f² × R
Standard rule of thumb: pick a resistor with at least 2× the
calculated power rating for thermal headroom. The calculator
above already adds this margin when recommending 1/4 W, 1/2 W, etc.
Common LED forward voltages
| Color | Typical Vf | Range |
| Infrared | 1.5 V | 1.2–1.7 V |
| Red | 2.0 V | 1.8–2.2 V |
| Orange / Yellow | 2.1 V | 2.0–2.2 V |
| Green (traditional) | 2.2 V | 2.0–2.4 V |
| Green (high-brightness) | 3.0 V | 2.8–3.3 V |
| Blue | 3.2 V | 2.8–3.5 V |
| White | 3.2 V | 2.8–3.5 V |
| UV (385–405 nm) | 3.7 V | 3.5–4.0 V |
These are typical 5 mm indicator LEDs at 20 mA. High-power LEDs
(1 W, 3 W, COB) have higher Vf and need much more current —
consult the datasheet.
Series vs Parallel: choose series
Series (recommended): One resistor for all LEDs in the
chain. Same current through every LED — perfect uniform brightness. Total
voltage drop adds up, so you need enough supply voltage.
Parallel (avoid): Each LED in parallel must have its own
resistor. LEDs have slightly different Vf even within the same
batch — without individual resistors, the LED with the lowest Vf
hogs all the current and burns out first, then the next, etc. Cascade failure.