Ohm's Law Resistance Calculator

Created by Luis Hoyos
Reviewed by Arturo Barrantes
Last updated: Aug 23, 2022

If you're searching for how to calculate resistance from Ohm's Law, you're in the right place. Don't you know what we're talking about? Don't worry! In the following sections, you can read more about what is the resistance of Ohm's law and the formulas for its calculation.

What is the resistance of Ohm's law?

Ohm's law relates the electrical current (I) to the voltage difference (V) between two points of an electrical conductor. Mathematically:

I = V/R

V, I, and R, the parameters of Ohm's law. Attribution: Waveguide2, Public domain, via Wikimedia Commons
V, I, and R, the parameters of Ohm's law. Attribution: Waveguide2, Public domain, via Wikimedia Commons

In the previous equation, R represents the resistance of the conductor, an extensive property that indicates how averse that conductor is to the flow of electrons through it. The higher the resistance, the harder it will be to transport current through the conductor and, therefore, the more voltage we'll need. This extensive property depends on the cable material and geometry, and you can learn more about this dependence in our wire resistance calculator.

If we already know the required current and the available voltage, we can solve for resistance in the Ohm's law equation and obtain the value of R that will allow that current. We'll see that in the following section.

Ohm's law equation for resistance

The formula for resistance from Ohm's law is:

R = V/I

where:

  • R — Resistance, in ohms (Ω);
  • V — Voltage, in volts (V); and
  • I — Current, in amps (A);

You can also express R in terms of the electrical power (P):

R = P/I² = V²/P

You can learn more about those relationships in our Ohm's law calculator, which you can find in the following section.

Other tools like this Ohm's law resistance calculator

FAQ

What is the resistance of a 10 AWG copper wire?

In different units, the resistance of a 10 AWG copper wire is:

  • 0.0009733 ohms per foot (Ω/ft);
  • 0.9733 ohms per every 1000 feet (Ω/1000 ft);
  • 0.003193 ohms per meter (Ω/m); and
  • 3.193 ohms per kilometer (Ω/km).

How do I calculate resistance from Ohm's law?

There are various ways to calculate resistance from Ohm's law:

  • Divide the voltage by the current (V/I). That is the most common way to know what is resistance from Ohm's law formula.
  • Divide the power by the square of the current (P/I²).
  • Divide the square of the voltage by the power (V²/P).

How do I find total resistance in a parallel circuit from Ohm's law?

We know from Ohm's law that the total resistance in a parallel circuit is the reciprocal of the sum of reciprocals of the individual resistances. If it sounds confusing, follow these steps:

  1. Find the inverse of each of the resistances.
  2. Add all the inverses of each resistance.
  3. Take the inverse of the previous result.

Alternatively, you can use the formula for the total parallel resistance (R):

1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃ + ... + 1/Rₙ

Luis Hoyos
Electric circuit model with a resistor
Voltage (V)
V
Current (I)
A
Power (P)
W
Resistance (R)
Ω
Are you interested in finding the resistance using another method? Here are some calculators that you might find helpful:

Series resistor calculator

Parallel resistor calculator

Wire resistance calculator
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