Engineering · Electrical Engineering · Circuit Analysis
Ohm's Law Calculator
Calculate voltage, current, resistance or power using Ohm's Law and the power formulas. Enter any two known values to find the others.
Calculator
Formula
Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R). Combined with the power formula P = VI, this gives three equivalent expressions for electrical power. Given any two of the four quantities (V, I, R, P), the remaining two can be derived.
Source: Ohm G.S. Die galvanische Kette mathematisch bearbeitet. Berlin, 1827. Standard reference: IEC 60050.
How it works
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality: V = IR. This relationship holds for ohmic (linear) materials at constant temperature.
The power dissipated in a resistor is given by P = VI. Substituting Ohm's Law, this yields two equivalent forms: P = I²R (useful when current and resistance are known) and P = V²/R (useful when voltage and resistance are known).
The four quantities form a square of relationships — knowing any two allows derivation of the other two. This is the basis of the Ohm's Law wheel or triangle, a common reference tool in electronics.
Worked example
A resistor in a 12 V circuit carries a current of 2 A:
Resistance = V ÷ I = 12 ÷ 2 = 6 Ω
Power = V × I = 12 × 2 = 24 W
This 24 W is dissipated as heat in the resistor. A resistor rated below 24 W would overheat and fail.
Limitations & notes
Ohm's Law applies to linear (ohmic) components at constant temperature. Non-linear components — diodes, transistors, thermistors, and many semiconductor devices — do not obey Ohm's Law. In AC circuits, the equivalent of resistance is impedance (Z), which includes reactive components (capacitive and inductive reactance) and requires complex number analysis.
Frequently asked questions
What units are used in Ohm's Law?
In SI units: voltage is measured in Volts (V), current in Amperes (A), resistance in Ohms (Ω), and power in Watts (W). These are the standard units for electrical calculations. Ohm's Law holds exactly in these units: 1 V = 1 A × 1 Ω.
Does Ohm's Law apply to AC circuits?
In AC circuits, the relationship V = IZ applies, where Z is impedance — a complex quantity that includes resistance, capacitive reactance, and inductive reactance. For purely resistive AC circuits, Z = R and Ohm's Law applies directly. For reactive circuits, phase angles must be accounted for.
What is the power rating of a resistor?
The power rating of a resistor specifies the maximum power it can continuously dissipate without damage. Common ratings are 0.25 W (quarter-watt), 0.5 W (half-watt), 1 W, and 2 W. Calculate the actual power dissipation using P = I²R or P = V²/R, and select a resistor rated for at least twice the calculated value for safety margin.
What is the difference between resistance and impedance?
Resistance (R) is the opposition to current flow in purely resistive components, independent of frequency. Impedance (Z) is the generalised opposition to AC current flow, including both resistance and reactance. Reactance arises from capacitors (capacitive reactance XC = 1/(2πfC)) and inductors (inductive reactance XL = 2πfL). Impedance is a complex quantity: Z = R + jX.
How do I calculate resistance if I only know voltage and power?
From the power formula P = V²/R, rearranging gives R = V²/P. For example, a 60 W light bulb operating at 230 V has a resistance of 230² ÷ 60 ≈ 882 Ω (at operating temperature).
Last updated: 2025-01-15 · Formula verified against primary sources.