LED Resistor Calculator
Size the series resistor an LED needs, with the nearest standard value, its colour bands and the power rating to buy.
Circuit
In series they share one current, so one resistor serves them all. Never put LEDs in parallel on a single resistor — small differences between them make one hog the current.
20 mA is the usual maximum for an ordinary indicator LED. Running at 10 mA costs little brightness and leaves headroom.
LED
Typically 1.8–2.2 V. Forward voltage varies between parts, so the datasheet for your actual LED beats this table.
Resistor
nearest standard value at or above the calculated 150 Ω
The standard value is rounded up, never down: more resistance means less current, which keeps the LED inside its rating. The power rating allows roughly double the calculated dissipation, which is normal practice for a part that runs continuously.
How It Works
An LED does not limit its own current. Past its forward voltage it conducts hard, so connecting one straight across a supply destroys it. The resistor exists to drop the leftover voltage and set the current.
R = (supply voltage − total LED forward voltage) ÷ current
Everything the resistor drops is turned into heat rather than light, so a 12 V supply driving one 2 V LED wastes about five sixths of the power. For long strings or battery work a constant-current driver is a better answer than a bigger resistor.
Frequently Asked Questions
- What resistor do I need for an LED?
- Subtract the LED forward voltage from the supply voltage, then divide by the current you want. A red LED at about 2 volts run at 20 milliamps from a 5 volt supply needs (5 - 2) / 0.02, which is 150 ohms. Round up to the next standard value rather than down, because more resistance means less current.
- Why does an LED need a resistor at all?
- An LED does not limit its own current. Once the supply passes its forward voltage it conducts heavily, and the current rises until something fails, usually the LED. The resistor drops the leftover voltage and sets the current to a safe value.
- What forward voltage does my LED have?
- It depends mostly on colour, because colour follows from the semiconductor used. Red is around 2 volts, yellow and standard green a little above that, and blue, white and UV around 3 to 3.4 volts. Infrared is lowest at roughly 1.4 volts. These are typical figures and individual parts vary, so use the datasheet where accuracy matters.
- Can I put several LEDs on one resistor?
- In series, yes: the same current flows through all of them, so one resistor sets it for the string. Add the forward voltages together. In parallel, no. Small differences between the LEDs mean one takes more current than the others, gets hotter, drops its forward voltage further and takes more still. Give each parallel branch its own resistor.
- What power rating resistor should I use?
- Multiply the voltage across the resistor by the current to get the dissipation, then pick a standard rating with roughly double that headroom. Most small indicator LED circuits dissipate well under a tenth of a watt, so an ordinary quarter watt resistor is ample. Higher currents or large supply voltages can push you to half a watt or more.
- Why can I not just use a higher supply voltage?
- You can, but everything above the LED forward voltage is dropped by the resistor and turned into heat rather than light. Driving one 2 volt LED from 12 volts wastes about five sixths of the power. For long strings or anything battery powered, a constant-current driver is far more efficient than a larger resistor.
- What if the supply voltage is below the LED forward voltage?
- No resistor can help. The LED will not conduct meaningfully below its forward voltage, so a single blue LED at 3.2 volts cannot run from a 3 volt coin cell through a resistor. Raise the supply, use fewer LEDs in series, or use a boost converter or constant-current driver.