Every circuit question in hobby electronics reduces to four letters: V, I, R and P. Know any two, and the other two are arithmetic, if you pick the right formula from the twelve rearrangements.
Updated August 15, 2026, after a fresh review, with updated visuals and links.
The calculator above holds all twelve: fill any two boxes, leave the rest empty, and it solves the grid. Below, the cheat-sheet and when each version matters.
The twelve formulas, organized
The triangle everyone learns: V = I times R, I = V over R, R = V over I. Adding power extends the family: P = V times I, and from there P = I squared times R and P = V squared over R follow directly.
Practical example one: a 12V LED strip drawing 2A consumes P = 12 times 2, which is 24 watts, and presents 6 ohms of resistance. Sizing its supply or its fuse starts from those numbers.
Practical example two: a 60W bulb on 120V pulls I = P over V, half an amp, and its hot resistance reads R = V over I, 240 ohms. Automotive and home questions ride the same algebra with different scenery.
Worth remembering
- Battery projects: mAh capacity divided by milliamps drawn estimates runtime hours directly, the closest cousin of these formulas.
- When numbers look impossible (resistance of zero, runaway current), re-check units: milliamps versus amps causes a thousand-fold class of error.
Frequently asked
What is Ohm's law in simple terms?
Voltage equals current times resistance: push harder (more V) through the same path and more flow results (more I), while a tighter path (more R) throttles the flow.
How do I calculate watts from volts and amps?
Multiply them: P equals V times I. A 12V motor drawing 3A consumes 36 watts.
