LED Resistor Calculator finds the series resistor needed to protect one LED or a string of identical LEDs. It also estimates resistor power, LED power, circuit current and efficiency.
Calculate an LED series resistor
LED Resistor Calculator formula
Resistance: R = VR ÷ I
Resistor power: PR = VR × I
LED power: PLED = N × VF × I
The supply voltage must be higher than the total forward voltage of the series LED string. Current is converted from milliamps to amps before resistance and power are calculated. The LED Resistor Calculator then rounds upward to a common resistor value so the estimated current does not exceed the target.
Worked example
Suppose three red LEDs each have a 2 V forward voltage and operate from a 12 V supply at 20 mA. The LEDs use 6 V, leaving 6 V across the resistor. The ideal resistance is 6 ÷ 0.02 = 300 Ω. With a five-percent design margin, the target becomes 315 Ω, and the next suitable E24 value is 330 Ω. Estimated current becomes 18.18 mA. The resistor dissipates about 0.109 W, so a 0.25 W resistor provides useful headroom.
Choosing LED and resistor values
Forward voltage is not perfectly fixed. It varies with LED color, manufacturing tolerance, current and temperature. Red and amber devices commonly have lower forward voltages than white, blue and many green LEDs, but the component datasheet is the correct source. Use a conservative voltage range when the circuit must work across temperature and supply variation.
For multiple LEDs, series wiring uses the same current through every LED and generally wastes less power than giving each LED its own resistor. However, the supply must exceed the combined forward voltage plus enough voltage for current control. Parallel branches should normally use a separate resistor in each branch because small forward-voltage differences can cause uneven current sharing.
Power rating and safety margin
A resistor should not be operated continuously at its exact maximum rating. The calculator suggests a standard power rating above twice the estimated dissipation where possible. Check ambient temperature, enclosure ventilation, pulse conditions and the manufacturer derating curve. For higher-power LEDs, a constant-current driver is usually more stable and efficient than a simple resistor.
Standard resistor series
E12, E24 and E96 series provide progressively finer value spacing. The LED Resistor Calculator selects the next standard value at or above the design target. A higher resistance slightly reduces current, brightness and heat. Selecting a lower value can exceed the intended LED current, particularly when the supply is high or the actual forward voltage is low.
Accuracy and limitations
This ideal model assumes a steady DC source and identical series LEDs. Battery voltage can change during discharge, automotive systems can exceed their nominal voltage, and unregulated adapters may rise when lightly loaded. Confirm the worst-case supply and LED voltage, then measure the finished circuit. Review practical LED fundamentals from SparkFun and always follow the LED datasheet.
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Frequently asked questions
Can several LEDs share one resistor?
Series LEDs can share one resistor because the same current flows through them. Parallel branches should normally have individual resistors.
Why round the resistor upward?
The next higher standard value keeps calculated current at or below the design target and adds protection against variation.
What resistor wattage should I use?
Choose a standard rating comfortably above calculated dissipation. Twice the calculated power is a practical minimum starting point.
Is a resistor suitable for a high-power LED?
Usually a regulated constant-current driver is the better choice for high-power LEDs because it controls current as voltage and temperature change.