Boneyard Tools

The Ohm's law triangle and power wheel

How the V, I, R triangle and the twelve formula power wheel let you rearrange Ohm's law for any unknown without memorising every equation.

Open the Ohm's Law Calculator →

Reading the Ohm's law triangle

The classic memory aid places voltage at the top of a triangle with current and resistance side by side beneath it. Cover the quantity you want and the layout shows the formula: cover V to see I times R, cover I to see V over R, and cover R to see V over I. It works because the three quantities are tied together by the single relation V = I x R. Once the triangle clicks, you rarely need to look up the rearranged forms again.

Adding power to the picture

Ohm's law on its own does not mention power, so electricians extend it with the power wheel, a circle split into four quadrants for V, I, R and P. Each quadrant lists the three ways to compute that quantity from pairs of the others, giving twelve formulas in total. Power appears as P = V x I, P = I^2 x R and P = V^2 / R, which are the same relations this calculator uses internally. The wheel captures the fact that knowing any two quantities pins down all four.

Choosing the right pair to measure

In practice you measure whatever is easiest with the equipment on hand. A multimeter reads voltage across a component and resistance when the circuit is off, so Voltage plus Resistance is a common starting pair. If you can only measure current and you know the supply voltage, use Voltage plus Current instead. The tool covers all six pairings so you are never forced to measure a quantity that is awkward to reach.

A worked check with a resistor

Suppose a 5 volt supply drives a small indicator through a 500 ohm resistor. Choosing Voltage plus Resistance and entering 5 V and 500 ohm returns a current of 0.01 A and a power of 0.05 W. That confirms the resistor only dissipates a fraction of a watt, so a standard quarter watt part is more than adequate. Running the numbers this way before building a circuit is a quick sanity check that nothing will overheat.

Frequently asked questions

Do I have to memorise all twelve power wheel formulas?

No. The calculator applies the correct one for your chosen pair, so you only need to know which two quantities you have measured.

Why does the power wheel have three formulas per quantity?

Because each quantity can be reached from any of the three pairs of the others. For power that means V times I, I squared times R, or V squared over R, all giving the same watts.

Is the triangle valid for non ohmic devices?

The triangle assumes resistance stays constant, which holds for ordinary resistors. Components like diodes and lamps change resistance with conditions, so the simple triangle is only an approximation for them.