Ohm’s Law Calculator

Find 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.

Using Ohm’s Law to Find Voltage, Current, Resistance, and Power

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, determine the power and resistance if the voltage and current is known. For the calculator to work at least two of the four values are needed. A single value will not work since it’s not enough to work with. This calculator uses Ohm’s Law to determine voltage, amperage, wattage, and ohms of an electric circuit. This is great for finding how many light bulbs can be powered on a circuit or how much amperage is needed to power a device given its voltage and wattage.

What is Ohm’s Law

According to Wikipedia, “Ohm’s law states that the current through a conductor between two points is directly proportional to the potential difference across the two points.” There is an equation that defines this, I = V / 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.

Ohm’s Law Triangle

The following diagram expresses this formula and is called the Ohm’s Law triangle.

Ohm's Law triangle visualizing the formula to find voltage, current, and resistance

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 voltage part of the equation, which is the E in the triangle. When covering the E, notice that the I and R are next to each other, which shows that the equations to solve voltage is I × R. I stands for current and R stands for resistance. *Note: voltage is sometimes expressed as E or V and the symbols are used interchangeably. Alternatively, to find current, cover the I in the triangle. Covering the I leaves E / R, expressed as the voltage divided by the resistance.

Watt’s Law

Electric power can be found using Watt’s Law: Power = Current × Voltage, or watts equals amps times volts. This formula is very similar to Ohm’s law and can help solve for power/wattage.

Watt's Law triangle visualizing the formula to find power, voltage, and current

Determining Voltage, Power, Current, and Resistance

Use Ohm’s Law to find voltage(volts), power(watts), current(amps), and resistance(ohms) if any two values are known. This can be done using the two 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

Calculating Volts

  • Voltage = Current x Resistance
  • Voltage = Power / Current
  • Voltage = √ Power x Resistance

Calculating Watts

  • Power = Voltage x Current
  • Power = Voltage2 / Resistance
  • Power = Current2 x Resistance

Calculating Amps

  • Current = Voltage / Resistance
  • Current = Power / Voltage
  • Current = √ Power / Resistance

Calculating Ohms

  • Resistance = Voltage / Current
  • Resistance = Voltage2 / Power
  • Resistance = Power / Current2

Ohm’s Law in the Real World

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