Buying guide
How to compare watercolor brush sizes
If the size 8 round you already own feels right, use it as a reference when you shop. Write down its series, shape, and approximate head width before comparing another brush marked 8.
What to compare
Separate the size label from the measurement
A number identifies a size within a series. For a straight swap, match the maker, series, head shape and size. For a new series, compare the head width and the length of the exposed bristles if the manufacturer gives both. Overall brush length is more useful for checking how the handle will fit in a case.
Compare the exact brush format
Princeton lists its Neptune studio round size 8 at 20 mm long by 4.75 mm wide at the head. Its Neptune travel round size 8 is listed at 23.11 mm by 5.08 mm. The format can change the head even when the series name and size number match.
Pick the size for the marks you paint most
A pointed round can make a fine start and a wider body within one stroke. For small botanical studies, look for enough point control to place the stem and enough body to fill a petal. If the brush already handles those marks but feels slow on a background, keep it and compare wash shapes.
Questions before buying
What should I measure on a brush I like?
Measure the exposed bristles from their base to the tip and note the head width. Compare those figures to a maker's head dimensions. Measure the entire brush separately if you are checking case fit.
Does a head width in millimeters tell me the width of my stroke?
It gives you a physical reference. The mark also depends on the head shape, pressure, angle, and how wet the brush is.
Can a travel round replace my studio round in the same size?
Compare the head dimensions and try the grip position you expect to use. If they suit your marks, the travel format can do the same job on a smaller setup.