オームの法則計算ツール
The detailed guide below is currently available in English.
Two equations, four unknowns
Ohm's law ties voltage, current, and resistance together: V = I × R. Power adds a fourth player with P = V × I, which can be rewritten as P = I²R or P = V²/R by substituting Ohm's law. Fix any two of the four quantities and the other two are locked in — that is exactly what this calculator does, whichever pair you choose to enter.
Choosing the right pair
Measure what a meter can actually tell you. A multimeter reads voltage across a component and current through it directly, while resistance is only reliable with the circuit powered down. Power is often the quantity you care about — heat, brightness, battery drain — and solving from voltage and current is the most common lab scenario.
よくある質問
Why can I not enter zeros?
Ohm's law works fine with zero, but solving from it does not: with zero current, resistance becomes 12 ÷ 0 — undefined — and power would be zero no matter what. Requiring positive inputs keeps every answer meaningful.
Does Ohm's law apply to AC circuits?
Yes, with impedance standing in for resistance and the quantities treated as RMS values. Purely reactive loads add a phase angle, so real power can differ from the simple V × I product.
What units does the calculator assume?
Volts, amperes, ohms, and watts — the base units. Convert first: 100 mA is 0.1 A, and 4.7 kΩ is 4,700 Ω, otherwise results will be off by factors of a thousand.