Science & Engineering

Power Calculator

Calculate electrical power in watts using voltage, current and resistance with this free online Power Calculator.

Power Calculator

Select the values you know, enter them below, and calculate electrical power in watts.

Calculate power from

Voltage

Enter the electrical potential difference.

Current

Enter the electrical current.

Calculated Power

0 W

Formula: P = V × I

What Is a Power Calculator?

A Power Calculator is an online tool for determining electrical power from known electrical quantities. This calculator can determine power when voltage and current are known, when current and resistance are known, or when voltage and resistance are known.

Electrical power is measured in watts (W) and represents the rate at which electrical energy is transferred or converted. The calculator is useful for basic electrical calculations, electronics, engineering work, circuit analysis and checking values in electrical systems.

Why Use the Power Calculator?

This calculator provides several standard electrical power formulas in one interface, allowing you to calculate power from the values available to you.

Three Power Formulas

Calculate power using voltage and current, current and resistance, or voltage and resistance.

Watts Output

Results are calculated and displayed in watts, the standard SI unit of electrical power.

Multiple Units

Enter voltage, current and resistance using common metric units such as millivolts, amps and kilohms.

Fast Calculations

Calculations are performed immediately in your browser without requiring an account or external software.

Clear Formulas

The result identifies the formula used so you can verify the calculation.

Free to Use

Use the Power Calculator as often as required without registration or payment.

How to Use the Power Calculator

Choose the electrical quantities you know and enter their values. The calculator automatically applies the corresponding power formula.

Select a calculation method — Choose voltage and current, current and resistance, or voltage and resistance.
Enter the known values — Enter each value and select the appropriate unit.
Calculate the power — Select the Calculate Power button to perform the calculation.
Read the result — The calculated electrical power is displayed in watts together with the formula used.

Electrical Power Formula

The basic electrical power formula is P = V × I. In this equation, P represents power in watts, V represents voltage in volts, and I represents current in amperes. For example, a circuit operating at 12 volts and drawing 2 amps has a power of 24 watts.

Power can also be calculated using resistance when the appropriate electrical values are known. Ohm's law allows the standard power equation to be rearranged into P = I² × R or P = V² ÷ R.

Power From Voltage and Current

When voltage and current are known, multiply the voltage in volts by the current in amperes. The result is power in watts. The formula is P = V × I.

Power From Resistance

When current and resistance are known, use P = I² × R. When voltage and resistance are known, use P = V² ÷ R. Resistance should be entered in ohms, voltage in volts and current in amperes before applying these formulas.

Power Calculation Reference

The following standard formulas are used by this calculator for electrical power calculations.

Known Values	Formula	Units
Voltage and current	P = V × I	Volts × amps = watts
Current and resistance	P = I² × R	Amps² × ohms = watts
Voltage and resistance	P = V² ÷ R	Volts² ÷ ohms = watts

The calculator converts supported metric prefixes into their base SI units before performing the calculation. Results are rounded for display while the calculation itself uses the entered numerical precision.

This tool calculates electrical power using the standard relationships between voltage, current and resistance. It does not account for additional factors that may apply to specialised AC power calculations, such as power factor, phase angle or reactive power.

Created by

James William Steven Parker

Creator & Developer of YesTools

James William Steven Parker is a web developer and technology professional with extensive experience in web development, web design, SEO, web hosting, systems analysis and cybersecurity. He created YesTools to provide fast, practical and free online tools for everyday tasks and technical work.

Learn more about James and YesTools

Frequently Asked Questions

What does a power calculator calculate?

A power calculator determines electrical power in watts from known electrical values. This calculator supports voltage and current, current and resistance, and voltage and resistance.

What is the formula for electrical power?

The basic electrical power formula is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes.

Can I calculate power from voltage and resistance?

Yes. When voltage and resistance are known, electrical power can be calculated using P = V² ÷ R, where voltage is measured in volts and resistance in ohms.

Can I calculate power from current and resistance?

Yes. When current and resistance are known, power can be calculated using P = I² × R, where current is measured in amperes and resistance in ohms.

What unit is electrical power measured in?

Electrical power is measured in watts (W). One watt is equal to one joule of energy transferred or converted per second.

Is this Power Calculator free to use?

Yes. The YesTools Power Calculator is free to use and performs calculations directly in your browser without requiring an account.

Explore More Free Tools

Explore the YesTools collection of free online tools, calculators, converters, generators and utilities.

Browse All Categories