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My mathematical Watch Sizing guide!

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wrist size
125K views 215 replies 151 participants last post by  Spiffy 
#1 · (Edited)
I'm sure that some mathematician has worked this out before but I worked out the following ratio system for assessing the appropriate size for a watch on a wrist.

Its pretty simple

1/ Convert wrist size into mm

2/ Divide total wrist size in mm by the size of the watch

eg A 7 inch wrist and a 38 mm watch = 178mm divided by 38mm= Ratio of 4.7


This brings me to the controversial part. What is an acceptable ratio? I think using this formula, that an acceptable ratio falls between 4.0 and 5.0. Anything below 4.0 is too big and anything above 5 is too small.

Here is an example using a 44mm watch which should be too big for some with smaller wrists

6.5 inch ( 165 mm) Ratio=3.75 Too big
7.0 inch (178mm) Ratio=4.0 Just ok- acceptable
8.0 inch (203 mm) Ratio=4.6 Great fit


This brings me to interesting observation that wearing a 39 mm watch on a 7 inch wrist has a a very similar Ratio as wearing a 44mm watch on an 8 inch wrist. That is, a ratio of about 4.6.

For reference, here are the conversions for wrist size
6 inch = 152.5 mm
6.5inch = 165mm
7inch = 178mm
7.5inch= 190.5mm
8.0inch = 203mm
8.5 inch = 216mm

I realize I am opening myself up to ridicule for being too analytical but I think it's a pretty useful way to work out a rough guide and obviously does not take into account lug shape etc. What do you think? Has someone worked this out before? Have I got too much time on my hands? Can this concept be improved on?

Cam
 
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#5 ·
Interesting...

I like it!

I did it for my watches and there may be some truth to it.

My wrist size: 7.25 inches = 185 mm

Ratios:
Seiko Monster - 4.40
Citizen BN0000-04H - 4.51
Bulova Precisionist Claremont - 4.20
Casio G-Shock Mudman - 4.02
 
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#6 · (Edited)
Makes sense to me. Let's assume the human wrist is approximately an ellipse with major axis say twice the minor axis (ie twice as wide as thick.)Then assume that the watch diameter is approx the same as the width of the wrist (major axis). Then if you work out the circumference of the ellipse you'll find that the ratio of circumference to major axis is approx. 5. You'd probably want the ratio to be less than that as you don't want the watch to overhang your wrist.
 
#97 · (Edited)
With this premise, the ratio should be about 1.58*pi = 4.96... but as you have noted, you probably want the ratio to be a bit smaller so that the watch doesn't overhang, so probably about 4.5 is ideal.
 
#10 ·
I like it.

A 4.5 ratio on my 7 1/8 inch wrists puts me at a 40-41mm watch which is exactly what I prefer.

Furthermore your upper and lower range would equate to 36mm to 45mm, which would be extremes for me.

Well done.
i was just in the shower where i do all my best thinking and it dawned on me that dividing the total wrist by the ratios 4 and 5 gives you a minimum and maximum ! You just beat me to pointing that out!

eg 7 inch (178mm) gives me a minimum of 35.6mm and a maximum of 44.5 mm
 
#12 ·
No No No No!!! It's Length times diameter plus weight over girth............
 
#14 ·
I think you can impose certain conditions to account for various watch styles.

1. Dive watches should use bezel diameter should be used instead of case diameter. Some divers have tapered cases so a 44mm Oris Diver wears like a 40mm due to the 40mm bezel.
2. Cushion cased watches should use vertical case length rather than the formal 4 to 10 measurement for case diameter.
 
#24 · (Edited)
Lug to lug length also pays a crucial role, some watches have long lugs and tend to hang or occupy the whole wrist without necessarily being attached to a large diameter case. A refined algorithm should take into the account even the angle of the lugs.

Also, as a European and software developer thinking with an analytical mind most of the time, I chuckled a bit when I saw this thread:-d
 
#26 ·
Lug to lug length also pays a crucial role, some watches have long lugs and tend to hang or occupy the whole wrist without necessarily being attached to a large diameter case. A refined algorithm should take into the account even the angle of the lugs.

Also, as a European and software developer thinking with an analytical mind most of the time, I chuckled a bit when I saw this thread:-d
You are entering territory that is out of my league now!
 
#33 ·
For weird shaped watches, I propose that we calculate the area of the watch case and find the circle that has the same area.

A = pi*r^2
A = pi*(d/2)^2
A = pi*d^2/4

d^2 = 4*A/pi
d = sqrt(4*A/pi)

So a 40mm x 40mm square watch would be the equivalent of a circular watch with diameter:

d = sqrt(4*(40*40)/pi)
d = 45mm

I realise that the area of a Panerai watch is very hard to calculate.
 
#38 ·
I'm wearing a 44mm watch right now with only a 6.5" wrist. That gives me a ratio of 3.75, which should be too big according to your theory. I'd say the watch just about fits (no hanging lugs either). Judge for yourself.
(slightly blurry pic)

View attachment 523876
No problem, Im not saying you shouldn't have it. the fact that you just said it "just about fits" sits nicely with the system to me. Perhaps large pilots watches are supposed to be on the larger side of things. Having said that i just tried on an IWC pilot on a 7 inch wrist at about 40mm and it fitted like a glove to me.
 
#36 ·
Your formula is too simple and is missing several important criteria including: shape of lugs, lug-to-lug distance, height of the watch, how the strap/bracelet fits, shape of the case(round, square, rectangle, how the individual likes the look of the watch, etc.
 
#39 ·
Works well for me and I think it's a great formula for use as a general guide. For watches that come close to the upper or lower end of your range you'd need to consider the other factors such as lug to lug length, dial diameter, case shape and thickness etc. Perhaps some of these variables could be included in the equation (I'm no mathemetician!) but provided you have at least some experience in wearing different types of watches I think you can make your own conclusions regarding their influences.
 
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