How to read a watch specification sheet

A watch specification sheet is a marketing document that happens to contain facts. Nobody is usually lying to you, but the lines are chosen and ordered by someone who wants the watch to look good, and the most flattering number goes at the top. Reading one properly means knowing which figures constrain your experience of wearing the watch, which are neutral, and which are there because the format demands a number.

Three figures dominate every product page and every forum argument: diameter, water resistance and jewel count. All three are close to useless for predicting what a watch will be like to own.

Case dimensions

Case diameter is the width across the middle in millimeters, excluding the crown on almost every sheet you will meet. Some brands include the crown or its guards, which flatters a small watch and is rarely flagged. The number also says nothing about proportion: a wide bezel on a narrow dial makes a 40 mm watch read as 37, and the reverse is just as common.

Lug-to-lug is the distance from the tip of one lug to the tip of the opposite lug. This is the measurement that decides whether the watch fits, because a lug span longer than the flat top of your wrist forces the case to perch and the lugs to overhang. It is the most useful dimension on the sheet and the one most often omitted. Our guide to sizing and fit sets out how to measure your wrist against it.

Thickness is quoted caseback to the top of the crystal, and the trap is the domed crystal: a watch with a tall box sapphire can measure 13 mm on paper and wear like an 11 mm one, because the dome sits above the wrist rather than lifting the case off it. Thickness quoted "excluding crystal" is a figure to distrust. Under about 11 mm is genuinely slim for a mechanical watch with a date.

Lug width is the gap where the strap fits, almost always an even number of millimeters, with 18, 20 and 22 the common sizes. It tells you how easy the watch is to re-strap: odd sizes and proprietary fittings both narrow your options considerably.

Materials

Case material is where vocabulary does a lot of work. "Stainless steel" almost always means 316L, an austenitic grade with good corrosion resistance; a few makers use 904L and market the difference heavily. "Titanium" needs a grade: grade 2 is softer and grayer, grade 5 (an alloy) is harder and takes a better polish. Gold must carry a fineness, such as 18 carat, meaning 750 parts per thousand. "Steel" without a grade, or a case described only as "premium alloy", usually means a cheaper metal or a plated base. Our guide to case materials covers the trade-offs.

Bezel material and, more importantly, bezel function deserve their own line. Ask three questions: does it rotate, in which directions, and what is the insert made of. A dive bezel should rotate one way only, counterclockwise, in defined click positions, so an accidental knock can only shorten your indicated remaining time and never lengthen it. A bidirectional bezel is fine on a GMT and wrong on a dive watch.

Crystal type should say sapphire, mineral or acrylic. Sapphire is synthetic corundum, extremely hard and correspondingly brittle. Acrylic (often called hesalite) scratches easily but absorbs impacts and polishes out with a cloth. Mineral glass sits in between and does neither job especially well. The related line is anti-reflective coating: "AR coated" usually means one internal layer, while "coated both sides" gives better legibility at the cost of a coating that can wear on the outer face.

Water resistance and sealing

Water resistance is quoted in meters, feet, bar or ATM, and it is not a depth. It is shorthand for a static pressure the case survived on a test bench when new. Movement through water, temperature change and aging gaskets all subtract from it in ways the number does not capture. The full explanation is in our guide to water resistance ratings, but the practical hierarchy is: 30 m survives splashes, 50 m survives a shower you should not have taken, 100 m is a swimming watch, and 200 m with a screw-down crown is a swimming watch you can stop worrying about.

The related lines are crown type and caseback type. A screw-down crown threads onto a tube and compresses a gasket; a push-pull crown relies on gaskets alone and is why many 100 m watches are less waterproof in practice than they read. A screw-down caseback seals better than a snap-on one, and an exhibition back held by four small screws is a decorative choice that usually costs some pressure rating.

The movement block

This is the part worth reading twice.

Movement name and supplier should be specific: ETA 2824-2, Sellita SW200-1, Miyota 9015, Seiko NH35. A named caliber means parts availability and a watchmaker who has seen one before. "Swiss automatic movement" with no name is a deliberate omission, usually of a base the brand would rather not have compared, or of a grade below the one you would assume.

Automatic or manual is straightforward, but check separately whether an automatic can also be hand-wound: some low-cost automatics cannot, which makes restarting a stopped watch a matter of shaking it. Check too whether it hacks, meaning the seconds hand stops when the crown is pulled out, which is what lets you set to a time signal. Both are covered in our guide to automatic versus manual winding.

Jewels is the line to ignore. Synthetic ruby bearings have been industrially cheap for a century, and 21 to 27 is normal for a time-and-date automatic. A higher count on a simple movement means jewels were added where they do nothing.

Frequency is given in vibrations per hour (vph) or hertz, where 1 Hz is two vibrations a second, so 28,800 vph is eight vibrations a second, or 4 Hz, and is the modern default. Higher frequencies recover faster from a knock; lower ones waste less energy and support longer reserves. The mechanism behind that trade-off is set out in how a mechanical watch works.

Power reserve is the running time from full wind to stop. Around 38 to 42 hours is the classic single-barrel figure, and 70 to 80 hours is achievable with a longer spring, a wider barrel or a lower frequency. Treat a quoted reserve as a maximum from a fresh, fully wound movement.

Accuracy is the line where wording changes everything. "Chronometer certified (COSC)" means an independent laboratory tested the movement under ISO 3159 and it met a mean daily rate of minus 4 to plus 6 seconds a day. "Master Chronometer" adds a METAS test on the cased watch, including magnetic exposure. Anything else, including "high precision", "adjusted in five positions" and a stated tolerance such as plus or minus 10 seconds a day, is the manufacturer's own unaudited claim. Our guide to chronometer standards sets out what each certification covers.

Dial, hands and the rest

Lume should name a material. Super-LumiNova and its licensed equivalents are strontium aluminate pigments that charge from light and fade over hours. Tritium gas tubes, sold as H3 or GTLS, glow continuously for years without charging and are a different technology entirely. "Luminous" with no material named usually means an inexpensive pigment applied thinly.

Dial and hand finishing rarely gets a spec line and should. Applied indices are separate metal pieces fixed to the dial; printed indices are ink. Faceted, polished hands catch light in a way flat painted hands do not. "Sunray" and "guilloché" describe surface texture, while a lacquered dial is flat color under a clear coat. This is where a watch looking smart or looking cheap is usually decided.

Strap or bracelet should name the material and, on a bracelet, the construction: solid links and solid end links are structurally better than folded steel, and the difference is visible and audible. The clasp line matters more than people expect. A pin buckle is simple and reliable; a folding clasp preserves the strap and adds bulk under the wrist. Look for on-the-fly micro-adjustment, the feature that copes with a wrist that swells in summer.

Weight in grams raises one question: does it include the bracelet, which can double it.

Warranty is two years as the historic standard, with five increasingly common. Read what it covers: a warranty on the movement is not a warranty on water resistance, and almost none cover accidental damage.

The decode table

What is missing, and what that implies

Omissions are information. A sheet that gives diameter and thickness but not lug-to-lug is usually describing a watch with a long lug span. A movement described by country rather than by name is usually one the brand does not want compared on price. A missing accuracy line means the watch has not been specified to a rate at all, which is ordinary at the low end and a warning sign at the high end.

Other common gaps, with the safe assumption in each case: no crystal material (assume mineral), no lume material (assume a basic pigment), no clasp detail (assume stamped), no link construction (assume folded), and no crown type on a watch claiming 100 m or more (assume push-pull).

Comparing two watches fairly

The method is dull and it works.

1. Build one table with both watches as columns and every line above as a row, including the lines each sheet omits. Write "not stated" rather than skipping a row: the not-stated cells are half the comparison.
2. Fill the gaps from the movement supplier's own documentation rather than the retailer's page. ETA, Sellita and Citizen all publish caliber data sheets.
3. Convert everything to the same units first. Bar to meters, feet to millimeters, hertz to vph.
4. Sort the rows into three groups: fit (diameter, lug-to-lug, thickness, lug width, weight), function (water resistance, crown, bezel, crystal, lume, clasp) and mechanism (movement, frequency, reserve, accuracy, warranty).
5. Decide which group you are actually buying on. Most disappointments are fit failures dressed up as taste, and fit is the group you can settle before spending anything.
6. Only then compare price, against the group you chose in step 5 rather than against the total number of specification lines.

Two watches that differ by one jewel and 200 meters of unusable water resistance are, for most buyers, the same watch. Two watches that differ by 6 mm of lug-to-lug are not remotely the same watch, however similar their diameters look online.