Enter your LED's voltage and current to get the exact value.
What happens without a resistor?
An LED has very little internal resistance, so without a current-limiting resistor it draws far too much current and quickly overheats and fails, often instantly on a normal supply. Always include a resistor (or a proper constant-current driver).
What power rating should the resistor be?
Above the dissipation the tool shows (P = current² × resistance). For typical indicator LEDs that's well under ¼ W, so a standard ¼ W resistor is fine; higher currents may need ½ W or more.
What is an LED's forward voltage?
The voltage dropped across the LED when it's lit, which depends on its color/chemistry, roughly 1.8 V (red), 2.0 V (yellow/green), 3.0-3.4 V (blue/white). It's on the datasheet; use it as the "LED forward voltage" input.