R = (Vsupply − N × Vforward) ÷ Iforward. Pick a resistor at or above this value. Power dissipation must be ≤ resistor wattage rating (¼ W is standard).
Common Vf: Red 1.8–2.2 V, Yellow 2.0–2.2 V, Green 2.1–3.4 V, Blue/White 3.0–3.6 V. Standard If = 20 mA for 5 mm indicator LEDs.
Resistor value
0 Ω
Next E12 value
0 Ω
Voltage across R
0 V
Power in R
0 W
Recommended wattage
¼ W
Notes
Pick R at or above the calculated value — slightly higher = dimmer but safer. E12 series rounds up to standard stocked parts.
Multiple LEDs in series share the same current — voltage across them adds. Multiple LEDs in parallel need a separate resistor for each — they don’t share current evenly.
Power dissipation: P = I² × R = VR × I. Use a resistor rated at least 2× this value to be safe.
If R is negative, your supply voltage is below total Vf — the LED won’t light. Add more supply or fewer LEDs.