I wanted to start a thread on this because there have been a few queries on how to do this. I am interested in frame design, geometry and fit and many that follow the sport know that stack and reach makes it easy to compare frames and also determine a suitable frame for a given rider once the rider knows his best fit. But calculating stack and reach is a bit tedious for many who aren't into math and not all geometry charts have these values. Another poster the other day wanted to know his BB drop for example which wasn't posted and yet stack was. BB drop is easy to calculate once you know the math for how to calculate Stack.
So I thought I would start with Stack first. Reach as it turns out is easy to calculate once stack is known and I will add that a bit later.

Because I ride a Specialized Roubaix which btw does publish Stack and Reach and BB drop, I thought I would use that as an example. Then all you have to do is substitute your frame dimensions in for the values I used. Please feel free to ask any questions if its been a while since you made these type of calculations...

A last note. The calculated value for stack varies slightly from Specialized posted value for Stack. Hard to reconcile this. I rechecked the math a couple of times and yet there is a slight disparity in calculated versus published value. I am quite certain the math is correct and so I have to believe the discrepancy is due to a slight measurement error or one of the published metrics maybe a bit off...like fork rake. I did measure Fork crown to axle center which wasn't published and this maybe the underlying reason.

Stack calculation below and hope it helps others:

Thank you!

I haven't read you're whole spiel but, you can't relate BB drop to stack or reach without additional information. Stack and reach are the vertical and horizontal distances between BB and top center of the head tube when the dropouts are level. BB drop is the distance of the BB below a line connecting the dropouts. This is unrelated to stack and reach.

You're welcome. I was curious myself and just decided to do it. Thing is, even if Stack isn't listed on a geometry chart, sometimes fork crown to axle center and/or fork rake aren't listed. Now most road bike forks are in the 375mm range for crown to axle dimension and about 45mm rake...Roubaix fork has a bit more rake to reduce trail with 72.5 deg HTA so just a bit different.
I hope others post their stack calculations based upon the crib sheet I posted.

I was really excited to calculate the Colnago C59 in the 58s geometry, but they don't provide BB drop.

Ummm spiel? I think the only spiel is your post. BB drop IS the portion of Stack below the front and back fork axle centers by definition.
You need a bit more education...lol.

You can contact them of course...or post in a Colnago forum and somebody would know the BB drop so you can calculate stack. You can also measure BB drop if you own the bike.

And lastly, solving for Reach. What is Reach? Reach is the horizontal distance from the center of the BB to the centerline of top of head tube. To determine this horizontal distance, one has to first establish Stack because Reach is measured alone the top of the Stack dimension at the centerline of top of head tube.
If you know STA and Top tube length and have solved for Stack, then Reach is quite easy to calculate as follows:

Here's a calculator, which does the hard work for you.

https://bb2stem.blogspot.com/2011/10/...ator-v100.html

Thanks for linking that calculator.