A board is sold as four numbers: length, width, thickness and volume. The first three describe the shape; the last one, liters, is the one that tells you whether it will float you.
A Pyzel Ghost Round Tail at 5'10" reads 5'10" × 19" × 2 7/16" and 27.0 liters. Go one size up and width, thickness and liters all grow together; that is why the same model suits a 60 kg surfer at 5'8" and an 85 kg surfer at 6'2".
Feet and inches, nose to tail. Longer paddles faster and holds a line; shorter turns tighter. It is the number everyone quotes and the least useful on its own.
Inches at the widest point. Wider is more stable and planes earlier in small waves; narrower goes rail to rail faster. Half an inch is a noticeable change.
Inches at the thickest point, usually under the chest. Thickness is where most of the volume lives; a quarter inch changes the float more than two inches of length.
Liters of foam, the measure every brand now prints on the board. It is what the calculator gives you and what the finder filters on, because it is the one number that lets you compare a 6'0" fish with a 6'4" shortboard. A board without a stated volume gets an estimate in the catalog, marked ≈, from its dimensions and the same model at other sizes.
Volume chart by weight and skill →
More guides: Beginner board size · Soft-top or hard board · How many liters is too many · Foamie to fiberglass.