LED Resistor Calculator
Calculate the series resistor an LED needs from supply voltage, forward voltage and target current, with the nearest E24 standard value, the resulting real current and the required power rating.
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An LED is a current device with a voltage habit
An LED does not have a resistance in any useful sense. Above its forward voltage the current rises almost vertically, so connecting one directly across a supply lets the current run to whatever the supply can deliver, and the die fails in milliseconds. The series resistor exists to set the current, and its value comes from the voltage left over after the LED takes its forward drop, divided by the current you want.
Forward voltage depends on colour
The forward drop is a property of the semiconductor, so it varies by material and therefore by colour. Red and infrared parts sit around 1.8 to 2.2 volts, yellow and green around 2.1 to 2.4, and blue, white and ultraviolet around 3.0 to 3.4 because they use a different die chemistry. Using a red LED figure for a white LED gives a resistor that runs the part well under its intended brightness, or the reverse if you go the other way and burn it out.
Standard values and the real current
The calculated resistance is almost never a value you can buy. Rounding up to the next E24 standard part is the safe direction, because it lowers the current slightly rather than raising it. This calculator reports the exact value, the nearest standard part above and below, and the actual current each would produce, so you can see whether the substitution matters for brightness before soldering it in.
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Frequently Asked Questions
What forward voltage should I use if I do not have a datasheet?
Use the typical figure for the colour: about 2.0 V for red, 2.2 V for yellow and green, and 3.2 V for blue and white. The calculator preloads these. If the exact brightness matters, measure the drop across the LED at your target current.
Why round the resistor up rather than down?
Rounding up reduces the current, which reduces brightness slightly. Rounding down increases the current, which can push the LED past its maximum rating. When in doubt, up is the safe direction.
Can I put several LEDs on one resistor?
Wire them in series if the supply voltage allows, and add the forward voltages together; one resistor then sets the current for the whole string. Do not wire them in parallel on a single resistor, because small differences in forward voltage make one LED take most of the current.
What power rating does the resistor need?
The dissipation is the voltage across the resistor times the current. A typical 20 mA indicator on a 5 V supply dissipates well under a tenth of a watt, so a quarter watt part is ample. Higher supply voltages or higher currents change that quickly, which is why the figure is shown.
Does this apply to high power LEDs?
Only loosely. Above about half a watt an LED should be driven by a constant current driver rather than a resistor, because a resistor wastes the excess voltage as heat and offers no thermal regulation as the die warms up.
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How to Use
Enter supply voltage, LED forward voltage and desired current to get the resistor value and rating.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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