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.


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How to Use Ohm’s Law

Ohm’s Law defines the relationship between electric current, resistance, and voltage. More specifically, it 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 calculate voltage, current, power, and resistance of an electrical circuit if you know at least two of the other values.

For example, if you know the voltage and resistance, use the calculator above to find power and current using Ohm’s Law. Alternatively, you can use the calculator to solve power and resistance if you know the voltage and current.

Ohm’s Law Formula

The Ohm’s Law formula is 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.[2]

I = V / R

The formula states that the current I is equal to the voltage V divided by the resistance R.

Components of Ohm’s Law

The following table shows the electrical quantities and units used in Ohm’s Law.
Quantity Unit Symbol
Voltage (V) volt V
Current (I) ampere A
Resistance (R) ohm
Power (P) watt W

Ohm’s Law Triangle

The Ohm’s Law triangle illustrates how to calculate voltage, current, or resistance. To use it, cover the unit you want to calculate to reveal the formula to solve it.

Ohm's Law triangle with the formulas to calculate voltage, current, and resistance
Ohm’s Law triangle, where V represents voltage, I represents current, and R represents resistance.

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

What do the Letters in the Ohm’s Law Formula Mean?

In the Ohm’s Law formula, V represents 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 voltage, current, and resistance units were defined.

It wasn’t until 1881 that the Volt, Ampere, and Ohm were defined, 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 Power Formula

Watt’s Law defines the power formula, which states that electric power measured in watts is equal to the circuit’s current times the voltage.[4] This formula is very similar to Ohm’s Law and can help solve for power or wattage.

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

P = I × V

Thus, the power formula states that power P is equal to the current I times the voltage V.[5]

Power Formula Triangle

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

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

Power formula triangle with the formulas to calculate power, voltage, and current
Watt’s Law power triangle where P represents power, I represents current, and V represents voltage.

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

Ohm’s Law Wheel

We can use Ohm’s Law to calculate the volts, watts, amps, or ohms, given that at least two measurements are known. The formula allows us to derive the equations for calculating any measurement given two other known values.

The Ohm’s Law wheel shows all the formulas you can use to find volts, watts, amps, or ohms. Note that all of the formulas in the wheel assume a power factor of 1.[6]

Ohm's Law wheel showing all the formulas to find the voltage in volts, power in watts, current in amps, and resistance in ohms
Ohm’s Law wheel with all the formulas that can be used to calculate the voltage in volts, current in amps, resistance in ohms, or power in watts.

Ohm’s Law Wheel Formulas

The Ohm’s Law wheel illustrates the following formulas:

Voltage Formulas

  • Voltage = Current × Resistance
  • Voltage = Power ÷ Current
  • Voltage = Power × Resistance

Power Formulas

  • Power = Voltage × Current
  • Power = Voltage2 ÷ Resistance
  • Power = Current2 × Resistance

Current Formulas

  • Current = Voltage ÷ Resistance
  • Current = Power ÷ Voltage
  • Current = Power ÷ Resistance

Resistance 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.

Formula Examples

We can use the following examples to learn how certain electrical circuit parameters can be calculated:

Example #1 – Let’s calculate the voltage when a current of 10 amps flows through a resistance of 2 ohms.

Voltage = Current × Resistance

Voltage = 10 A x 2 Ω
Voltage = 20 V

Example #2 – Let’s find the power consumed by a device supplied with 5 volts and a current of 2 amps.

Power = Voltage × Current

Power = 5 V × 2 A
Power = 10 W

Example #3 – Let’s find the resistance of a circuit if it consumes 20 watts of power when supplied with 5 volts.

Resistance = Voltage ÷ Current

Resistance = 52 V ÷ 20 W
Resistance = 25 V ÷ 20 W
Resistance = 1.25 Ω

Example #4 – Let’s find the current flowing through a circuit if it consumes 40 watts with a 10 resistance of Ω.

Current = Power ÷ Resistance

Current = 40 W ÷ 10 Ω
Current = 4
Current = 2 A

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.


  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-present
  3. Encyclopeadia Britannica, Ohm's Law, https://www.britannica.com/science/Ohms-law
  4. Honore, P., Basic Electronic Circuits Part-1, Analog Circuits, https://www.google.com/books/edition/Basic_Electronic_Circuits_Part_1_Analog/-J6rAgAAQBAJ
  5. Mike Holt, Electrician’s Math and Basic Electrical Formulas, https://www.mikeholt.com/instructor2/img/product/pdf/1302643872-sample.pdf
  6. Miller, C., NFPA's Electrical References, National Fire Protection Association, 2004, Jones & Bartlett Learning, 54. https://www.google.com/books/edition/NFPA_s_Electrical_References/raUyIi7i-asC