Not the case width and not the old strap at its widest point. Use a ruler on the gap.
It is the sort of detail that only matters once, and only if it is wrong. Every listing on this site carries the figure the supplier gave us for exactly this reason.
Where the number comes from
One wrong millimetre is the most common ordering mistake here, and it is entirely avoidable.
Lug width in millimetres is the first thing we check on a strap listing, because a 20 mm strap on a 22 mm lug is the single most common ordering mistake in this category.
What to measure at your end
Before ordering nylon and silicone watch bands, take the measurement the product page uses and compare like for like. Where our page is silent on a figure, the supplier did not give it, and we would rather leave the line blank than fill it with an estimate.
If it still does not fit
Tell us within the return window and we will sort it out. A change of mind is a customer-paid return; a wrong item sent by us is ours to pay for. Both are on the return policy page in full.