Ohm's Law Calculator

Pick the two values you know and the calculator solves for the other two using Ohm's Law and the power equation. Best for DC circuits and purely resistive AC loads (heaters, incandescent bulbs, resistor networks). For motors, HVAC, chargers, and other reactive or non-linear loads, see Watts to Amps or VA to Watts, which take power factor directly.

I know

Voltage (V)

Current (A)

Resistance (Ω)

Power (W)

R = 12 Ω P = 1,200 W

See full breakdown for 120V and 10A

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What Is Ohm's Law?

Ohm's Law defines the relationship between voltage, current, and resistance in an electrical circuit. It states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance. The law is named after German physicist Georg Simon Ohm, who published it in 1827.

The core formula is V = I × R (voltage equals current times resistance). Combined with the power equation P = V × I (power equals voltage times current), these two relationships produce 12 derived formulas that connect all four electrical quantities: voltage (V), current (I), resistance (R), and power (P).

The Power Wheel: All 12 Formulas

Given any two of the four values, you can calculate the other two. These 12 formulas are grouped by what you are solving for:

Solving for Voltage (V)

V = I × R | V = P ÷ I | V = √(P × R)

Solving for Current (I)

I = V ÷ R | I = P ÷ V | I = √(P ÷ R)

Solving for Resistance (R)

R = V ÷ I | R = V² ÷ P | R = P ÷ I²

Solving for Power (P)

P = V × I | P = I² × R | P = V² ÷ R

How to Use This Calculator

Enter any two known values in the fields above. The calculator automatically solves for the other two. For example:

Know voltage and current? Enter 120V and 10A to find resistance (12Ω) and power (1,200W).
Know voltage and resistance? Enter 240V and 24Ω to find current (10A) and power (2,400W).
Know current and power? Enter 15A and 1,800W to find voltage (120V) and resistance (8Ω).

The calculated fields are highlighted with a blue background so you can see which values were solved. Click "See full breakdown" for step-by-step math on any result.

DC vs AC: When Does Ohm's Law Apply?

Ohm's Law in its simple form (V = I × R, P = V × I) applies directly to DC circuits and to purely resistive AC loads: space heaters, toasters, hot-water elements, incandescent bulbs, and resistive cooktop surfaces. For these the voltage and current stay in phase, power factor is 1.0, and the calculator on this page returns the real power the load consumes.

For AC circuits with reactive components (motors, compressors, transformers, capacitor banks) or non-linear loads (LED drivers, switch-mode power supplies, phone and laptop chargers, EV chargers, variable-speed drives), the relationship becomes more complex:

Resistance is replaced by impedance (Z), which includes both resistance and reactance. Z is measured in ohms, and Ohm's Law holds as Z = V / I.
The product V × I gives apparent power (S) in volt-amperes (VA), not real power in watts.
Real power is S × power factor: P = V × I × PF. Typical power factors range from 0.65 (desktop computers without PFC) to 0.80-0.85 (induction motors) to 0.95 (modern LED lighting).

So the DC Ohm's Law calculator on this page is the right tool when you are working with resistive DC circuits, battery or automotive systems, solar / RV wiring, resistor networks, heating elements, or incandescent bulbs. For motor, HVAC, charger, and general appliance sizing, use Watts to Amps or kW to Amps (both take power factor) or VA to Watts for apparent vs real power.

Common Calculations

boltI know V and ISolve for R and P tuneI know V and RSolve for I and P electric_boltI know I and RSolve for V and P powerI know V and PSolve for I and R

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Frequently Asked Questions

What is Ohm's Law?expand_more

Ohm's Law states that voltage equals current times resistance: V = I × R. Combined with the power equation P = V × I, it allows you to solve for any two electrical values given the other two.

What are the 12 Ohm's Law formulas?expand_more

From V=IR and P=VI: V=IR, V=P/I, V=√(PR) | I=V/R, I=P/V, I=√(P/R) | R=V/I, R=V²/P, R=P/I² | P=VI, P=I²R, P=V²/R.

How do I use the power wheel?expand_more

The power wheel arranges V, I, R, and P in a circle. Enter any two known values and the calculator solves for the other two. For example, knowing 120V and 10A gives R=12 ohms and P=1,200W.

Does Ohm's Law apply to AC circuits?expand_more

The DC form of Ohm's Law (V = I × R, P = V × I) only gives accurate real-power results for purely resistive AC loads such as heaters, toasters, incandescent bulbs, and resistive cooktop elements. For reactive or non-linear loads (motors, compressors, SMPS chargers, LED drivers, HVAC, pumps), use impedance Z = V/I and apparent power S = V × I in volt-amperes, then apply power factor to get real power: P = S × PF. For those loads see the Watts to Amps and VA to Watts calculators, which take power factor directly.

This calculator provides estimates for reference purposes only. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before performing any electrical work.