The Knot Diameter gives you an indication of the amount of hair that has been packed into the handle. It is usually expressed in Millimetres (mm) and you’ll find that they vary from around 20mm to around 30mm in diameter. With 24mm being considered the average and 26mm being a popular choice.
Premium HD & SHD knots are packed with extra fibres to give the knot a denser softer feel.
A brushes Diameter, in conjunction with its Loft, the depth the knot is set into the handle and the size of the glue bump gives a brush its "backbone".