Science

Ohm's Law Calculator

Enter any 2 of voltage (V), current (A), resistance (Ω), and power (W) to calculate the other 2. Includes an Ohm's wheel diagram and wire gauge reference.

Quick Answer

V = I × R. P = V ×I. Enter any two known values below and the calculator will solve for the remaining two using Ohm's Law and the power equation.

Ohm's Law Wheel

V = I×RP = V×IR = V/II = V/RVI×RP/I√(P×R)IV/RP/V√(P/R)PV×IV²/RI²×RRV/IV²/PP/I²

Each quadrant shows 3 formulas to solve for that quantity using any two of the other three.

Calculate

Click to select 2 known values, enter their numbers, and the other 2 will be calculated.

Voltage
12 V
Current
2 A
Resistance
6 Ω
Power
24 W

Common Wire Gauge Reference (AWG)

AWGMax AmpsTypical Use
14 AWG15ALighting, general outlets
12 AWG20AKitchen outlets, bathroom outlets
10 AWG30ADryers, window AC units
8 AWG40ARange tops, large appliances
6 AWG55ACentral AC, electric stoves
4 AWG70AFeeder cables, sub-panels
2 AWG95ALarge sub-panels, service entrance

About This Tool

The Ohm's Law Calculator solves for any two unknown electrical quantities when you provide the other two. It combines Ohm's Law (V = IR) with the power equation (P = VI) to cover all possible combinations of voltage, current, resistance, and power. This is essential for electrical engineering, electronics design, and household wiring calculations.

The Four Electrical Quantities

Voltage (V) is the electrical pressure that pushes current through a circuit, measured in volts. Current (I) is the flow rate of electrons, measured in amperes (amps). Resistance (R) opposes current flow, measured in ohms. Power (P) is the rate of energy consumption, measured in watts. These four quantities are interconnected: knowing any two allows you to calculate the other two.

Practical Applications

Electricians use Ohm's Law to size wiring and circuit breakers. Electronics designers use it to select resistors and calculate heat dissipation. Homeowners use it to understand how many appliances a circuit can support. For example, a 120V circuit with a 20A breaker can safely handle 2,400W of load (120 × 20 = 2,400). Understanding these relationships prevents circuit overloads and electrical hazards.

Power and Heat

Power dissipated in a circuit becomes heat. The formula P = I²R shows that power increases with the square of the current, which is why high-current circuits generate significantly more heat. This is critical for selecting wire gauge (thicker wire for higher current) and designing cooling systems for electronic components. A 10A current through a 1Ω resistor generates 100W of heat.

Frequently Asked Questions

What is Ohm's Law?
Ohm's Law states that voltage (V) equals current (I) times resistance (R): V = I × R. It describes the fundamental relationship between voltage, current, and resistance in an electrical circuit. Combined with the power equation (P = V × I), you can calculate any two electrical quantities from the other two. It was formulated by German physicist Georg Simon Ohm in 1827.
How do I calculate watts from volts and amps?
Multiply volts by amps to get watts: P = V × I. For example, a 120V outlet powering a device drawing 5 amps uses 600 watts (120 × 5 = 600W). This is useful for determining if an appliance will overload a circuit, sizing power supplies, and calculating electricity costs.
How do I find resistance if I know voltage and current?
Divide voltage by current: R = V / I. For example, if a 12V battery drives 0.5A through a circuit, the resistance is 12 / 0.5 = 24 ohms. This is the direct application of Ohm's Law and is commonly used to troubleshoot circuits, size resistors, and verify component specifications.
What wire gauge do I need for my circuit?
Wire gauge depends on the maximum current the wire must carry. For residential wiring: 14 AWG handles 15A (lighting circuits), 12 AWG handles 20A (kitchen and bathroom outlets), 10 AWG handles 30A (dryers, large AC units), and 8 AWG handles 40A (ranges). Always follow local electrical codes and use the appropriate wire gauge to prevent overheating and fire hazards.
What is the difference between AC and DC in Ohm's Law?
Ohm's Law (V = IR) applies directly to DC circuits. In AC circuits, the concept extends to impedance (Z) instead of simple resistance: V = I × Z. Impedance includes resistance plus reactance from capacitors and inductors. For purely resistive AC loads (like heaters and incandescent bulbs), the basic Ohm's Law formulas work fine. For motors, transformers, and other inductive/capacitive loads, impedance calculations are needed.

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