Ohm's Law Calculator: Solve V, I, R & Watts From Any Two Values (DC)

Ohm’s law is the backbone of introductory circuit math: V = I × R. Add power in watts and you get a full square of relationships — P = V × I, P = I² × R, and P = V² ÷ R — that show up everywhere from bench power supplies to residential wiring.

The Tooladex Ohm’s Law Calculator does what usually takes several separate searches: pick any two of the four quantities you already know, enter your values, and get all four back instantly — voltage, current, resistance, and power — with scaled prefixes (mV/kV, mA/kA, kΩ, mW/kW) for easy reading. Nothing leaves your browser.

Below is a practical guide to all six combinations, real-world examples of each, and how this fits with our broader electrical calculator suite.

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What Is Ohm’s Law?

Georg Simon Ohm published his findings in 1827 after experimenting with wire lengths and battery voltages. His key insight: for a given conductor at a constant temperature, current is directly proportional to voltage. Double the voltage, double the current. The constant of proportionality is resistance.

V = I × R

• V — Voltage in volts (V). The electrical “pressure” pushing charge through the circuit.
• I — Current in amps (A). The rate of charge flow. (The symbol I comes from the French intensité de courant.)
• R — Resistance in ohms (Ω). The opposition to current flow.

This holds for linear resistive elements — resistors, heating elements, incandescent bulbs, and simple wire runs. It does not directly apply to non-linear components like diodes or transistors, or to reactive components like capacitors and inductors where frequency matters.

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⚡ Three Ways to Write the Same Relationship

Rearranging V = I × R gives you three equivalent forms:

• V = I × R — find voltage from current and resistance
• I = V ÷ R — find current from voltage and resistance
• R = V ÷ I — find resistance from voltage and current

A handy memory aid is the VIR triangle: cover the quantity you want to find, and the remaining two show whether to multiply or divide.

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🔋 Adding Power: Joule’s Law

Ohm’s law alone covers V, I, and R. Bring in power — the rate at which energy is consumed — and the relationship expands.

P = V × I

Substitute V = I×R or I = V÷R and you get the full set:

• P = V × I
• P = I² × R — useful when you know current and resistance (e.g. calculating heat dissipation in a wire)
• P = V² ÷ R — useful when you know voltage and resistance (e.g. rating a resistor in a known supply circuit)

Because P is derived from V, I, and R, all four quantities are linked. Know any two and the other two are determined — as long as the values are physically consistent.

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🧮 The Six “Two Knowns” Cases

• V + I: R = V ÷ I · P = V × I
• V + R: I = V ÷ R · P = V² ÷ R
• V + P: I = P ÷ V · R = V² ÷ P
• I + R: V = I × R · P = I² × R
• I + P: V = P ÷ I · R = P ÷ I²
• R + P: V = √(P × R) · I = √(P ÷ R)

The R + P case is the least intuitive — it requires square roots because you’re working backwards from power and resistance without a direct voltage or current reference. It still holds as long as both inputs are positive.

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🔌 Real-World Examples

V + I → R, P

A 12 V car battery powers an accessory drawing 3 A. R = 12 ÷ 3 = 4 Ω. P = 12 × 3 = 36 W.

V + R → I, P

A 230 V outlet feeds a 52.9 Ω heating element. I = 230 ÷ 52.9 ≈ 4.35 A. P = 230² ÷ 52.9 = 1,000 W (a 1 kW heater).

I + R → V, P

A circuit carries 0.5 A through a 220 Ω resistor. V = 0.5 × 220 = 110 V. P = 0.5² × 220 = 55 W.

R + P → V, I

A resistor rated 10 Ω dissipates 40 W. V = √(40 × 10) = 20 V. I = √(40 ÷ 10) = 2 A.

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🔗 How This Connects to Our Other Electrical Calculators

The Ohm’s Law Calculator bundles all six pairs in one place. Our specialised calculators cover the same underlying math but add AC-specific options:

• V + I → P — Amps to Watts / Volts to Watts (DC path)
• P + V → I — Watts to Amps
• V + R → I — Volts to Amps (Ohm’s law mode)
• I + R → V — Amps to Volts (Ohm’s law mode)

If you’re working in DC or with a purely resistive AC load at PF = 1, both this calculator and those tools give identical results. Use the dedicated AC calculators when power factor or three-phase wiring is involved.

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🏠 What About AC Circuits?

Ohm’s law still applies to the instantaneous values of voltage and current in AC circuits, but for real power you need to account for power factor — the phase difference between voltage and current caused by inductive or capacitive loads.

• DC or resistive AC: P = V × I
• Single-phase AC: P = V × I × PF
• Balanced three-phase (line-to-line): P = √3 × V_L-L × I × PF

The Ohm’s Law Calculator is intentionally scoped to DC resistive calculations — clean, unambiguous, and useful for the majority of troubleshooting and learning scenarios. For AC real power with power factor, use our dedicated Watts, Amps, and Volts calculators.

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Common Mistakes to Avoid

• Using peak voltage instead of RMS in AC circuits. Wall outlet ratings (120 V, 230 V) are RMS values, not peak. Plugging peak voltage (~170 V or ~325 V) into the DC formula will give you incorrect results.
• Ignoring temperature effects on resistance. Ohm’s law assumes constant temperature. In practice, resistance rises as conductors heat up — relevant when calculating wire sizing or fuse ratings under load.
• Mixing up VA and watts. For AC loads with a power factor below 1, apparent power (VA) and real power (W) are different. The Ohm’s Law Calculator works in watts (real power) for resistive DC loads only.

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✅ Summary

• Ohm’s law: V = I × R and its rearrangements.
• Power: P = V×I = I²R = V²÷R for the same resistive model.
• Any two of V, I, R, P determine the other two — six combinations, one calculator.
• For AC circuits with power factor, use our dedicated electrical calculators.

Try the Tooladex Ohm’s Law Calculator — pick your known pair, enter values, and get V, I, R, and P in one step.