How to choose a wristwatch size
Watch size is usually quoted as case diameter, but diameter alone is a poor guide to how a watch wears. A 40 mm watch with long, curved lugs can overhang a small wrist, while a 44 mm watch with short lugs can sit neatly. The numbers that matter are your wrist’s flat top width, the watch’s lug-to-lug length and its thickness. This calculator links them.
Start with the flat top of your wrist
The top of your wrist is a flat surface only so wide. When the lugs extend past its edges, the watch tilts, the strap pulls at an angle and the case looks too big. As a rule, the lug-to-lug length (the distance between the tips of the lugs, measured along the strap) should never exceed the width of your flat wrist, and it looks best at about 90% of that width.
Case diameter ≈ 0.74 × flat width + style offset + look offset
Style changes the picture. Dress watches look best slightly smaller and thin, sports and diver watches are traditionally larger, and pilot or field watches are bigger still because of their big, legible dials. Your personal taste matters too, so the tool lets you choose a subtle, balanced or bold look.
Case thickness
A thick watch sticks up from the wrist and catches on cuffs. As a rough guide, dress watches are under 10 mm thick, everyday watches 10–12 mm, sports watches up to 13–14 mm and divers up to 15 mm. Smaller wrists look better with slimmer cases, so the limit is scaled to your flat wrist width.
Strap length and lug width
The strap should be snug enough not to slide over the wrist bone but loose enough for one finger to slip underneath. The finished loop is usually the wrist circumference plus about 12 mm, a range of 8 mm (snug) to 20 mm (loose) for a bracelet that can expand during the day. Lug width, the gap between the lugs, is usually about half the case diameter, rounded to an even number of millimetres.
| Wrist circumference | Typical strap length (buckle side / tail side) |
|---|---|
| 14.5 – 16.5 cm (5.7 – 6.5 in) | 110 / 70 mm (short) |
| 16.5 – 19 cm (6.5 – 7.5 in) | 120 / 75 mm (standard) |
| 19 – 21 cm (7.5 – 8.3 in) | 125 / 80 mm (long) |
| 21 – 23 cm (8.3 – 9 in) | 130 / 85 mm (extra long) |
Accuracy
Every watch gains or loses a little time. A quartz watch is typically within ±15 seconds a month, which is about ±0.5 seconds a day. Mechanical watches are less precise: certified chronometers run within −4/+6 seconds a day, high-grade modern watches within ±2, and many everyday mechanicals within ±10. Rates also change with position, temperature and how fully the watch is wound, so measure over a week.
Days to reach one minute = 60 ÷ |rate|
Power reserve
An automatic watch winds itself as you move. The power reserve tells you how long it runs when fully wound and left off the wrist. If you wear it for most of the day it stays wound, but if you rotate between several watches it may stop in the drawer, and you will need to wind it by hand and reset the time.
Water resistance
A rating such as 30 m or 3 ATM does not mean the watch can be used to that depth. It is a laboratory pressure test on a new watch. Gaskets age, crowns can be left unscrewed and dynamic pressure from a dive or a swimming stroke is higher than static pressure. Choose a rating well above your activity: 30 m for splashes, 50 m for showers, 100 m for swimming, 100–200 m for snorkeling or surfing and 200 m or more for scuba (ISO 6425).