How to Read a Watch Spec Sheet: Case Size, Water Resistance, Crystal and Materials

Case size: diameter, lug-to-lug, and thickness

The headline number is the case diameter, measured across the watch from the 9 o'clock to the 3 o'clock side, usually excluding the crown. Modern watches range from roughly 34 mm for many dress styles to 40–46 mm for most sports and tool watches, and larger for some dive and aviation pieces. Diameter alone is misleading, because two watches with the same diameter can wear very differently.

Lug-to-lug distance is the span from the tip of one lug to the tip of the lug directly across the case. It's the single most important number for fit: if the lugs overhang your wrist, even a modest 40 mm watch will feel oversized. A common rule of thumb is that the case width should occupy about 60–75% of the flat wrist width, and the lug-to-lug should fall within roughly 75–95% of that same measurement. The strap fits between the lugs, so lug width (the gap between the two lug horns) tells you which strap sizes to buy; 18–20 mm is common on smaller or vintage-style watches, 22–24 mm on larger sports models.

Case thickness matters more than buyers expect. Most dress and everyday tool watches sit between 8 mm and 14 mm thick. A 12 mm watch slides under a shirt cuff easily; a 15 mm tool watch often doesn't. On a spec sheet, thickness is usually listed in millimetres, sometimes alongside the crystal height, which can add 1–2 mm on top of the case itself.

Water resistance: what the metres really mean

Water-resistance ratings are among the most misunderstood numbers in watchmaking. The metres printed on a dial or spec sheet refer to a static overpressure test in a laboratory, not to a depth you can safely dive to. ISO 2281 is the long-standing ISO standard for everyday water-resistant watches, while ISO 6425 is the dedicated standard for dive watches.

Here's how the common ratings actually translate to use:

A 30 m watch isn't a 30-metre watch. It's rated to withstand the static pressure equivalent of 30 metres of water in a lab, which translates to splashes and hand washing in real life. A 100 m rating is widely described as suitable for swimming, but it doesn't make a watch a dive watch unless it's also ISO 6425 certified. ISO 6425 sets the minimum water-resistance threshold for dive watches at 100 m, with 200 m or more preferable for regular water use. The standard also requires specific features such as a unidirectional rotating bezel (or an equivalent internal indicator) and a static overpressure test at 125% of the rated pressure; a 200 m dive watch must therefore withstand 250 m of static pressure during testing.

One more detail worth knowing: helium-release valves aren't required on every ISO 6425 dive watch. They're specified only for the saturation (professional) dive sub-class, designed for commercial divers working in pressurised chambers. If you aren't doing saturation diving, you don't need one. Finally, "water-resistant" without any depth marking may indicate only minimal splash resistance under ISO 2281, so a bare "WR" marking isn't a green light for swimming.

Crowns, casebacks, and gaskets: how water resistance is actually built

Two spec-sheet details hint at the sealing system inside the watch: whether the crown screws down, and whether the caseback is solid or display-style. A screw-down crown mechanically compresses a gasket (a rubber O-ring) against the case tube. The crown provides the mechanical force; the gasket provides the seal. A worn or missing gasket can defeat even a properly threaded screw-down crown, and gaskets are considered service parts that degrade with age. ISO 6425 also specifies a static overpressure test, and many dive watches add a screw-down caseback as part of the same sealing strategy.

Crystal: mineral, sapphire, and acrylic

The crystal is the transparent cover over the dial. Three materials dominate. Mineral glass is a hardened, heat-tempered glass formulated specifically for watch crystals. Sapphire crystal isn't technically a glass at all; it's synthetic sapphire, a lab-grown crystalline form of corundum (aluminium oxide), chosen for hardness. Acrylic, sometimes called plexiglass or Hesalite, is a clear plastic that many vintage and a few current watches still use.

The key trade-off is hardness versus shatter resistance. Sapphire measures about 9 on the Mohs hardness scale, compared with roughly 5–6 for mineral glass and 3 for acrylic. That makes sapphire extremely difficult to scratch with everyday objects, but it's more brittle than mineral or acrylic and can crack on a hard impact. Mineral glass scratches more easily but is also tougher to shatter. Acrylic scratches very easily but can be buffed out with polishing compound, and it tends to crack rather than shatter, which is one reason some space and military watches have stuck with it.

Sapphire's main optical drawback is higher reflectivity. Watchmakers apply an anti-reflective (AR) coating to the inside, the outside, or both sides of the crystal to reduce glare. Single AR coating is a noticeable improvement; double AR is better, but the outer coating can show fingerprints and wear over years of use. Mineral glass has a naturally lower reflectivity, which is part of why it doesn't strictly need a coating to look clean in everyday use.

Case materials: stainless steel grades, titanium, and ceramic

Most watches are made from 316L stainless steel, an industry-standard alloy valued for corrosion resistance and biocompatibility (suitability for prolonged skin contact). A growing number of brands use 904L stainless steel, often called a "superalloy," which contains higher levels of chromium, molybdenum, and nickel than 316L, giving it greater corrosion resistance, particularly against salt water and chlorides. Both 316L and 904L are considered high-quality grades of stainless steel used in watchmaking, and the practical difference for most wearers is small.

Titanium Grade 5 (also written Ti-6Al-4V) is the most common titanium alloy in watch cases. It's roughly half the weight of steel for the same volume, hypoallergenic, and corrosion-resistant. The trade-off is that titanium scratches more visibly than steel (the surface oxide layer scuffs) and can be harder to refinish. Ceramic cases, usually zirconium oxide, offer scratch resistance that steel, titanium, and gold can't match; a ceramic case can look new for years. Ceramic is, however, brittle and can crack or chip on a sharp impact, and it can't be polished back to a perfect finish if it does.

Gold, bronze, carbon composites, and precious-metal cases appear on many spec sheets as well. For comparison purposes, the practical questions are always the same: how heavy is it, how will it age (patina, scratches, polishing), and how does it interact with skin and water. A spec sheet answers the first question; the rest you learn over time.