Ohm’s Law Calculator

Related tools: Watts Amps Volts Calculator, Voltage Divider Calculator, and Power Factor Calculator.

An Ohm’s Law Calculator solves the relationship between voltage, current, and resistance in a fixed-resistance circuit. It produces the selected electrical value and reports the associated power in watts.

For the values shown, 120 V applied across 24 ohm produces 5 A of current and 600 W of power. That current value can support an early branch-circuit load review, conductor ampacity review, voltage-drop calculation, or comparison against the rating of an existing component. The calculator does not select AWG or kcmil conductors, determine overcurrent protection, or establish NEC compliance.

Updated August 22, 2026

Electrical Values

Choose the electrical quantity to solve for and provide the known values required by that mode. This is an educational fixed-resistance estimate, not an equipment-selection or code-compliance result.

Field	Unit	Electrical meaning
Solve for	—	Selects the missing electrical quantity: Voltage, Current, Resistance, or Power. For Power, provide the known Voltage and Current.
Voltage	V	Electrical potential across the load or circuit section
Current	A	Current through the resistance
Resistance	ohm	Opposition to current flow in the assumed fixed-resistance load
Power	W	Electrical power associated with the calculated voltage and current

Use the inputs that apply to the selected Solve for mode. Solving for Current requires known Voltage and Resistance; solving for Voltage requires known Current and Resistance; solving for Resistance requires known Voltage and Current; and solving for Power requires known Voltage and Current.

Ohm’s Law Relationship

Ohm’s Law defines the relationship among voltage, current, and resistance:

\(\displaystyle V = I \times R\)

Where:

• (V) = Voltage in volts
• (I) = Current in amperes
• (R) = Resistance in ohms

Rearranging the same relationship produces the calculator’s solve modes:

\(\displaystyle I = \frac{V}{R}\)

\(\displaystyle R = \frac{V}{I}\)

The calculator also reports electrical power:

\(\displaystyle P = V \times I\)

Where (P) is power in watts.

For a fixed resistive load, power may also be expressed as:

\(\displaystyle P = I^2 \times R\)

\(\displaystyle P = \frac{V^2}{R}\)

These forms should produce the same result when the values describe the same steady-state resistance.

Calculation Example

Select Current under Solve for, then enter:

Input	Value
Voltage	120 V
Resistance	24 ohm

Calculate current:

\(\displaystyle I = \frac{120\text{ V}}{24\text{ ohm}} = 5\text{ A}\)

Calculate power:

\(\displaystyle P = 120\text{ V} \times 5\text{ A} = 600\text{ W}\)

Result	Value
Voltage	120 V
Current	5 A
Resistance	24 ohm
Power	600 W

A 24-ohm fixed-resistance load connected to 120 V draws 5 A and converts 600 W of electrical power. In a field workflow, 5 A may be added to other known loads during a preliminary branch-circuit or feeder load review. The 600 W result can also be compared with the load’s marked wattage or expected heat output when evaluating a resistive appliance or heating element.

Applying the Result

Ohm’s Law is most useful when the circuit behavior is primarily resistive and the voltage at the load is known or reasonably assumed.

Common applications include:

• Estimating current drawn by a fixed-resistance heating element.
• Checking whether stated voltage, current, wattage, and resistance values agree.
• Estimating resistance from measured voltage and current during basic troubleshooting.
• Establishing a preliminary current value before conductor ampacity and voltage-drop review.
• Estimating the wattage of a resistive load supplied at a known voltage.
• Comparing expected current with clamp-meter readings after accounting for measurement conditions.

The calculated Current is not a conductor size. Conductor selection requires a separate review of ampacity, conductor material, AWG or kcmil size, insulation temperature rating, terminal rating, ambient correction factor, adjustment factor for current-carrying conductors, installation method, and the applicable branch-circuit or feeder requirements.

Likewise, the calculated power does not establish a circuit load calculation by itself. Continuous-load treatment, utilization conditions, motor characteristics, connected equipment ratings, and the AHJ-approved code basis must be evaluated separately.

Field Verification

The calculation assumes a stable supply voltage and a fixed resistance. Actual current can differ when the load is not purely resistive or when resistance changes in service.

Motors, transformers, electronic power supplies, LED drivers, variable-frequency equipment, and other inductive or nonlinear loads cannot be fully characterized by a simple fixed-resistance Ohm’s Law result. Their current may depend on power factor, impedance, starting conditions, operating speed, waveform characteristics, or internal controls.

Resistance also changes with temperature. A heating element, incandescent filament, or conductor may have a materially different hot resistance than its cold resistance. A calculation based on a cold resistance measurement can therefore overstate the expected steady-state current at operating temperature.

Verify the final condition with appropriate measurements and equipment data. Confirm actual voltage at the load, measured current under normal operation, connection integrity, conductor temperature, terminal conditions, and voltage drop before using the result in an installation, troubleshooting, or equipment decision.