Calculate the series resistor for one or more LEDs using supply voltage, forward voltage, and target current. Includes nearest E24 resistor value and expected current.
Basic formula: R = (Vs - N×Vf) / I. If remaining voltage is not positive, the selected number of LEDs cannot be driven from that supply in series. Choose resistor power with safety margin (commonly at least 2× calculated dissipation).
This calculator handles one or more LEDs wired in series by multiplying LED count by forward voltage before subtracting from supply voltage. For LEDs wired in parallel, calculate the resistor for one branch only - do not combine parallel LED currents into a single shared resistor, since parallel LEDs do not share current evenly unless their forward voltage is closely matched.
Usually no. Without current limiting, LED current can rise quickly and damage the LED.
Real circuits use standard resistor values. E24 provides practical parts you can buy easily.
Not recommended. LEDs in parallel do not share current evenly because of small differences in forward voltage, so one LED can hog current and fail. Use a separate resistor per parallel LED branch, or wire the LEDs in series instead.
LED current rises above the target value, which increases brightness but also heat and long-term degradation risk. Always check the calculated current against the LED's maximum forward current rating.
LED Resistor 12V Guide · Voltage Divider · SMD Resistor Code