Ohm's Law Calculator

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V = I × R · P = V × I

Fill in exactly two fields and leave the others blank.

Voltage (V)

Current (I)

Resistance (R)

Ω

Power (P)

For DC and purely resistive AC loads (use RMS values). Reactive AC circuits need the power factor — see the notes below.

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Enter any two values to solve the other two

How to Use the Ohm's Law Calculator

Enter any two values — volts, amps, ohms or watts — and clear the other two fields (or hit Clear and start fresh).
Read the solved pair — the calculator picks the right formulas automatically and shows its work.
Check the formula box to see exactly which of the 12 Ohm's-law relationships were used.

⚡ Any two in, all four out

V&I, V&R, V&P, I&R, I&P or R&P — every pair is supported, no need to rearrange formulas yourself.

🧮 Shows the math

Each answer comes with the formula used — perfect for homework checks, electronics projects and electrical exams.

🔒 Instant & private

Pure in-browser arithmetic — no upload, no sign-up, results update as you type.

Ohm's Law: The Formula Wheel Explained

Ohm's law describes the relationship between voltage, current and resistance in a circuit: V = I × R. Add the electric power formula P = V × I and the four quantities lock together — knowing any two determines the other two. That gives twelve rearranged formulas, which this calculator picks between automatically:

To find	From V & I	From V & R	From I & R	From P pairs
Voltage (V)	—	—	V = I×R	V = P/I · V = √(P×R)
Current (A)	—	I = V/R	—	I = P/V · I = √(P/R)
Resistance (Ω)	R = V/I	—	—	R = V²/P · R = P/I²
Power (W)	P = V×I	P = V²/R	P = I²×R	—

Everyday US examples: a 1,500 W space heater on a 120 V outlet draws I = 1500/120 = 12.5 A — most of a 15 A circuit, which is why heaters want their own breaker. A 100 W bulb at 120 V draws only 0.83 A through a hot filament resistance of about 144 Ω. A 12 V automotive accessory rated 60 W pulls 5 A, so its wiring and fuse must handle at least that. For continuous loads the NEC’s 80% rule applies: a 15 A breaker should carry no more than about 12 A continuously (≈1,440 W at 120 V), and a 20 A circuit about 16 A (≈1,920 W).

AC caveat: Ohm's law as used here is exact for DC and for purely resistive AC loads (heaters, kettles, incandescent bulbs) using RMS voltage and current. Motors, compressors, LED drivers and anything with coils or capacitors have impedance and a power factor, so their real power is P = V × I × PF and these formulas become an upper-bound approximation. This tool is for education and project planning — actual mains wiring should always follow the NEC and be done or checked by a licensed electrician.

Ohm's Law Calculator FAQ

What is Ohm's law? ▾

Ohm's law states that current through a conductor is proportional to the voltage across it: V = I × R, where V is volts, I is amps and R is ohms. Combined with the power formula P = V × I, any two of the four electrical values determine the other two.

How do I calculate watts from volts and amps? ▾

Multiply them: P = V × I. A 120 V circuit drawing 5 A delivers 600 W. The same relationship rearranges to I = P / V, which is how you find the current a 1,500 W space heater pulls on a 120 V outlet — 12.5 A.

How do I find resistance from voltage and current? ▾

Divide voltage by current: R = V / I. If a 12 V supply pushes 0.5 A through a component, its resistance is 24 ohms. From power instead, R = V² / P or R = P / I².

What are all 12 Ohm's law formulas? ▾

Each quantity has three forms. Voltage: V = IR, P/I, √(PR). Current: I = V/R, P/V, √(P/R). Resistance: R = V/I, V²/P, P/I². Power: P = VI, V²/R, I²R. The calculator picks the right pair automatically from whichever two values you enter.

Does Ohm's law work for AC circuits? ▾

For purely resistive AC loads like heaters and incandescent bulbs, yes — use RMS voltage and current. Circuits with motors, coils or capacitors have impedance and a power factor, so real AC power is P = V × I × PF and the simple formulas only approximate.

How many amps does a 100 W bulb draw at 120 V? ▾

I = P / V = 100 / 120 ≈ 0.83 A. At Europe's 230 V the same bulb draws only 0.43 A. This is why high-power appliances in 120 V countries need thicker wiring or dedicated circuits.

Why does my calculated current matter for wire and breaker sizing? ▾

Conductors and breakers are rated by amps. A continuous load should not exceed 80% of the breaker rating — so a 15 A breaker safely carries about 12 A continuous, or roughly 1,440 W at 120 V. Always verify against the NEC and consult an electrician for actual installations.

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