Buying a watch online means you cannot try it on. The good news is that four numbers on a listing – case diameter, lug-to-lug, thickness and lug width – tell you almost everything about how a watch will sit on your wrist. Here is how to read them.

Start by measuring your wrist

Wrap a soft tape measure, or a strip of paper you then lay against a ruler, snugly around your wrist just above the wrist bone, where you would wear a watch. Most adults fall between 6 and 8 inches (15 to 20 cm). Note the number; everything below refers to it.

Case diameter

This is the width of the case excluding the crown. Broad guidelines:

• Under 6.5 in (16.5 cm): 34-38 mm for a classic look, up to 40 mm for a sports watch.
• 6.5-7.5 in (16.5-19 cm): 38-42 mm suits most styles.
• Over 7.5 in (19 cm): 40-46 mm; smaller cases can look lost.

Style shifts these numbers. Dress watches traditionally run smaller (36-39 mm); dive and field watches run larger because their bezels and bold dials need room. A thin bezel and a large dial make a watch wear bigger than a thick-bezelled watch of the same diameter.

Lug-to-lug: the number that matters most

Lug-to-lug is the distance from the tip of the top lugs to the tip of the bottom lugs. If it is longer than the flat part of your wrist, the lugs will overhang and the watch will feel too big no matter what the diameter says. As a rule: up to 46 mm lug-to-lug for wrists under 6.5 in, up to 50 mm for 6.5-7.5 in, and up to 54 mm above that.

Thickness

Thickness decides whether a watch slides under a shirt cuff. Under 10 mm is slim, 10-13 mm is typical for automatics and chronographs, and over 14 mm is chunky.

Lug width and strap choice

Lug width (18, 20 or 22 mm are the most common) is the gap between the lugs where the strap fits. It determines which replacement straps you can buy, and it also affects proportion: a 20 mm strap on a 40 mm case looks balanced; a narrow strap on a wide case looks top-heavy.

A quick test at home

Cut a piece of card to the listed diameter and lug-to-lug, lay it on your wrist and look in a mirror. It takes a minute and is remarkably accurate.

Bracelet sizing

Metal bracelets arrive on full length and are sized by removing links; a jeweller or watch repairer will do this for a few dollars, and many clasps have micro-adjustment holes for fine tuning. Keep the removed links, and remember that you should not resize a watch until you are sure you are keeping it.

Every watch listing on Jouelanity states the case diameter and thickness and, where the manufacturer publishes it, the lug-to-lug and lug width. If a measurement you need is missing, email us and we will check the piece for you before you order.