💡 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.