Lug to lug: the measurement that decides

The question asked in every shop is what size case suits me, and the honest answer is that case diameter is the wrong variable. A 40 mm diver with long straight lugs wears larger than a 42 mm dress watch with short curved ones, and neither number predicts it. What predicts it is the distance from the tip of the top lug to the tip of the bottom lug, and whether that distance fits on the flat part of your wrist.

Measure your wrist properly

Not the circumference, which is what a strap needs. The flat: the width across the top of your wrist, from the outer edge to the inner edge, where the case will actually sit. On most people this is between 40 and 55 mm and it is considerably less than the circumference suggests.

A watch whose lug to lug exceeds that flat has to bridge the curve, which is what produces the look of a watch perched on a wrist rather than sitting on it, with daylight visible under the lugs and a strap that will not lie flat.

The working ranges

• Wrist circumference under 16 cm: lug to lug under 46 mm. Case diameter typically 34 to 38.
• 16 to 17 cm: lug to lug up to 48 mm. Case diameter 36 to 40.
• 17 to 18 cm: lug to lug up to 50 mm. Case diameter 38 to 42.
• Over 18 cm: most things work, and it is worth noticing that this is a minority of wrists and the majority of watch photography.

Thickness, the second forgotten number

Thickness decides whether a watch lives under a shirt cuff. Under 10 mm is genuinely thin and will disappear. Ten to 12 is comfortable under most cuffs. Twelve to 14 catches occasionally. Above 14 the watch is an outerwear item, and above 16 it will not pass a buttoned cuff at all.

Automatic movements with a rotor add two to three millimetres over a hand wound equivalent, which is why dress watches are so often hand wound. A domed sapphire crystal adds another millimetre and changes nothing about how the watch behaves except that it now catches on a doorframe.

The watch that never leaves the drawer is almost always a millimetre and a half too thick, not five millimetres too wide.

Lug shape, which changes everything

Straight lugs project the full length outward. Curved or downturned lugs hug the wrist and effectively shorten the span. A watch with the same lug to lug as another can sit entirely differently depending on the curve, which is why the only real test is putting it on, and why online measurements need the curve described as well as the number.

Lug width also decides strap options, and the common widths, 18, 20 and 22 mm, have a vastly larger market than the odd ones. A 19 or 21 mm lug width is a lifetime of limited choice.

The dial, which changes apparent size

Two watches of identical dimensions can read very differently. A wide bezel and a small dial opening make the watch look smaller and more formal. A thin bezel with a large open dial makes the same case look substantially larger. A dark dial recedes and reads smaller, a white or silvered dial advances and reads larger.

Which means the traditional advice, that a small wrist needs a small watch, is only half right. A small wrist needs a short lug to lug, and within that constraint a thin bezel and a light dial can give a presence that a smaller looking watch would not.

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