Ohm’s Law Calculator

Calculate electrical voltage, current, resistance, or power using Ohm’s Law. Enter any two known values to find the other two. For example, enter voltage and wattage to find the current and resistance.

How to Use Ohm’s Law to Find Voltage, Current, Resistance, or Power

Ohm’s Law defines the relationship between electric current, resistance, and voltage. More specifically, “Ohm’s law states that the current through a conductor between two points is directly proportional to the potential difference across the two points.” (Wikipedia).

Ohm’s Law and the calculator above can be used to find voltage, current, power, and resistance if at least two of the values are known. For example, if voltage and resistance is known, then the calculator will find power and current. Alternatively, the power and resistance can be solved if the voltage and current is known.

Ohm’s Law provides the equation I = E ÷ R, where I is the current through a conductor measured in amperes, V is the potential difference across the conductor measured in volts, and R is the resistance of a conductor measured in ohms.

Current(I) = Voltage(E) ÷ Resistance(R)

Ohm’s Law Triangle

The Ohm’s Law triangle can help solve all of the Ohm’s law formulas to find voltage, current, or resistance. To use the triangle, cover the value being solved for and use the remaining two values to calculate it.

For example, to find the voltage, cover the E in the triangle, which leaves I and R are next to each other. This shows that the equations to solve voltage is I × R (current × resistance).

Alternatively, to solve for current, cover the I in the triangle, which leaves E ÷ R, expressed as the voltage divided by the resistance.

Ohm's Law triangle visualizing the formula to find voltage, current, and resistance
Ohm’s Law triangle visually represents the formula above. E stands for voltage, I stands for current, and R stands for resistance in this diagram.

Watt’s Law

Watt’s Law states that electric power measured in watts is equal to the of the circuit current times the voltage. This formula is very similar to Ohm’s law and can help solve for power/wattage.

Watt’s Law is often used in conjunction with Ohm’s Law to solve electrical properties when the power of the circuit is known.

Power(P) = Current(I) × Voltage(E)

Our watts to amps calculator uses this formula to convert between power and current in electrical circuits, for example.

Watt's Law triangle visualizing the formula to find power, voltage, and current
Watt’s Law triangle visually represents the Watt’s Law formula. P stands for power, I stands for current, and E stands for voltage in this diagram.

Ohm’s Law Formulas to Solve Voltage, Power, Current, and Resistance

Ohm’s Law can be used to find volts, watts, amps, or ohms if any two values are known. This can be done using the Ohm’s Law and Watt’s Law formulas above. Refer to this diagram to see all of the formulas used to find volts, watts, amps, or ohms.

Chart showing all the formulas to find volts, watts, amps, and ohms using the Ohm's Law and Watt's Law
Illustration shows all of the formulas defined by Ohm’s Law and Watt’s Law.

Formulas to Calculate Volts

These three formulas can be used to solve for voltage.

Voltage = Current × Resistance

Voltage = Power ÷ Current

Voltage = Power × Resistance

Formulas to Calculate Watts

Use these formulas to solve for power.

Power = Voltage × Current

Power = Voltage2 ÷ Resistance

Power = Current2 × Resistance

Formulas to Calculate Amps

These three formulas can be used to solve for current.

Current = Voltage ÷ Resistance

Current = Power ÷ Voltage

Current = Power ÷ Resistance

Formulas to Calculate Ohms

These formulas that can be used to solve for resistance.

Resistance = Voltage ÷ Current

Resistance = Voltage2 ÷ Power

Resistance = Power ÷ Current2

Ohm’s Law is used for many things, from understanding how large of a microwave can be used without blowing a circuit breaker to knowing how many light fixtures can be safely installed on a circuit without creating a fire hazard.

Our lighting cost calculator can help find lighting energy usage, and our electricity cost calculator can help find costs of powering electric devices.

Use Ohm’s Law to help size an electrical circuit or find out how large of a heater can be safely used on a regular outlet. You might also find our voltage drop calculator to determine voltage drop, the minimum wire size needed, and the maximum wire length for your next electrical project.