How to Use the Ohm’s Law Calculator

The Ohm’s Law Calculator can calculate voltage, current, resistance, and electrical power when any two of these values are known.

Select the two electrical values you already know, enter the values, and the calculator will determine the remaining values automatically.

The calculator supports the following combinations:

Voltage and Current
Voltage and Resistance
Current and Resistance
Voltage and Power
Current and Power
Resistance and Power

Ohm’s Law Formula

The basic Ohm’s Law formula is:

V = I × R

Where:

V = voltage in volts
I = current in amperes
R = resistance in ohms

The equation can be rearranged depending on the value you need to calculate.

To calculate voltage:

V = I × R

To calculate current:

I = V ÷ R

To calculate resistance:

R = V ÷ I

Ohm’s Law and Power Formulas

Electrical power can also be calculated together with Ohm’s Law.

The basic power formula is:

P = V × I

Where:

P = power in watts
V = voltage in volts
I = current in amperes

By combining the power equation with Ohm’s Law, several useful formulas can be derived.

Example: Calculate Resistance from Voltage and Current

Suppose a circuit operates at 12 volts and carries 2 amps.

Use:

R = V ÷ I

R = 12 ÷ 2

R = 6 Ω

The resistance is 6 ohms.

Power can also be calculated:

P = V × I

P = 12 × 2

P = 24 W

Therefore:

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

Example: Calculate Current from Voltage and Resistance

Suppose a circuit has 24 volts across a 12-ohm resistance.

I = V ÷ R

I = 24 ÷ 12

I = 2 A

Power is:

P = V² ÷ R

P = 24² ÷ 12

P = 48 W

Example: Calculate Voltage from Current and Resistance

Suppose 3 amps flow through a resistance of 10 ohms.

V = I × R

V = 3 × 10

V = 30 V

Power is:

P = I² × R

P = 3² × 10

P = 90 W

What Is Voltage?

Voltage is the electrical potential difference between two points and is measured in volts.

Voltage provides the electrical pressure that causes current to flow through a circuit when a conductive path is available.

What Is Current?

Current is the flow of electric charge through an electrical circuit.

Current is measured in amperes, commonly called amps.

Using Ohm’s Law:

I = V ÷ R

For a fixed resistance, increasing voltage increases current.

What Is Resistance?

Resistance is the opposition to electrical current flow.

Resistance is measured in ohms and uses the symbol Ω.

Using Ohm’s Law:

R = V ÷ I

For the same voltage, greater resistance results in lower current.

What Is Electrical Power?

Electrical power represents the rate at which electrical energy is transferred or converted.

Power is measured in watts.

A common power formula is:

P = V × I

You can perform more general DC, single-phase AC, and three-phase power calculations with our

Electrical Power Calculator
.

Ohm’s Law Formula Wheel

The common Ohm’s Law and electrical power relationships can be summarized as follows.

Voltage Formulas

V = I × R

V = P ÷ I

V = √(P × R)

Current Formulas

I = V ÷ R

I = P ÷ V

I = √(P ÷ R)

Resistance Formulas

R = V ÷ I

R = V² ÷ P

R = P ÷ I²

Power Formulas

P = V × I

P = V² ÷ R

P = I² × R

Ohm’s Law Reference Table

How Voltage, Current, and Resistance Are Related

Ohm’s Law describes the relationship between voltage, current, and resistance.

If resistance stays constant, increasing voltage increases current.

If voltage stays constant, increasing resistance decreases current.

For example, a 12 V source connected to a 6 Ω resistance produces:

I = 12 ÷ 6 = 2 A

If the resistance increases to 12 Ω:

I = 12 ÷ 12 = 1 A

Ohm’s Law and DC Circuits

Ohm’s Law is commonly used for DC circuit calculations involving resistive components and loads.

Examples include resistors, DC heaters, basic electronic circuits, LED resistor calculations, battery-powered circuits, and other resistive applications.

Can Ohm’s Law Be Used for AC?

The basic resistance-based calculator on this page is most straightforward for DC and resistive circuits.

AC circuits containing inductors or capacitors can involve impedance and phase relationships rather than resistance alone.

For AC power calculations involving power factor, use an appropriate AC power calculation instead of treating all loads as purely resistive.

Frequently Asked Questions

What is Ohm’s Law?

Ohm’s Law describes the relationship between voltage, current, and resistance.

V = I × R

How do I calculate voltage?

If current and resistance are known:

V = I × R

How do I calculate current?

If voltage and resistance are known:

I = V ÷ R

How do I calculate resistance?

If voltage and current are known:

R = V ÷ I

How do I calculate watts using Ohm’s Law?

When voltage and current are known:

P = V × I

Other useful formulas include:

P = V² ÷ R

P = I² × R

What current flows through 6 ohms at 12 volts?

I = 12 ÷ 6

I = 2 amps.

What resistance is needed for 12 volts and 2 amps?

R = 12 ÷ 2

R = 6 ohms.

What voltage is required for 2 amps through 10 ohms?

V = 2 × 10

V = 20 volts.

How many watts is 12 volts at 2 amps?

P = 12 × 2

The result is 24 watts.

How many amps is 24 watts at 12 volts?

I = P ÷ V

I = 24 ÷ 12

The result is 2 amps.

How many ohms is a 24-watt load at 12 volts?

R = V² ÷ P

R = 12² ÷ 24

R = 6 ohms.

What does Ω mean?

The symbol Ω represents ohms, the unit used to measure electrical resistance.

Can I calculate watts from amps and resistance?

Yes.

P = I² × R

Can I calculate voltage from watts and resistance?

Yes.

V = √(P × R)

Can I calculate current from watts and resistance?

Yes.

I = √(P ÷ R)

Electrical Calculation Disclaimer

This Ohm’s Law Calculator is provided for informational, educational, and preliminary planning purposes. Calculated values may be rounded.

The calculator uses standard Ohm’s Law and electrical power relationships and is most directly applicable to DC and resistive circuits.

Actual electrical circuits may include resistance changes, component tolerances, temperature effects, inductance, capacitance, impedance, power factor, waveform distortion, voltage variation, and other operating conditions.

Do not use this calculator alone to determine conductor sizes, protective devices, component ratings, or safe working limits.

Electrical installations and equipment should follow manufacturer specifications, applicable electrical codes, component ratings, and appropriate safety requirements.

Consult a qualified electrical professional when making final electrical installation, wiring, circuit protection, component selection, or safety decisions.