The Ohm’s Law

As the most used law in electricity, Ohm’s law establishes that the intensity of the current flowing between two points of an electrical circuit is proportional to the voltage between these two points. Thus, it is used to explain the relationship between the current, voltage and resistance, where “I” is the current that flows through the object in amperes, “V” is the potential difference of the objects’ terminal in volts and “R” is the resistance in ohms.

That is to say, “R” is the constant and independent unit of the current; “V” has a minor current fluency at a higher resistance, and “I” is directly proportional to the applied voltage and inversely proportional to its resistance. In consequence, the Ohm’s law focuses on the property of some materials. However, it is not an electromagnetic law as Gauss’ law. In maths language, it’s translated as V = IR

Ohm’s Law Calculator 1


The History of Ohm’s Law

Before any resistance calculator was created, the Ohm’s Law was born in 1827 by the German physicist Georg Simon Ohm. He did a vast investigation in the galvanic series field, discovering some voltage and current values that were flowing through simple electric circuits. Nowadays, this investigation led to a law which carries his name.

Due to that, Ohm received many acknowledgments and tributes: Munich University granted him the Chair of Professor of Physics in 1849, and the Royal Society – London acknowledged him with the Copley medal in 1941. Nevertheless, the top-notch tribute was naming the electric resistance unit after him, Ohm.


Features of Ohm’s law

  • Electric resistance: it is the opposition or difficulty found by a current in a closed circuit, which reduce the free flow of electrons. The unit of resistance is the ohm (R o Ω), meaning that the resistance offered by a conductor when an ampere (intensity) circulates through it and between its extremes, gives a potential difference (voltage) of one volt.
  • Ohm: it is the unity of electric resistance, and one ohm is equal to one ampere of current that flows when a voltage of one volt is applied. All circuits have a degree of opposition (or resistance) to the current flow, resulting in the Ohms formula R = V/I. Put in other words, increasing the current flow with the same voltage will reduce the resistance.
  • Volt: it is the unit of electromotive force or electric pressure (B) regularly applied to a circuit having a resistance of one ohm, which produces a current of one ampere. In a nutshell, the water flowing through a copper tube may be considered equal to the voltage flowing through the electric cable; because it requires a force to drive it, and the resistance to this flow is measured in amperes.

Ampere: it is the standard unit of electrical current, which is produced by a pressure of one volt in a circuit having a resistance of one ohm.


The Watt formula, Ohms formula, and Ampere formula – Understanding Ohm’s Law

Due to the existence of materials reduce the electric current flows through them, and when their resistance value changes, the value of the current intensity in amperes also varies inversely proportional. As the resistance increases, the current decreases, and as the resistance decreases, the current increases. In both cases, the value of the voltage requires constant maintenance.

Consequently, Ohm’s law works for circuits and passive circuit sections that a) have exclusively resistive loads (but not inductive or capacitive) or b) have a permanent regime. In both cases, the value of the conductor’s resistance could be affected by temperature. Therefore and from a Physics point of view, any device or material inserted into an electric circuit provokes resistance in the current flow. This resistance can be increased or decreased depending on the material used.

To calculate the resistance given by a material with specific length and thickness, we must apply the Ohm’s formula:

Ohm’s Law Calculator 2

Meaning, R is equal to rho (ρ) multiplied by the conductor length (L) and divided by the conductor section or thickness (area S). Where rho (ρ) is a constant called resistivity; L is the length in meters of the conductor cable, and S the section or thickness in mm2 of the conductor cable.

For further information, we share a table with some values of rho (ρ), depending on the type of conductive material:

Ohm’s Law Calculator 3

To calculate the resistor values​​, we already know the resistivity constant (ρ), so we must identify both the conductor length (L) and section (S). Meaning:

  • The longer the length, the higher the resistance.
  • The smaller the length, the smaller the resistance.
  • The longer the section, the smaller the resistance.
  • The smaller the section, the higher the resistance.

Once we analyze these four statements, we deduce that the resistance value is directly proportional to the conductor length and inversely proportional to its section.

Ohm’s Law Calculator 4

Resistance calculator - volt, ampere, watt calculator

This device will allow you to calculate the voltage, current, resistance and power in an electrical circuit through Ohm’s Law and its derived formulas.



R = U / I =

R = U2 / W =

R = P / I2 =


I = U / R =

I = P / U =

I = √ ( P / R ) =


U = R * I =

U = P / I =

U = √ ( P * R ) =


P = U * I =

P = U2 / R =

P = R * I2 =

Three phase Triangle


R = ( √3 * U ) / I =

R = ( 3 * U2 ) / P =

R = P / I2 =


I = ( √3 * U ) / R =

I = P / ( √3 * U ) =

I = R * U2 =


U = ( R * I ) / √3 =

U = R * I2 =

U = √ ( ( P * R ) / 3) =


P = √3 * U * I =

P = ( 3 * U2 ) / R =

P = R * I2 =

Three phase Star


R = ( U / √3 ) / I =

R = U2 / P =

R = P / ( 3 * I2 ) =


I = ( U / √3 ) / R =

I = P / ( √3 * U ) =

I = √( P / ( 3 * R2 ) ) =


U = √3 * R * I =

U = P / ( √3 * I ) =

U = √ ( P / R ) =


P = √3 * U * I =

P = U2 / R =

P = 3 * R * I2 =