What size metal button should I use for a shirt made of cotton?

For a typical cotton shirt, I would recommend using size 16 or size 20 metal snap buttons:

Size 16 snap buttons

These are about 3/8 inch or 10mm in diameter. This is a good size for most lightweight to mid-weight cotton fabrics. Size 16 snaps provide enough holding power for a cotton shirt but are small enough to appear proportional and not overwhelm the fabric.

Size 20 snap buttons

These are about 1/2 inch or 12mm in diameter. Size 20 snaps are suitable for most cotton shirt fabrics and provide a bit more durability than size 16 snaps. However, on a lightweight cotton, size 20 snaps may appear a bit large.

In the end, it depends on the specific weight and crispness of your cotton fabric:

Lightweight and soft cottons like voile, lawn or crepe

Use size 16 snaps. Size 20 snaps could slightly distort and weigh down these delicate fabrics.

Mid-weight cottons like poplin, twill or lightweight denim

Either size 16 or 20 would work well. Personal preference.

Heavier and stiffer cottons like canvas, duck cloth or denim

Size 20 snaps are probably best suited to the weight and hand of the fabric. Size 16 snaps may appear too small.

So in summary, for most cotton shirts, sizes 16 and 20 metal snap buttons are good options:

• Size 16 for lightweight, delicate cottons
• Size 20 for mid-weight to heavier cotton fabrics
• When in doubt, size 20 snaps are a safe general-purpose choice