Choosing A Quill Stem Length

I've always chosen my stem length by trial and error until it felt right. But I'm building up a new (to me) frame and wondering if there's a more scientific approach. What measurements of my body can help me pick the ideal stem length?

 

There are several free bike fit calculators available to help with that.
Just do a net search for "bike fit calculator".
Competitivecyclist.com and wrenchscience.com are two decent ones.
But I've always found it more successful to measure a bike that I'm comfortable on, and then see what type of stem I'd need to recreate that position on a new bike.

 

110 - 120cm, if the frame is the right size for you. Tough to calculate that without a ton of information about your fit & exacting geometry of the frame.

 

There are no body measurements you can take to help decide what stem length to buy.
Best option is a professional fit on a bike, which is then easy to replicate on others. If this isn't an option, then you'll need to take several measurements off an existing bike that 'feels' right to you.
In my experience, the best way to ensure near identical fit on different bikes with different tube lengths and angles, is to use the bb as the reference point. This is the one point that everything else depends on, as it can't move, while the other contact points can. You need to ensure that your leg angles will be equal on any bike you ride. Once that is set correctly, then you can change stem lengths/height, handlebar reach/drop etc. to ensure you can get your back angle as close as possible to that on the 'feel-good' bike.
I regularly ride 5 different bikes, and all feel 99% identical despite ranging in sizes and geometries.

 

My rule of thumb: ride the bike, hands on hoods, look at bar tops, does it appear in line with front axle, if yes then stem length is roughly correct. This is for a drop bar road bike. Then get in drops and crouch with forearms horizontal, do knees come roughly to elbows, if yes then stem length is roughly correct. You could fine tune from here, especially if you feel strain or pain.

 

I make sure my saddle setback is adjusted so I feel in pretty good balance over the BB, with low hand pressure. Then I ride mostly with hands on hoods. If my hands tend to move back towards to the upper ramps as the ride goes on, I need either a shorter bar reach or shorter stem. I also try to make sure my arms and shoulders are relaxed when in the hooks, with a straight back. If I can't achieve that relaxation, I need more reach. I don't use any criteria like "10 cm is too short."

 

Sometimes you gotta buy a bunch of stems and try each one