Ohm’s Law Calculator

Use Ohm’s law to calculate the voltage, current, resistance, or power in an electrical circuit. 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 Calculate 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 circuit element is directly proportional to the potential difference applied to it and inversely proportional to the resistance.[1]

Ohm’s law allows you to find voltage, current, power, and resistance if at least two of the values are known.

For example, if voltage and resistance are known, then the calculator will find power and current. Alternatively, power and resistance can be solved if the voltage and current are known.

What is the Ohm’s Law Formula?

The Ohm’s law formula is I = E/R, where I is the current through a conductor measured in amperes, E is the potential difference across the conductor measured in volts, and R is the resistance of a conductor measured in ohms.[2]

I = ER

Thus, the formula for Ohm’s law states that the current I is equal to the voltage E divided by the resistance R.

Ohm's law triangle visualizing the formula to find voltage, current, and resistance
Ohm’s law triangle where E represents voltage, I represents current, and R represents resistance.

The Ohm’s law triangle illustrates how to find volts, amps, or ohms. To use it, cover the unit you want to calculate to reveal the formula to solve it.

For instance, to solve for volts, cover the E with your thumb, which reveals that voltage is equal to I × R.

What do the Letters E, I, and R Mean in the Ohm’s Law Formula?

In the Ohm’s law formula, E represents electromotive force, or voltage, I represents intensity, or current, and R represents resistance.

Georg Simon Ohm created ohm’s law in a paper published in 1827,[3] well before the units of voltage, current, and resistance were defined.

It wasn’t until 1881 that the Volt, Ampere, and Ohm were defined, which is over 50 years after Ohm’s law was published. This explains why the letters are not related to modern-day units used in the formula.

Watt’s Law and the Power Formula

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.

We often use the power formula is in conjunction with Ohm’s law to solve electrical properties when the power of the circuit is known.

P = I × E

Thus the power formula states that power P is equal to the current I times the voltage E.[4]

Power formula triangle visualizing the formula to find power, voltage, and current
Power formula where P represents power, I represents current, and E represents voltage.

The power triangle illustrates the formula to find watts, volts, or amps. Just like the Ohm’s law triangle, cover the unit you want to find to reveal the formula to solve it.

For instance, to solve for amps, cover the I with your thumb, which reveals that current is equal to P/E.

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

Ohm’s Law Formula Wheel

Ohm’s law is used to determine the volts, watts, amps, or ohms, given that at least two measurements are known. The Ohm’s law formula allows us to derive the equations for calculating any measurement given two other known values.

The Ohm’s law wheel shows all of the formulas that can be used to find volts, watts, amps, or ohms. You can also see the formulas below.

Ohm's law formula wheel showing all the formulas to find volts, watts, amps, and ohms
Ohm’s law formula wheel with all of the formulas that can be used to solve volts, amps, ohms, or watts.

Formulas to Calculate Volts

Solve for voltage using these formulas:

Voltage = Current × Resistance

Voltage = Power ÷ Current

Voltage = Power × Resistance

Formulas to Calculate Watts

Solve for power using these formulas:

Power = Voltage × Current

Power = Voltage2 ÷ Resistance

Power = Current2 × Resistance

Formulas to Calculate Amps

Solve for current using these formulas:

Current = Voltage ÷ Resistance

Current = Power ÷ Voltage

Current = Power ÷ Resistance

Formulas to Calculate Ohms

Solve for resistance using these formulas:

Resistance = Voltage ÷ Current

Resistance = Voltage2 ÷ Power

Resistance = Power ÷ Current2

We use Ohm’s law for many things, such as determining the maximum microwave size or the maximum number of light fixtures a circuit can safely handle without creating a fire hazard.

Our lighting cost calculator can help find lighting energy usage, and our electricity cost calculator can help find the 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.

References

  1. Brandon Mitchell, Robert Ekey, Roy McCullough, and William Reitz, A Fan-tastic Quantitative Exploration of Ohm’s law, https://aapt.scitation.org/doi/full/10.1119/1.5021431
  2. National Institute of Standards and Technology, Ampere: The Present, https://www.nist.gov/si-redefinition/ampere/ampere-present
  3. Encyclopeadia Britannica, Ohm's law, https://www.britannica.com/science/Ohms-law
  4. Mike Holt, Electrician’s Math and
    Basic Electrical Formulas, https://www.mikeholt.com/instructor2/img/product/pdf/1302643872-sample.pdf