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.
Thickness matters more than people expect. Straps around 4 to 5mm at the lugs can hit the case or stop the strap folding down on watches with short lugs or tight lug-to-case clearance. Do you know whether your watch has drilled lugs or a tight gap between strap and case? That tells you how much thickness is safe.
Document spring-bar diameter, end-tip fit and the strap’s clearance using appropriate measuring tools. If dimensions or compatibility are uncertain, ask a watchmaker rather than forcing a bar or substituting an unsuitable part.
Correction to a common shortcut: don't measure lug width on an existing strap. Leather and rubber straps compress or swell, and some are made slightly undersized on purpose. Always measure between the lugs themselves, with the strap removed if possible.
Summary order of importance. 1) Lug width, measured between the lugs. 2) Lug-end thickness against clearance. 3) Spring bar diameter. 4) Taper and buckle width. 5) Length, which is usually given as long side / short side in mm and should be compared with a strap that already fits you.
Clarification for “Strap dimensions: lug width, taper, thickness and spring bar diameter explained”: the point beginning “Lug width is the essential one: the inside distance between…” belongs to the question as written. It should not be read as a report that a new check, purchase or experiment has been completed. Review focus: scope. Keep the stated question separate from any broader claim.