Dimensionless number

From Example Problems
Jump to navigation Jump to search

In dimensional analysis, a dimensionless number (or more precisely, a number with the dimensions of 1) is a pure number without any physical units. Such a number is typically defined as a product or ratio of quantities which have units of identical dimension, in such a way that the corresponding units can be converted to identical units and then cancel.

For example: "one out of every 10 apples I gather is rotten." -- the rotten-to-gathered ratio is (1 apple) / (10 apples) = 0.1, which is a dimensionless quantity. Another more typical example in physics and engineering is the measure of plane angles with the unit of "radian". An angle measured this way is the length of arc lying on a circle (with center being the vertex of the angle) swept out by the angle to the length of the radius of the circle The units of the ratio is length divided by length which is dimensionless.

Dimensionless numbers are widely used in the fields of mathematics, physics, and engineering but also in everyday life. Whenever one measures anything, any physical quantity, they are measuring that physical quantity against a like dimensioned standard. Whenever one commonly measures a length with a ruler or tape measure, they are counting tick marks on the standard of length they are using, which is a dimensionless number. When they attach that dimensionless number (the number of tick marks) to the units that the standard represents, they conceptually are referring to a dimensionful quantity. But, ultimately, we always work with dimensionless numbers in measuring and manipulating even dimensionful quantities.

The CIPM Consultative Committee for Units toyed with the idea of defining the unit of 1 as the 'uno', but the idea was dropped. [1] [2] [3] [4]


  • A dimensionless number has no physical unit associated with it. However, it is sometimes helpful to use the same units in both the numerator and denominator, such as kg/kg, to show the quantity being measured.
  • A dimensionless number has the same value regardless of the measurement units used to calculate it. It has the same value whether it was calculated using the metric measurement system or the imperial measurement system.
  • However, a physical quantity may be dimensionless in one system of units and not dimensionless in another system of units. For example, in the nonrationalized cgs system of units, the unit of electric charge (the statcoulomb) is defined in such a way so that the permittivity of free space ε0 = 1/(4π) whereas in the rationalized SI system, it is ε0 = 8.85419×10-12 F/m. In systems of natural units (e.g. Planck units or atomic units), the physical units are defined in such a way that several fundamental constants are made dimensionless and set to 1 (thus removing these scaling factors from equations). While this is convenient in some contexts, abolishing of all or most units and dimensions makes practical physical calculations more error prone.

Buckingham π-theorem

According to the Buckingham π-theorem of dimensional analysis, the functional dependence between a certain number (e.g., n) of variables can be reduced by the number (e.g., k) of independent dimensions occurring in those variables to give a set of p = nk independent, dimensionless numbers. For the purposes of the experimenter, different systems which share the same description by dimensionless numbers are equivalent.


The power consumption of a stirrer with a particular geometry is a function of the density and the viscosity of the fluid to be stirred, the size of the stirrer given by its diameter, and the speed of the stirrer. Therefore, we have n = 5 variables representing our example.

Those n = 5 variables are built up from k = 3 dimensions which are:

  • Length L [m]
  • Time T [s]
  • Mass M [kg]

According to the π-theorem, the n = 5 variables can be reduced by the k = 3 dimensions to form p = nk = 5 − 3 = 2 independent dimensionless numbers which are in case of the stirrer

  • Reynolds number (This is the most important dimensionless number; it describes the fluid flow regime)
  • Power number (describes the stirrer and also involves the density of the fluid)

List of dimensionless numbers

There are infinitely many dimensionless numbers. Some of those that are used most often have been given names, as in the following list of examples (in alphabetical order, indicating their field of use):

Dimensionless physical constants

The system of natural units chooses its base units in such a way as to eliminate a few physical constants such as the speed of light by choosing units that express these physical constants as 1 in terms of the natural units. However, the dimensionless physical constants cannot be eliminated in any system of units, and are measured experimentally. These are often called fundamental physical constants.

These include:

See also

External links

de:Dimensionslose Kennzahl fr:nombre sans dimension he:מספר חסר ממד it:gruppo adimensionale pl:Liczby podobieństwa ja:無次元数 sl:brezrazsežno število nl:dimensieloos getal