A number of other approximate formulae for the characteristic impedance have been advanced by other authors.

Dulong suggests the following approximate formula for Q when the oxygen content is less than 10%:

It is also sometimes called the Blasius friction factor, after the approximate formula he proposed.

The second way is through an approximate formula which can be derived using some calculus and the theory of chemical thermodynamics.

The symmetric form is: and the approximate formula is:

The approximate formula is: and in the degree measure:

Another Indian influence was an approximate formula used for timekeeping by Muslim astronomers.

An approximate formula can be used to calculate the response time distribution for a network of fork-join queues joined in series (one after the other).

The focus is usually on developing approximate formulas for counting these objects in various contexts.

Another approximate formula by Eggleton is as follows: