Voltage calculator
Voltage is current times resistance (V = IR), power divided by current (V = P/I), or the square root of power times resistance (V = √(PR)). Half an amp through 24 ohms means 12 volts and 6 watts dissipated.
How to find voltage
Ohm’s law and the power relationship together give twelve rearrangements of four quantities, which is why the "Ohm’s law wheel" exists as a poster on workshop walls. Knowing any two gives the other two. The practical point is that voltage is almost never the thing you measure indirectly. A meter gives it to you directly, so this page mostly earns its place in design work: choosing a supply for a known load, or working out what a divider or dropper resistor will present downstream.
Questions
V = I × R from Ohm’s law. Combined with P = V × I you also get V = P ÷ I and V = √(P × R).
Yes. Voltage is a difference between two points; a single point has no voltage on its own without a reference.
Usually a unit slip. Milliamps entered as amps, or kilohms as ohms. Power is a good error detector because it scales with the square.
Yes, by exactly I × R. In a series chain the individual drops always add up to the supply voltage.
For resistive AC loads, yes, using RMS values. Anything with significant inductance or capacitance needs impedance rather than resistance.