Oil painting brushes come in all different sizes. In fact, there really is not a consistent standard throughout the industry. Sizes are usually given as numbers. For certain brands and certain sets of brushes these numbers can run from 1 to 24, 10 to 100, or 0 to 30, etc.
You will also find size markings such as 00 (or 2/0). A 00 brush is one size smaller than a 0 brush. Similarly, a 000 (or 3/0) brush is one size smaller than a 00 brush or two sizes smaller than a 0 brush.
Note that not all like numbered brushes are the same size. The actual size depends on the manufacturer and has also to do with the fact that some brushes are manufactured using the imperial system of units and others the metric one. However, you can count on the fact that a higher numbered brush will always be larger than a lower numbered brush. Also, the brushes of a particular series or set will generally be properly sized relative to each other.
The best thing you can do is to physically inspect a set either in an art store or on the Internet (if the merchant shows actual size pictures of the brush set).
One system of measuring brushes in actual measuring units such as millimeters or inches that is in fairly widespread use proceeds as follows:
* Diameter of Brush Head – In this system the diameter of a round brush head is measured at the top edge of the ferrule. The ferrule is the metal ring that holds the brush hair in place on the handle.
* Depth of Brush Head – When you lay a flat brush on the table, then the depth of the brush head is the vertical height of the brush head measured at the top edge of the ferrule.
* Width of Brush Head – The width of a flat brush head is the length of the hair at the top edge of the ferrule measured across the hair. For the flat brush on the table you measure the horizontal width at the top edge of the ferrule. Of course, the width of a round brush is the same as its diameter.
* Length of Brush Head – The length of the brush head is the length measured from the top edge of the ferrule to the tip of the hair along the center of the brush head. For a brush on the table you measure the middle line segment that starts at the top edge of the ferrule and ends at the tip of the brush head.
Brushes should always be measured when dry because when they are wet they expand. Also, do not confuse the width of a brush head with the width of the brush stroke. The width of the brush head is fixed once and for all but the width of a brush stroke varies with pressure, the kind of paint used, the angle of the brush head, and the flexibility of the brush hair.
This brush head measuring information is probably more than you’ll ever want to know. However, it’s a good thing to go through this at least once in a life time.