Ohm's Law with Power Calculator

Enter any two values.

How to use it

Review the methodology below the calculator to make sure it aligns with your project's requirements, then enter exactly two of:

  • Voltage (V): the line-to-line or line-to-neutral voltage, in volts.
  • Current (I): the circuit load current, in amperes.
  • Resistance (R): the measured or expected resistance, in ohms.
  • Power (P): the total load power, in watts.

The calculator determines the other two quantities from Ohm's law and the power law, and updates as you type.

Important: enter only two values at a time; with three or more the results would not be consistent. Values must be greater than zero, since HVAC electrical loads are treated as real, resistive and unidirectional.

Results

Enter any two values.

Ohm's law wheel: the equations for voltage, current, resistance and power

V = I × R; P = V × I = I² × R = V² ÷ R.

Methodology

This calculator determines electrical power, voltage, current and resistance for HVAC systems, motors and control circuits.

Ohm's law defines the linear relationship between voltage (V), current (I) and resistance (R):

V = I × R

When power (P) is introduced, the set of equations expands to:

P = V × I = I² × R = V² ÷ R

These formulas let engineers estimate current and resistance for resistive loads such as electric heaters, and the heat output of resistive HVAC elements. They hold exactly only for resistive loads: motors and other inductive loads also need their power factor, and three-phase loads a factor of √3. Conductor and breaker sizing follow the NEC.

Calculation methods

Depending on which two quantities are known, the calculator applies:

Known values Calculations performed
Voltage (V), current (I) P = V × I, R = V ÷ I
Voltage (V), resistance (R) I = V ÷ R, P = V × I
Voltage (V), power (P) I = P ÷ V, R = V ÷ I
Current (I), resistance (R) V = I × R, P = V × I
Current (I), power (P) V = P ÷ I, R = V ÷ I
Power (P), resistance (R) I = √(P ÷ R), V = I × R

Worked example

Given: a 208-volt electric duct heater rated at 4,500 watts. Find the operating current and resistance.

  • I = P ÷ V = 4,500 ÷ 208 = 21.63 amps
  • R = V ÷ I = 208 ÷ 21.635 = 9.61 ohms (with the unrounded current; 208 ÷ 21.63 would give 9.62)

This matches standard calculations for single-phase resistive HVAC loads, such as electric coils or reheat elements.

Further reading

  • National Fire Protection Association (NFPA). NFPA 70: National Electrical Code (NEC), 2026 edition.
  • ASHRAE. Standard 90.1-2025: Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings.
Top