Paint brushes

Paint brushes come in a variety of sizes and shapes. The brushes are numbered differently between manufacturers making it difficult to tell the difference. I am going to help you clarify that in this blog post.​

Brushes are usually given numbered sizes, but they are not typically standard in their physical dimensions. 

Brushes for oil painting have longer handles, and brushes for watercolor often have shorter handles below is a diagram showing the different parts of a paint brush to pay attention to.

Brushes made in Europe often run smaller than the brushes made in the U.S. Also, a manufacturer might also add a measurement, a 4/0 brush is approximately the same as a 1/64 in or 0.4 mm. They can differ in not only bristle size but shape. Also, the same manufacturer may make 2 of one size brush with different quality making them more affordable for beginners and students.
small brushes increase by increments of 1 and larger brushes increments of 2.
From smallest to largest, the sizes are:
20/0, 12/0, 10/0, 7/0, 6/0, 5/0, 4/0 (also written 0000), 000, 00, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 18, 20, 22, 24, 25, 26, 28, 30.

Decorator brush sizes are given in millimeters or inches, which refers to the width of the bristles of the brush. Common sizes are:
10 mm, 20 mm, 40 mm, 50 mm, 60 mm, 70 mm, 80 mm, 90 mm, 100 mm.
​1⁄8 in,​1⁄4 in, ​3⁄8 in, ​1⁄2in, ​5⁄8 in, ​3⁄4 in, ​7⁄8 in, 1 in, ​11⁄4 in, ​11⁄2 in, 2 in, ​21⁄2 in, 3 in, ​31⁄2 in, 4 in.

I hope this will help you in the future find a brush that meets your needs without so much confusion.