LED Resistor Calculator
Series resistor for LEDs, rounded to E12/E24 values, with power rating.
About this tool
Enter the supply voltage, choose the LED colour or type its forward voltage, and set the current, the number of LEDs in series and the number of parallel strings. The calculator shows the standard resistor value, the exact value, the current you will actually get, the resistor's power and a safe wattage rating, the total current and how much power reaches the LEDs.
How to choose an LED resistor
An LED needs its current limited, and for small LEDs a series resistor does the job: R = (supply voltage − forward voltage × LEDs in series) ÷ current. A red LED of about 2 V on a 5 V supply at 20 mA needs (5 − 2) ÷ 0.02 = 150 Ω, and the resistor dissipates 60 mW. Three white LEDs of about 3.2 V in series on 12 V need (12 − 9.6) ÷ 0.02 = 120 Ω. When the exact value falls between standard values, the calculator rounds up to the next E12 or E24 value so the current stays at or below your target. Choose a resistor rated for at least twice the power it dissipates. Forward voltage varies by colour and by part, so use the datasheet figure when you can. Modern LEDs are often bright enough at 5–10 mA. Give each parallel string its own resistor, never one shared resistor. Honest limits: high-power LEDs are better driven by a constant-current driver than a resistor. The Ohm's law calculator checks the maths, the resistor colour code tool identifies the part and the battery life calculator estimates runtime.
Frequently asked questions
What resistor do I need for an LED on 5 V?
For a red LED at 20 mA, (5 − 2) ÷ 0.02 = 150 Ω. For a white or blue LED of about 3.2 V, (5 − 3.2) ÷ 0.02 = 90 Ω, so use 100 Ω.
Can I use one resistor for several LEDs in parallel?
No. Small differences in forward voltage make one LED take most of the current. Give each string its own resistor.
What wattage resistor do I need?
Power = current² × resistance. Choose a rating at least double that. Most small single-LED circuits are fine with a ¼ W resistor.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.