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In chemistry and biology, the dilution ratio is the ratio of solute to solvent. It is often used for simple dilutions, one in which a unit volume of a liquid material of interest is combined with an appropriate volume of a solvent liquid to achieve the desired concentration. The diluted material must be thoroughly mixed to achieve the true dilution. For example, in a 1:5 dilution, with a 1:5 dilution ratio, entails combining 1 unit volume of solute (the material to be diluted) with 5 unit volumes of the solvent to give 6 total units of total volume.
This is often confused with "dilution factor" which is an expression which describes the ratio of the aliquot volume to the final volume. Dilution factor is a notation often used in commercial assays. For example, in a 1:5 dilution, with a 1:5 dilution factor, (verbalize as "1 to 5" dilution) entails combining 1 unit volume of solute (the material to be diluted) with (approximately) 4 unit volumes of the solvent to give 5 units of total volume. Note that some solutions and mixtures take up slightly less volume than their components.
The dilution factor can be expressed using exponents: 1:5 would be 5e−1 (5−1 i.e. one-fifth:one); 1:100 would be 10e−2 (10−2 i.e. one hundredth:one), and so on.
There is often confusion between dilution ratio (1:n meaning 1 part solute to n parts solvent) and dilution factor (1:n+1) where the second number (n+1) represents the total volume of solute + solvent. In scientific and serial dilution assays, the given dilution factor often means the ratio to the final volume, not to just the solvent. The factors then can easily be multiplied to give an overall dilution factor. Some have suggested that dilution factors should more clearly be written as a/total or a þ b, as the use of the colon symbol ":" is widely used to represent ratios in fields like mathematics, chemistry, or organic chemistry. Leave ratios for actual ratios 1:100 = 101. However, at this time, both dilution conventions are widely used at this time -which is why it is important for laboratory personnel to always clarify whether a "dilution ratio" or "dilution factor" ought to be used in performing dilutions.
or sometimes the inverse for other fields: 
In other areas of science such as pharmacy, and in non-scientific usage, a dilution is normally given as a plain ratio of solvent to solute. For large factors, this confusion makes only a minor difference, but in precise work it can be important to make clear which ratio is intended.