How it works
The resistor drops whatever voltage the LEDs don’t:
R = (Vs − n × Vf) / I
P = I² × R
The calculator then picks the next standard value up in the series you choose, so the real current is at or slightly below your target, never above. It recomputes the actual current and the power dissipated in the resistor with that value.
Choosing the rating
The suggested wattage is the smallest common rating at least twice the dissipated power. Running a resistor at half its rating keeps it cool and its value stable. An 0805 SMD part is typically rated 1/8 W; 1206 is 1/4 W.
Assumptions and limits
- Vf varies with current, temperature and batch. The color presets are typical values; use the datasheet for anything critical.
- With several LEDs in series, the supply must be comfortably above n × Vf: with little headroom, small Vf changes cause large current changes.
- For constant brightness across temperature or long strings, use a constant-current driver instead.