Ohm’s Law Calculator

Find Voltage, Current, Resistance, or Power using Ohm's Law

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

Find voltage, current, power, and resistance if you know at least 2 of the values. For example, if you know voltage and resistance then the calculator will find power and current. Alternatively, you can find the power and resistance if you know the voltage and current. In order for the calculator to work you need at least 2 of the 4 values. A single value will not work since it’s not enough to work with. This calculator uses Ohm’s Law in order to determine voltage, amperage, wattage, and ohms of your electric circuit. This is great for finding how many light bulbs you can power on a circuit or how much amperage you’ll need to power a device given it’s 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 can help visualize 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 you want to solve for and use the remaining two to find it. For example, if you want to find the voltage, you would cover the voltage part of the equation, which is the E in the triangle. Covering the E you notice that the I and R are next to each other, which means we would multiply I and R to solve. 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, if we want to find current we would 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 can help visualize the formula to find power, voltage, and current

Finding Voltage, Power, Current, and Resistance

We can use Ohm’s Law to find voltage(volts), power(watts), current(amps), and resistance(ohms) if we know any two values. This can be done using the 2 formulas above. Refer to this diagram to see all of the formulas used to find volts, watts, amps, or ohms.

Find volts, watts, amps, and ohms using the Ohm's Law and Watt's Law formulas on this chart

Finding Volts

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

Finding Watts

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

Finding Amps

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

Finding 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 you can use without blowing your circuit breaker to knowing how many light fixtures you can safely install 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 your electric devices. Ohm’s Law is a simple formula that you can easily size an electrical circuit or find out how large of a heater you can safely use on a regular outlet. I found this Sparkfun article very helpful for understanding electrical principles.