Ohm's law ties together the three quantities you measure most often on a board: voltage, current and resistance. Add the power equation and any two known values give you the other two. Fill in two fields; the calculator fills in the rest and converts units automatically, so you can type 4.7k, 220m or 2k2 directly.
How to use it
- Enter any two values — for example a rail voltage and a resistor value.
- Pick the unit next to each field, or type the prefix with the number (
10k,470m,2u2). - Read voltage, current, resistance and power in the result panel.
If you fill in more than two fields, the first two (in the order V, I, R, P) are used and the others are recalculated.
The formulas
- V = I × R — voltage across a resistance equals current times resistance.
- P = V × I — power is voltage times current.
- Combining them: P = I² × R and P = V² ÷ R.
These hold for resistors and for any part that behaves like a resistance at a given moment. They do not describe diodes, MOSFETs or capacitors in general, whose current does not rise in proportion to voltage.
Worked example: a pull-up resistor
A 3.3 V signal is pulled up through a 10 kΩ resistor, and the chip holding it low connects the line to ground. The current through the pull-up is 3.3 V ÷ 10 kΩ = 0.33 mA, and the resistor dissipates 3.3 V × 0.33 mA ≈ 1.1 mW — far below what even a tiny 0402 resistor can handle. If you measure much more current than that on a pull-up line, something else is loading it.
Where it matters on a laptop board
- Shorted rails — a rail at 3.3 V with 0.5 Ω to ground would try to draw 6.6 A; that is why the regulator shuts down instead of holding the voltage.
- Current from a voltage drop — a known resistor in a current path turns a millivolt reading into a current, which is how sense resistors work.
- Heat — the power result tells you whether a resistor or a shorted part will get hot.
The quantities themselves are explained in the glossary under Ohm's law and electrical power. For safe measurement technique on a live board, see How to Use a Multimeter on a Laptop Motherboard: A Beginner's Guide.
FAQ
Can I use Ohm's law for a short circuit?
Yes, as an estimate. Divide the rail voltage by the measured resistance to see the current the short would demand. In practice the regulator limits or shuts down long before that.
Why does my calculated current not match what I measure?
Real circuits have other paths in parallel and parts that are not simple resistances. Ohm's law is exact for a resistor, approximate for everything around it.
Which two values should I enter?
Whichever two you know or have measured. Voltage and resistance are the most common pair at the bench.