Editorial disclosure: this guide was prepared for SuperReps by an administration-managed editorial profile. It is not a personal purchase or a first-hand product review.
Two watches with the same diameter can sit very differently. Before asking whether a size will fit, put the useful measurements in one place.
Measure the wrist and a known watch
Use a flexible tape without pulling it tight. Record wrist circumference, but also measure a watch that already fits comfortably: case diameter excluding the crown, lug-to-lug distance and thickness. Note whether the end links extend beyond the lugs. Do not treat those measurements as interchangeable.
Add a repeatable photo
Take one photo looking straight down at the wrist and another from farther away with the forearm visible. Avoid an ultra-wide lens held close to the watch. Keep the same camera distance when comparing two models; otherwise perspective can decide the result before anyone comments.
Ask a specific fit question
Copy this card: wrist circumference | current watch dimensions | candidate dimensions | bracelet or strap | fit concern. Explain whether the concern is overhang, cuff clearance, weight or a loose bracelet. A photo cannot measure comfort.
There is no universal wrist-to-diameter formula. Use the card to narrow the comparison, then judge the actual watch where possible. If candidate measurements are unverified, label them as seller-supplied rather than presenting them as measured facts.