Voltage divider calculator

Vout = Vin times R2 over R1 + R2, taken between the resistors. A load in parallel with R2 shrinks the bottom resistance and drags the output down, which is why a divider feeding anything but a high impedance input needs its load in the math. Real resistors add tolerance on top: the ratio of two 1 percent parts is not 1 percent accurate in the worst case, it is up to 2 percent off. And a divider wastes power the whole time it is powered, so for battery work pick the largest resistances your input current budget allows.