Artists’ brushes are usually given numbered sizes, although there is no exact standard for their physical dimensions. The number on the side of the brush refers to the thickness, length, or width of the brush hair. Rather than accurately representing these measurements, the numbering system is meant to order the size of the brushes. The width is measured just above the ferrule as can be seen in the diagram above.
Depending on the manufacturer sizing follows the following numbering conventions, from smallest to largest, the sizes are: