Nines are an informal, logarithmic notation for proportions very near to one, or equivalently percentages very near 100%. Their common uses include grading the purity of materials.

Put simply, "nines" are the number of consecutive nines in a percentage such as 99% (two nines) or a decimal fraction such as 0.999 (three nines). The number of nines of a proportion x is

${\displaystyle \mathrm {nines} =-\log _{10}(1-x)}$

However, there are different conventions for stating a non-integer number of nines. 99.5% could be expressed as "two nines five" or 2.3 nines, as outlined below. A completely pure material (x = 1) would have an infinite number of nines.