Calculate the output voltage of a two-resistor divider, or find the resistor needed for a target output. Shows the current drawn too.
Vout is taken across R2: Vout = Vin · R2 / (R1 + R2). This holds when the load on the output draws far less current than flows through the divider; a heavy load pulls the output down, so keep the divider current well above the load current.
Two resistors in series split the input voltage in proportion to their values. The voltage across the lower resistor (R2) is Vin · R2 / (R1 + R2). It is the simplest way to scale a voltage down.
Whatever you connect to the output draws current and acts like a third resistor in parallel with R2, pulling the output down. The formula assumes a light load, so design the divider to carry much more current than the load needs, or buffer it with an op-amp.
Not for anything that draws meaningful current. A divider wastes power as heat and sags under load. It is for reference voltages and sensing, not for supplying power, which needs a regulator.