Snap Fastener Sizes – How to Choose the Right Diameter in mm

October 7, 2026

Snap fastener sizes are given in millimetres and refer to the diameter of the visible top cap. Choosing the right size affects more than looks: a snap that is too small on thick fabric pops open on its own, while one that is too large on a light blouse looks heavy and pulls the fabric out of shape. Below you will find the common sizes, how to measure an existing snap and which diameter suits which garment.

How to measure a snap fastener

The size always refers to the cap – the part you see on the front of the garment. Measure it across its widest point with calipers or a ruler. The lower parts (the stud and the socket) are smaller and are not used to define the size.

Measure the cap, not the spring part (socket).
On domed caps, measure the base diameter, not along the curve.
When replacing a snap, also check the post length – it has to match the thickness of the fabric.

You will often see "L" (line) sizing in the trade – one line is about 0.635 mm. So 20L is roughly 12.5–13 mm and 24L is around 15 mm. At Bul-tex sizes are listed directly in millimetres, which makes choosing easier.

Concealed snap fasteners 202D 13 mm nickel-free with hidden caps

Snap fasteners E-2278 Gun Metal 40 mm – 4 pieces, "Teardrop" design, Luxury model

Special HK 305 ring-type snap fasteners – 18 mm and 22 mm, Gun Metal

Chart: size by garment and fabric

Diameter	Suitable fabric	Typical use
6–9 mm	Very fine, delicate fabrics	Mini sew-on snaps, lingerie, doll clothes, hidden closures
10–12.5 mm	Jersey, cotton, poplin	Shirts, baby bodysuits, blouses, pockets
13–15 mm	Denim, corduroy, heavier cotton	Denim shirts, workwear shirts, light jackets
17–20 mm	Dense and padded materials	Jackets, parkas, windbreakers, bags
21–25 mm	Canvas, leather, many thick layers	Winter jackets, tarpaulins, cases, umbrellas
30–40 mm	Heavy fabrics	Statement details on coats and bags

Size also depends on the snap system

Apart from diameter, the fastening system matters. Four-part snaps are set with a press or pliers and usually come in 10 to 25 mm. Two-part sew-on snaps are smaller – often 6 to 12 mm – although there are larger 18–25 mm models for coats. Prong ring snaps (with a toothed ring) are used on knits and are usually 10–15 mm, with bigger versions up to 22 mm.

Snap 0009 standard – 13 and 20 mm, an all-round choice for shirts and jackets.
SB 0001 domed and flat – 11.5 to 25 mm in silver, gold, dark grey and black.
Mini models – 6.8, 8, 9 and 10 mm for fine garments and accessories.

Common sizing mistakes

A large cap with a short post on thick fabric – the snap will not close properly.
A small snap on a heavy jacket – it comes undone as you move.
Mixing sizes on one garment for no reason – it looks careless. A smaller snap on cuffs or pockets is the exception.
No reinforcement – whatever the size, there should be at least two layers of fabric or interfacing under the snap.

Size and colour – getting them to work together

Once you have the diameter, think about the finish. The same snap looks very different in silver, gold, gunmetal or black oxide. On light fabrics a smaller snap in silver or white is almost invisible, while on dark denim a larger gunmetal or antique cap becomes part of the design. If the garment has other metal details – a zip, eyelets or a buckle – pick the snap in the same tone and in a size that is in proportion to them.

A good rule of thumb is for the cap to be roughly the width of the placket minus 8–10 mm on each side. On a 3.5 cm placket a 15 mm snap looks balanced, while on a narrow 2.5 cm placket 10–12.5 mm is a better fit. Before setting, lay the snap on the fabric and look at the garment from a distance – it is the easiest way to judge the proportion.