I want to buy aftermarket straps but keep seeing 20/16, 22mm, 4mm thick and spring bar sizes. Which measurements do I actually need to take from my watch, and how do I measure them without the right tools?
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Lug width is the essential one: the inside distance between the lugs where the strap fits. Measure with a ruler marked in millimetres, ideally reading it under magnification. Most watches use even numbers (18, 20, 22mm), but odd widths exist. If you measure 19 or 21, measure again and check the manufacturer spec before ordering.
A notation like 20/16 means 20mm at the lugs, tapering to 16mm at the buckle. Taper affects look and comfort more than fit. A strong taper suits smaller and dressier cases, a straight strap suits sport watches. Your buckle or clasp also has to match the narrow end, so check that if you want to reuse an existing buckle.