Voltage DividerX
There are different voltage dividers. This app handles resisitive dividers, when the impedances, R1 and R2, are purely resistive. See Wikipedia's Voltage Divider article for other voltage dividers.

Enter 3 of the 4 values. When entering the 3rd value, the remaining 4th value will be calculated.

R1 Ω
R2 Ω