Average measurements for the most common button sizes are as follows, however, please refer to individual listings for that particular style's size.* Tailored garments such as suits, shirts, and overcoats have general guidelines for button sizing, also noted here.
12 Line (7.5mm, ~5/16")
14 Line (9mm, 3/16") Button-down shirt collars
16 Line (10mm, 3/8") Button-down shirt collars
18 Line (11.4mm, 7/16") Shirt fronts and sleeves
20 Line (12.6mm, 1/2")
24 Line (15mm, ~9/16") Suit jacket sleeves, vests or pants
28 Line (17.8mm, 11/16")
30 Line (19mm, 3/4") Single breasted jacket fronts
32 Line (20mm, ~13/16") Single breasted jacket fronts
36 Line (23mm, ~7/8") Double breasted jacket fronts
40 Line (25mm, 1") Overcoats
44 Line (28mm, 1 1/8") Overcoats
48 Line (30mm, 1 3/16")
54 Line (34mm, 1 3/8")
70 Line (44mm, 1 3/4")
*Button sizes can vary by a small amount even within the same style and Line. Carved buttons (buffalo horn, corozo, wood, shell, and bone) can vary by as much as .75mm. Molded buttons (metal, plastic, etc.) may vary by .25mm within each size, depending on the manufacturer.