A voltage divider is two resistors in series across a supply. The tap voltage is the supply multiplied by the lower resistor over the sum of both resistors.
Formula
For resistors R1 (top) and R2 (bottom) across Vin:
Vout = Vin × R2 / (R1 + R2)
The voltage divider calculator solves for any one unknown when you know the other three.
Loading matters
A divider only holds its ratio when the load draws negligible current compared with the divider branch. For a stable rail, use a linear regulator or buffer instead of a bare divider driving a heavy load.