The Watch Guide: Movements, Water Resistance & Sizing
Confused by a term? Dotted terms in our reviews open a plain definition, and the full glossary covers every word from bezel to value score.
Can't decide? Answer three questions and the Watch Finder shortlists three watches for you — or browse by where you're going in What to Wear.
What powers a watch: the four movements
Every watch on this site runs one of four power sources, and each one changes what owning the watch is like:
- Quartz — a battery drives a vibrating crystal. The most accurate and cheapest to own; the battery is the only recurring cost. The Casio F-91W and nearly every watch under $100/₹2,000/£50 is quartz.
- Automatic (self-winding) — your wrist's motion winds a mainspring. No battery, but a service every 5-10 years and less accuracy than quartz (±15 seconds/day is normal). Seiko 5 is the classic entry point.
- Solar (Eco-Drive, Tough Solar) — light charges a rechargeable cell. Quartz accuracy with no battery changes for decades; Citizen Eco-Drive and Casio Tough Solar are the two names to know.
- Digital — an electronic module rather than hands. Timers, alarms and backlight at prices analog can't reach; G-Shock is the hardened end of this family.
Water resistance: what the numbers actually permit
The metre marking is a pressure test, not a diving depth. What each rating survives in real use:
| Marking | Survives | Avoid |
|---|---|---|
| 30M / 3ATM | Splashes, rain, hand-washing | Swimming, showering (heat and pressure differ) |
| 50M / 5ATM | Swimming in shallow water | Diving, high-speed water sports |
| 100M / 10ATM | Swimming, snorkelling | Scuba diving |
| 200M / 20ATM | High-impact water sports; dive-style use | Saturation diving (that needs ISO 6425) |
Leather straps and water are enemies regardless of the case rating — a 200M watch on a leather strap still should not be swum in.
What size fits: the wrist-to-case table
Measure your wrist with a tape (or a strip of paper) just above the wrist bone, then match:
| Wrist circumference | Comfortable case diameter |
|---|---|
| 14–16 cm (5.5–6.3 in) | 34–39 mm |
| 16–18 cm (6.3–7.1 in) | 38–42 mm |
| 18 cm and up (7.1 in+) | 40–46 mm |
Lug width: the number that decides your strap
Straps are sized by the gap between the lugs — measure it in millimetres before buying any replacement band:
| Lug width | Typical watches | Strap availability |
|---|---|---|
| 16 mm | Small women's and vintage dress watches | Good — the classic women's size |
| 18 mm | Men's dress watches, mid-size cases | Excellent — the most stocked size |
| 20 mm | Diver and field watches to 42 mm | Excellent — every style exists |
| 22 mm | Sports and G-Shock-class cases | Excellent, slightly fewer dress options |
| 24 mm+ | Oversized and tool watches | Limited — mostly rubber and nylon |
A spring-bar tool costs less than one ruined strap; quick-release straps skip the tool entirely.
Lug-to-lug (top to bottom of the strap attachments) matters as much as diameter: it should not overhang the wrist's flat top. When in doubt between two sizes, the smaller one ages better.
Case-size visualizer
Case diameter against your wrist's flat width — the visual test a spec sheet can't give you:
Cost per wear
What a watch really costs is price divided by how often it's worn:
Caring for any watch
Three habits cover most of it: rinse salt and chlorine off after water wear, keep magnets and speakers away from automatic and quartz movements, and replace a battery promptly — a dying battery left in place can leak and destroy the movement. Our care and comparison guides go deeper on specific models. When you're ready to browse, the men's and women's departments and the full brand index are one click away.
Frequently asked questions
Which is better, quartz or automatic?
For accuracy and low cost of ownership, quartz. For craft and the pleasure of a mechanical object, automatic. A quartz watch under $50 keeps better time than most $500 automatics — buy the automatic for what it is, not for accuracy.
Can I shower with a 50M watch?
No. Shower water under pressure and heat exceeds what a 50M rating tests. 30M and 50M watches are splash and rain watches; swim only from 100M up.
What size watch for a 7-inch wrist?
A 7-inch (17.8 cm) wrist sits at the top of the 16-18 cm band: 38-42 mm case diameter is the comfortable range, with 40 mm the safest single choice.
How often does a watch need service?
Quartz: only battery changes, every 2-5 years. Automatic: a full service every 5-10 years. Solar: typically nothing for 10-20 years — the cell outlives most ownership.