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Start with the size. To find the size of your zipper, measure the width of your zipper teeth when they are closed in millimeters (if possible). The measurement might not neatly line up with a zipper size, as the actual size of the zipper tape can vary, but it should roughly correlate.
Similarly, you may ask, what size zipper do I need?
The most common sizes are 3 mm, 5 mm, 7 mm, and 10 mm to match zipper-teeth widths. The number corresponds to the closed-teeth width in millimeters. A size 5 zipper, for example, has teeth that measure 5 mm wide when the zipper is zipped.
Accordingly, can you replace a zipper slider?
When a zipper pull breaks, or the slider simply wears out, you don't need to replace the entire zipper. Just replace the zipper slider and the stop that keeps it from sliding off. This is a quick repair that can make the difference between a zipper that doesn't work at all and one that works like it's brand new.
The second number, preceded by an actual number sign, tells you the width in millimeters of the chain. So a #3 zipper has a 3mm wide chain, and a #5 zipper has a 5mm wide chain. The #3 will be more flexible than the #5, but not as strong.