How the LED resistor is calculated

An LED needs a series resistor to limit its current: R = (V_supply − n·V_f) / I_f, where V_f is the LED forward voltage and I_f the desired current. Since resistors only come in standard values, the calculator rounds up to the nearest E12/E24 value — the safe direction, because it keeps the actual current slightly below your target. It also shows the power dissipated in the resistor and a recommended power rating with a 2× safety margin.

The voltage divider tab solves V_out = V_in·R2/(R1+R2) for whichever field you leave empty, suggests the nearest standard E24 resistor, and shows the divider current and per-resistor power. Keep in mind a divider is only good for light loads — anything drawing comparable current to the divider itself will pull V_out down. Related: Resistor Color Code Calculator.