Snaps are not one-size-fits-all. If your snaps are too long for your fabric, they won't close properly. If they're too short, they'll fall off. For this reason, it's important to choose the proper size / length for your material.
Which size works best for you will depend on a number of variables, such as the fabric you are using, its density and compression levels, how hard you are able to press, and your own personal preference. For these reasons, we are unable to provide specific size recommendations for every scenario.
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The 3 most commonly used sizes, in the order of popularity, are:
Size 20 regular length
Size 20 extra long (for thicker material)
Size 16 regular length (for thinner material)
Size 20 (12.4mm or 1/2" cap diameter)
Size 20 Regular Length (5.6mm prong) is often used for lightweight to mid-weight fabric that isn't prone to tearing including baby diapers, menstrual pads, bibs, regular clothing, and household projects. This is the most popular size of plastic snaps and is a good size to start with for most projects since most commonly used fabric fall within the desired thickness for this length. This size has a strong grip so if you're working with more delicate fabric at risk of tearing, consider size 16.
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Size 20 Extra Long (6.2mm prong) is recommended for slightly thicker & denser material like double layers of standard marine vinyl for key fobs, medium weight leather, and chinese prefold diapers. (Some versions of these material may be so thick that they'll need larger/longer snaps.)
Size 20 Extra Short (4.6mm prong) is good for thinner material where you still want a good, strong grip such as baby diapers made with just 1-2 layers of PUL.
Size 16 (10.7mm or 7/16" cap diameter)
Size 16 Regular Length (4.6mm prong) is most commonly used for projects that require a looser grip, such as doll clothes or reinforced stretch jersey.
Size 16 Extra Long is available in lengths ranging from 5.2mm - 7.2mm long for thicker material.
Size 14 (9.7mm or 3/8" cap diameter)
Size 14 Regular Length (3.5mm prong) is extremely short and should only be used on very thin material, usually single layers. The overall height of a complete size 14 plastic snap is about 9mm.
Size 14 Extra Long (5.0mm prong) for situations when the regular length is not long enough such as when you have more layers or thickness.
Size 22 (14.1mm or 9/16" cap diameter, 7.0mm prong)
A larger snap which can be used in similar situations as size 20 long-prong, or in situations where size 20 long-prong is still a little too short.
Size 24 (14.8mm or 9/16" cap diameter, 7.5mm prong)
Our largest, strongest snap which should only be used in very heavy duty applications with thicker material such as canvas tote bags, nylon webbing, dense leather and thick jackets. Because the grip is so strong, it can tear through more commonly used fabric.