LED Resistor Calculator
Enter your supply voltage, LED forward voltage, and target current to find the right resistor. Shows the exact value, the nearest standard (E24) size, power dissipation, colour bands, and a circuit diagram. Nothing uploaded.
Resistor Values
Resistor Colour Bands (4-band) ?
Circuit Schematic
Learn more: LED resistor calculation and Ohm's Law
The fundamental equation - Ohm's Law and current limiting
An LED is a light-emitting diode with a fixed forward voltage (Vf) at which it conducts current. If you connect an LED directly to a power supply, it burns out instantly because it allows unlimited current through itself. A current-limiting resistor drops the excess voltage and limits current to a safe level (typically 10-20mA for standard LEDs). The formula is R = (Vcc - Vf) / If, where Vcc is your supply voltage, Vf is the LED's forward voltage, and If is your desired current in amps.
LED forward voltage and color - why blue costs more voltage
Different LED colors have different forward voltages. Red, orange, and yellow LEDs require about 2.0-2.2V. Green is similar at 2.1-2.3V. Blue, white, and ultraviolet LEDs require 3.0-3.4V because of the physics of their semiconductor band gap. Infrared LEDs are lower at around 1.2V. Always check the datasheet for your specific LED - Vf varies between manufacturers and batches. Using the wrong voltage in your calculation can produce dim LEDs or burned-out ones.
E24 standard resistor values - getting what exists in the real world
The calculated resistor value from Ohm's Law might be 427 ohms, but resistors are not manufactured in every value - they come in standardised series. E24 is a common series with 24 values per decade (10, 11, 12, 13, 15, 16, 18, 20, 22... 91 before jumping to 100). The calculator shows your exact calculated value and recommends the nearest E24 standard value you can actually buy. Always use the E24 value or a standard value your supplier stocks - a nearby standard value will shift your LED current by a few milliamps, which is acceptable in practice.
FAQ
What if I use a resistor value slightly different from the calculated one?
A few ohms difference shifts the LED current by a couple of milliamps, which is usually fine - LEDs are robust components with a tolerance. If you use a value significantly different (e.g., half or double), the LED will be noticeably dim or overly bright. Use a standard value within 10-20% of your calculated value.
Can I use a smaller resistor to make the LED brighter?
Yes, but there is a limit. LEDs have a maximum rated current (often 20-30mA). Exceeding it shortens the LED's lifespan dramatically or destroys it instantly. Keep your current at or below the rated maximum - typically 10-20mA for standard LEDs.
Why does the calculator show power dissipation?
Power dissipation (P = I² × R) tells you how much heat your resistor will generate. A 1/4W resistor is rated for 0.25 watts continuous; if your calculation shows 0.4W dissipation, you need a larger resistor rating (1/2W or higher) to avoid damaging the resistor.