LED Series Resistor
Calculate a series resistor, E12/E24 choices and actual current with resistance tolerance.
About this tool
For n identical LEDs in series, Vᵣ=Vₛ−nVꜰ and ideal R=Vᵣ/I. With resistance tolerance t, nominal R must be at least R/(1−t) to keep the calculated current at or below the target when resistance is lowest. E12 and E24 are rounded upwards from this nominal minimum. The table gives actual nominal current, maximum current and corresponding resistor dissipation. The tolerance is a property of the chosen part, not automatically fixed by the E-series. Ideal LED power at the target is nVꜰI. This constant-forward-voltage model does not include supply tolerance, LED spread or thermal effects.
Presets are illustrative, not device ratings; use the specific LED data. A 1W example does not imply any arbitrary LED may operate at350mA. Dissipation is calculated, not a universal resistor power rating: package, ambient temperature and derating are separate. Each parallel LED string requires its own current-limiting arrangement. No automatic 2× rating rule is claimed. Inputs0.001…1000V supply,1…1000 LEDs,0.001…100V forward voltage,0.000001…10000mA,0…50% resistor tolerance are arithmetic bounds. All settings survive navigation/reload.