Wrists and Proportions
Lot of people don't quite realize this, but watch sizes should be proportional, and here's a nice illustration from Gentlemen's Gazette,
Ideal Dial and Lug Sizes
The Slender Wrist's excellent article, take this further with a reference table.
What I've done here is more a summary for myself and my table is a little different because my Excel formulas does not round and I've also considered ideal strap sizes,
Watch Case Size
Lug To Lug Distance
|Flat of Wrist*||Strap Size (common)|
|5.25 "||13.335 cm||26.67 - 33.3375 mm||26.67 - 42.2275 mm||44.45 mm||65/110 or 70/105|
|5.50 ”||13.97 cm||27.9 - 34.925 mm||34.925 - 44.23833333 mm||46.56666667 mm||65/110 or 70/105|
|5.75 ”||14.605 cm||29.21 - 36.5125 mm||36.5125 - 46.24916667 mm||48.68333333 mm||70/110 (my own preferred custom size)|
70/115 (works well too)
|6.00 ”||15.2 cm||30.48 - 38.1 mm||38.1 - 48.26 mm||50.8 mm||70/115|
|6.25 ”||15.9 cm||31.75 - 39.6875 mm||39.6875 - 20.27083333 mm||52.91666667 mm||70/115|
|6.50"||16.51 cm||33.02 - 41.275 mm||41.275 - 52.28166667||55.03333333||70/115|
|6.75"||17.145||34.29 - 42.8652 mm||42.8625 - 54.2925||57.15||75/120|
|7.00"||17.78||35.56 - 44.0375 mm||44.45 - 56.30333333||59.26666667||80/125|
|7.25"||18.415||36.83 - 46.0375 mm||46.0375 - 58.31416667||61.38333333||80/125|
|7.5"||19.05||38.1 - 47.625 mm||47.625 - 60.325||63.5||80/125|
The formula estimates the flat of your wrist. In some cases, you may actually have a smaller or larger "flat of the wrist" than the formula and should adjust accordingly.
We use the preferred ratio case diameter of 60% to 75% of the size of the flat of wrist. Rough estimate of flat of wrist is your wrist size (in mm) and divide it by 3.
Ideal lug to lug distance is 75% to 95% of wrist width.
Women may consider (if going more feminine) more tapered straps and bracelets.
I prefer quick release straps (to put reference and picture below) so you can change your straps without tools.
Also, consider most bracelets wear bigger than straps.
Again the Slender Wrist article, "The Ultimate Watch Size Guide: Factors Influencing The Size Of A Watch" has a comprehensive discussion and these are my short notes,
Colour - white looks bigger and black looks smaller) and lug to lug distance also plays an important role here,
Bezels and Hour Marker Circle - The watch design also visually impacts the eye to see a watch smaller than it is,
The Slender Wrist - "The Ultimate Watch Size Guide: Factors Influencing The Size Of A Watch"
Martu - Chart for Custom Watch Sizes