Ohm's Law Calculator — V, I, R & P

Four quantities, two equations, twelve ways to rearrange them. Ohm’s law ties voltage, current and resistance together as V = I × R, and power joins them as P = V × I. Know any two and the other two follow. This calculator does the rearranging for you, with the unit prefixes that real circuits use: millivolts, milliamps, kilohms and milliwatts.

There is no “solve for” menu. Whichever two boxes you typed in most recently are the knowns, and the other two fill in as dashed orange boxes. Type into one of those and it becomes a known, while the box you touched longest ago becomes the answer. Change a unit on a calculated box to see the same answer in milliamps instead of amps.

Worked Example: 12 V Across a 220 Ω Resistor

The calculator opens with 12 V and 220 Ω. It reports:

• Current: 54.55 mA, from I = V ÷ R
• Power: 654.5 mW, from P = V² ÷ R, both highlighted on the formula wheel
• Resistor rating: 2 W or higher, because a 1 W part would run at two thirds of its limit and get hot

Load Sample moves on to 5 V at 20 mA, which gives 250 Ω. That is not a stock value, so the tool suggests the nearest E24 resistor, 240 Ω, and shows that it would pass 20.83 mA. Next come a 60 W lamp on 230 V and 2 A through 4.7 Ω.

How the Math Picks Its Formula

Each pair of knowns has exactly one formula for each unknown. Voltage and resistance give I = V ÷ R and P = V² ÷ R. Power and resistance need square roots: V = √(P × R). The wheel shows all twelve and lights up the two in use. The standard resistor is the nearest E24 (±5%) value on a logarithmic scale, which is how the series is spaced. The wattage is the smallest common rating (1/8 W up to 100 W) at least twice the power, a margin that keeps a resistor comfortably cool.

Accuracy and Where Ohm’s Law Stops Applying

Results are shown to four significant figures. Ohm’s law describes resistors well, but not every component obeys it. An LED or a diode has a nearly fixed voltage drop rather than a resistance. A lamp filament’s resistance rises several times as it heats, so the 881.7 Ω of a 60 W, 230 V lamp is its hot resistance only. In AC circuits with motors or capacitors, use impedance and power factor instead. Every value must be positive, or the tool says, for example, “Resistance must be a number greater than 0.”