Electric circuit model with a resistor
Voltage (V)
Current (I)
Resistance (R)
Power (P)
You do know how to find resistance? If not, here are some calculators that you might find helpful:

Series resistor calculator

Parallel resistor calculator

Wire resistance calculator

Our Ohm's law calculator is a neat little tool to help you find the relationships between voltage, current and resistance across a given conductor. The Ohm's law formula and voltage formula are mainly used in electrical engineering and electronics. Also, if you know how to calculate power, you may find it very useful when studying electronic circuits. All of these calculations you be done with our Ohm Calculator.

In the rest of the article you'll find:

  • The Ohm's Law formula
  • How to use the voltage formula
  • What the equation for power is
  • How to calculate power
  • Ohm's law for anisotropic materials

Ohm's Law Formula

Ohm's Law is one of the basic laws of physics. It describes relationship between voltage, amperage (otherwise known as current) and resistance. Voltage refers to the potential difference between two points in an electrical field. Amperage is related to the flow of electrical charge carriers, usually electrons or electron-deficient atoms. The last term, resistance, is the substance's opposition to the flow of an electric current.

Ohm's law states that the current flows through a conductor at a rate that is proportional to the voltage between the ends of this conductor. In other words, the relationship between voltage and current is constant:

I/V = const

The Ohm's law formula can be used to calculate the resistance as the quotient of the voltage and current. It can be written as:

R = V/I


  • R - resistance
  • V - voltage
  • I - Current

Resistance is expressed in ohms. Both the unit and the rule are named after Georg Ohm - the physicist and inventor of Ohm's law.

Remember that the Ohm's law formula relates only to substances which are able to induce power. such as metals and ceramic. However, there are many other materials for which the Ohm's law formula cannot be used, such as semiconductors and isolators. Ohm's law is also only valid only under certain conditions, like a fixed temperature.

Looking for a real-life application of the Ohm's law? Make sure to check out the LED resistor calculator!

Voltage formula

The voltage formula is one of three mathematical equations related to Ohm's law. It is the formula provided in the previous paragraph, but rewritten so that you can calculate voltage on the basis of current and resistance, that is the voltage formula is the product of current and resistance. The equation is:

V = IR

This value is measured in volts.

What is the equation for power?

Another value you can calculate on the grounds of Ohm's law is power. Power is the product of voltage and curren, so the equation is as follows:

P = V x I

With this formula you can calculate, for example, the power of a light bulb. If you know that the battery voltage is 18V and current is 6A, you can that the wattage will be 108 with the following calculation:

P = 6A x 18V = 108 watts

How to calculate power?

If you are still not sure how to calculate power with the provided formulas, or simply want to save your time, you can use our Ohm's Law calculator. The structure of this tool is not too complicated, just input any two of the four values to receive the other two. The Ohm's law calculator is based on the power formula together with the Ohm's Law formula. All you need to do to get the value of power is to type:

  1. Voltage (expressed in volts)
  2. Current (expressed in amperes)

Then the Ohm's Law Calculator will give you two values - resistance, expressed in ohms, and power, expressed in watts. If you need this result in another unit, you can use our watts to amps calculator.

Ohm's law for anisotropic materials

There is another version of Ohm's law that uses the position of electrical properties within the conductor. Some prefer it over the previous formula due to its dimensional view. Conductive materials comply with Ohm's law when the specific resistance of the materials does not depend on the value and direction of the applied electric field.

You can find following formula if you click on the Advanced mode button:

ρ = E / J, where

  • ρ is the specific resistance of the conductive material.

  • E is the electric field vector.

  • J is the current density vector.

As far as isotropic materials go, it's best to use the first formula as it's much less complicated. Isotropic materials are those with the same electrical properties in all directions, such as metals and glass. This formula might come in handy when you work with anisotropic materials like wood or graphite.

Mateusz Mucha and Julia Żuławińska