RE: Strap dimensions: lug width, taper, thickness and spring bar diameter explained · RWI-15703

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.