Dimensional Analysis

Dimensional Analysis Basics

  • Dimensional analysis is a mathematical method used to check the validity of equations and to derive new equations.

  • The key principle is that in every valid equation the dimensions on the left-hand side must match the dimensions on the right-hand side.

  • Dimensions are typically represented by the symbols [L] for length, [T] for time, [M] for mass, and a few others according to the context.

  • In dimensional analysis, a physical quantity is expressed in the form [M]^{α}[L]^{β}[T]^{γ}, where α, β, γ are the powers of the fundamental quantities mass, length, and time.


  • Homogeneity of dimensions in an equation is the principle stating that the dimensions of the left side of the equation must be the equal to the dimensions of the right side.

  • This principle is used to validate physical equations, as any valid physical law must hold true irrespective of the units used for measurement.

Using Dimensional Analysis

  • Dimensional analysis is often used for deriving formulas. If you know the dependencies of a physical quantity, but not the exact equation, dimensional analysis can help you find the functional relationship.

  • Dimensional analysis can also be used to convert units from one system to another, such as from metric to imperial.

  • In problem-solving, dimensionless quantities often play key roles due to their property that they remain unchanged even when the units of measurement change.

Limitations of Dimensional Analysis

  • Dimensional analysis cannot help with dimensionless constants. For instance, the number π does not have any dimensions and hence dimensional analysis does not provide information about it in an equation.

  • It cannot detect if an equation is correct only in certain conditions. For instance, some equations are approximations valid only under specific circumstances, which is not detectable through dimensional analysis.

  • It does not replace or provide an alternative to actual experiment or observation in the field of physics or other sciences. It is a tool to be used alongside empirical methods.

Advanced Concepts

  • The Bridgman’s theorem talks about how every physically meaningful equation involving dimensional quantities will have the same form in every system of units.

  • The Buckingham Pi theorem is a key theorem in dimensional analysis used to compute sets of dimensionless parameters from given variables, even if the form of the equation is still unknown.

  • The Vaschy-Buckingham theorem is another form of the Pi theorem, providing a systematic method for identifying sets of dimensionless quantities in a physical system, which may then be used for scaling analysis.

Practice Techniques

  • Continue building fluency with dimensional analysis through regular practice, involving a variety of questions from simple unit conversions to complex applications in physics and engineering problems.

  • Review errors made in problem-solving. Identify if they come from misunderstanding fundamental principles or from calculation mistakes.

  • Develop a systematic approach to each problem to avoid careless errors. It’s advised to always write out the full steps while solving dimensional analysis problems.

  • Make a habit to perform a dimensional check on the final result in order to verify that you have used the right units throughout your calculations.

Remember, dimensional analysis is a powerful tool but it must be used with understanding of its capabilities and limitations.