LED Resistor Calculator

How to find the resistor for an LED

An LED passes whatever current the circuit lets it, so it needs something to limit the current. A series resistor does that job. The resistor takes up the part of the supply voltage that the LED does not use, and Ohm’s law gives its value:

Resistor = (supply voltage − LED forward voltage) ÷ current

The forward voltage is the voltage the LED holds across itself when it lights. It depends on the colour and the part, and the datasheet lists it at a test current, usually 20 mA. For a 5 V supply, a red LED of 2.0 V and a target of 20 mA, the resistor must drop 3.0 V at 20 mA, which is 150 Ω.

Pick a colour in the calculator and it fills in a forward voltage. Type your own forward voltage if you have the datasheet, and the colour switches to Custom.

Why the result is the next standard value up

Resistors come in standard values, not every value. The calculator rounds your exact result up to the next value in the E24 list, or the E12 list if you choose it. E24 parts are made in 5% tolerance and E12 parts in 10%. The lists are the published decade tables from Vishay: E24 has 24 values per decade and E12 has 12.

Rounding up is the safe direction. A bigger resistor passes a little less current than you asked for, so the LED runs at or under your target. Rounding down would push it over.

Forward voltage is not one number

A datasheet prints a typical forward voltage, and parts vary around it. The two makers we checked, Kingbright and Wurth, differ by several tenths of a volt for the same colour, and blue and white vary the most. The calculator uses a preset in the middle of the makers’ typical values, then shows what the current would be at the lowest and highest voltage in that range. A lower forward voltage means more current, and that is the failure people most often report: the LED runs hotter than expected.

The line above the notes tells you how far the current can move. If its top end is above 30 mA, the usual rating of a 5 mm LED, the lamp on the plate goes red and the calculator tells you to use a bigger resistor or check the datasheet.

LEDs in series and in parallel

LEDs in series share one current, and their forward voltages add. Three LEDs of 3.1 V need 3 × 3.1 V from the supply, and the supply must be higher than that total or the calculator reports an error.

LEDs in parallel do not share current evenly, because no two have exactly the same forward voltage. Both Kingbright and Vishay Precision Group say each string needs its own resistor. Enter the number of parallel strings and the calculator shows one resistor per string, the total current and the power the supply delivers.

The resistor’s power rating

The resistor turns the voltage it drops into heat: power = current² × resistance. A common rule picks a part rated at least twice the power it burns, so the plate shows that figure as the rating to look for. For the default example, 0.06 W of heat means a part rated 0.12 W or more. The rating is rounded up, never down.

LED strip power supplies

Switch to the strip tab for strip lights. Strips are rated in watts per metre or per foot, and the label gives the figure. Multiply by the length for the strip’s watts, and divide by the strip’s voltage for the current. The power supply should not run flat out, so the calculator divides the watts by 0.8 and rounds up to a whole watt. Some makers multiply by 1.2 instead, and the page shows that figure too so you can compare. Use a listed power supply and follow the strip maker’s instructions.

Long runs of strip lose voltage along the way, and the far end gets dim. Use the link under the result to check the cable in the voltage drop calculator.

Common mistakes

• Using the wrong forward voltage. A blue or white LED is not 2 V. Use the colour preset or your datasheet.
• Rounding the resistor down. The calculator rounds up so the current stays at or under your target.
• Sharing one resistor between parallel strings. Give each string its own.
• Ignoring the current range. If the top of the range is above 30 mA, change the resistor before you build.

Worked example

1. Inputs: 5 V supply, 1 red LED at 2.0 V, 20 mA, 1 string, E24.
2. Resistor = (supply − LED voltage) ÷ current = (5 V − 2.0 V) ÷ 20 mA = 150.0 Ω
3. Next value up in E24 = 150 Ω
4. Current = (supply − LED voltage) ÷ resistor = (5 V − 2.0 V) ÷ 150 Ω = 20.0 mA
5. Power = current² × resistor = (20.0 mA)² × 150 Ω = 0.06 W
6. Rating = 2 × power = 2 × 0.06 W = 0.12 W

Questions people ask