Electronics / LEDs
LED Resistor Calculator (Series Resistor for LEDs)
Work out the current-limiting resistor for an LED or a string of LEDs from the supply voltage, forward voltage, and current, rounded up to a standard E12 or E24 value, with the real current and the power rating to use.
LED Resistor Calculator (Series Resistor for LEDs): The resistor drops the voltage the LEDs do not use: R = (supply − forward voltage × number of LEDs) ÷ current. With a 5 V supply, a red LED at 2 V, and 20 mA, that is 3 V ÷ 0.02 A = 150 Ω. The result is rounded up to the next standard value so the current stays at or below the target, and the power, current squared times resistance, is compared with common ratings at twice the dissipation. Runs 100% locally in your browser with zero server file uploads.
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R = (supply voltage − forward voltage × number of LEDs) ÷ current. The resistor is rounded up to the next standard value, so the LED gets a little less than the target current rather than more. Forward voltages vary by LED; check the datasheet, and for several LEDs in parallel give each its own resistor.
A worked example
Three white LEDs at 3.2 V each in series on a 12 V supply at 20 mA: (12 − 9.6) ÷ 0.02 = 120 Ω, a standard value, dissipating 0.02² × 120 = 48 mW, so a ⅛ W resistor is enough.
To read the colour bands on the resistor you pick, use the resistor colour code tool; for other Ohm's law sums, the Ohm's law calculator.
Forward voltages
Red LEDs drop about 1.8 to 2.2 V, yellow and orange about 2.0 to 2.2 V, older yellowish-green about 2.2 V, and blue, white, and true green about 3.0 to 3.4 V. The exact figure changes with current and temperature, so use the datasheet's typical value.
For high-power LEDs of a watt or more, use a constant-current driver instead of a resistor: the resistor would waste a lot of power as heat.
How to use it
- Enter the supply voltage, or pick it with the slider.
- Choose the LED colour or type its forward voltage, and the current, usually 20 mA or less.
- Read the standard resistor to use, the current it gives, and the power rating it needs.
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Frequently asked questions
What current should I use?
20 mA is the classic maximum for a standard 5 mm LED; many modern LEDs are bright enough at 5 to 10 mA, which also saves power. Check the datasheet's maximum.
Can several LEDs share one resistor?
In series, yes: add their forward voltages, as long as the total stays below the supply. In parallel, give each LED its own resistor, because small differences in forward voltage make one LED take most of the current.
Why round up rather than to the nearest value?
A slightly larger resistor gives slightly less current, which is safe; a smaller one could push the LED past its limit.
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