LED Resistor Calculator
Find the current-limiting resistor value and wattage for one LED or a string in series.
About the LED Resistor Calculator
Enter the supply voltage, the LED's forward voltage (from its datasheet), the current you want and how many LEDs are in series. The calculator gives the exact resistor, the next standard E12 value up, the actual current and the power the resistor dissipates.
An LED needs a resistor to set its current. The resistor drops the voltage left over after the LEDs' forward voltages. Rounding up to the next standard value keeps the current at or below the target.
The formula
R = (Vsupply − n × Vf) ÷ I
Worked example
A red LED (2 V) at 20 mA from 5 V
- Voltage left for the resistor: 5 − 2 = 3 V
- R = 3 ÷ 0.020 = 150 Ω, a standard value
- Power: 3 × 0.02 = 60 mW, so a ⅛ W resistor is fine
Questions people ask
Can LEDs share one resistor in parallel?
Not reliably; each parallel LED needs its own resistor.
What current should I use?
Often 5–20 mA for indicator LEDs; check the datasheet's maximum.
How many LEDs can I put in series?
As many as leave at least a volt or so for the resistor: with 12 V and 3 V white LEDs, three.