Common jobs:
About this tool
The formula
Vout = Vin ร R2 รท (R1 + R2). Anything connected to the output acts like a resistor in parallel with R2 and pulls the voltage down, so add the load if you know it.
Dividers are for signals, not power
A divider is fine for level shifting a slow signal or scaling a voltage into an ADC. Don't use one to power a circuit: its output sags as soon as current is drawn. Use a regulator instead.
Choosing the total resistance
Lower values waste more current; higher values are more easily disturbed by the load and by an ADC's input. 10 kฮฉ to 100 kฮฉ total suits most microcontroller ADC inputs; add a 100 nF capacitor across R2 if the reading is noisy.