Ohms Law Wheel for Voltage, Current, Resistance & Power

An Ohms Law Wheel is a circular reference chart that arranges the relationships between voltage, current, resistance and power into four groups of three equations. The centre of the wheel holds the quantity you want to find, and the surrounding segment lists every formula that produces it from a different pair of known values. It combines Ohms Law with Joule's power law in a single diagram, which is why it covers twelve equations rather than the three most people memorise.

There are twelve: three for voltage, three for current, three for resistance and three for power. Each group of three exists because any of the remaining quantities can pair up in three different ways. For example, voltage can be found from current and resistance, from power and current, or from power and resistance.

V is voltage, measured in volts, and represents the electrical pressure pushing charge through a circuit. I is current, measured in amperes, and represents the rate of charge flow. R is resistance, measured in ohms, and represents how much the material opposes that flow. P is power, measured in watts, and represents the rate at which electrical energy is converted into heat, light or motion.

Ohms Law on its own connects only voltage, current and resistance. Power comes from Joule's Law, P = V × I. Because Ohms Law lets you replace V with I × R or replace I with V ÷ R, you can rewrite the power equation as P = I² × R or P = V² ÷ R. Merging the two laws is exactly what produces the full wheel.

It can solve for the two unknowns whenever you supply any two of the four quantities, provided the pair you enter does not create a division by zero. Entering a current of zero and asking for resistance, for instance, has no defined answer. The calculator on this page checks for those cases and explains the problem rather than showing an error value.

Ohms Law describes linear resistive behaviour. It does not describe semiconductors such as diodes and transistors, whose resistance changes with the applied voltage, and it does not account for reactance in AC circuits containing capacitance or inductance. It also assumes a constant temperature, while real conductors and filaments change resistance as they heat up.

Two of the four. The calculator watches how many boxes are filled and tells you whether you need to add or clear a value. If you enter three, it asks you to remove one, because a third value would either be redundant or contradict the other two.

Yes. Every numeric field has a unit selector covering the practical range, from microvolts to kilovolts and from milliohms to megohms. Values are converted to SI base units before any arithmetic happens, and results are then presented in the unit you selected alongside a readable engineering equivalent.

Every page is built to work from a 320-pixel-wide screen upward. Input fields, unit selectors and result panels reflow into a single column, and nothing scrolls sideways. Touch targets are sized so they can be used comfortably on site.

All arithmetic runs in JavaScript inside your browser, so once a page has finished loading it will keep calculating without a network connection. Nothing you type is sent anywhere, because there is no server-side calculation step at all.

They apply directly to DC circuits and to purely resistive AC loads such as heaters and incandescent lamps, where you can use RMS values. For loads containing motors, transformers, capacitors or electronic drivers, you also need impedance and power factor, which these simplified tools do not model.

No. Treat every figure as an engineering estimate for study, prototyping and planning. Real installations must follow local electrical codes, manufacturer specifications and ampacity rules, and any work on mains circuits should be designed and verified by a qualified electrician.