As a general rule are manufacturer's stated frame sizes measured from the center of
the crank up to the top of the seat tube or up to the center of the top tube?

I think it can vary, but generally I thought it was center of BB to top of top tube.

It depends on which manufacturer. Some go for center to top, and others went with c-c. I d prefer if they all went with c-c. They all do top tube c-c, so why not stick with that...

But then again, I would like to stick with inches OR mm, not both.

...to add a bit of complexity...sometimes (on frames with sloping top tubes) the measurement is to where the top tube ‘would have’ met the seat tube had it been level....

gotta love that needless complexity! Not like we don't already have enough in bicycle measurement "standards", which are anything but a uniform standard. But all true: "it depends"

Since you are in the Classic and Vintage forum, that implies that you are looking at understanding frame sizes made in a former era. In America right after the bike boom of 1970, many of our best frames came from England. Those frames were typically sized in inches and were measured from the center of the bottom bracket to the top of the seat tube. The problem is that "the top of the seat tube" is not a very specific defined point that can be identified clearly on any English made frame. It is usually where the seat lug ends right at the very side/center of the seat tube. A frame builder when he is finish filing the seat tube may scalp it a bit more or less. American bicycles like Schwinn and Trek of the same era followed the British example. For example steel Trek frames from the early 80's were 21", 22 1/2", 24", etc. Chicago made Schwinn frames had the added confusion of measuring to the very top of the end of the seat tube while at the same time the top tube came in quite a bit lower than where it would be on a lugged frame. I just measured a 24" 1973 fillet brazed Schwinn Super Sport. From the center of the BB to the very top of the seat tube (cut straight off and not scalloped) is exactly 24". To the top of the top tube is about 23". That is approximately where a British frame would be measured. The distance from the center of the BB to the center of its level top tube is around 58cm. The schwinn way of measuring threw Americans off about frame sizing back then because they might think they needed a bigger frame than what they really needed.

The Italian way to measure a frame was from the center of the BB to the center of a level top tube. This is the way that makes the most sense to a frame builder (like myself) because it is to an exact point that can be precisely replicated in another frame. Most other European frames that are described in centimeters are measured this same way. However there are exceptions like my Italian Masi I got in Italy in 1972. Faliero described his frames in centimeters (and he punched that number on the BB shell) and he measured them center to the very top. My frame is stamped a 58 cm model and he measured to the very tip of the seat lug. When I put it on my fixture and could read exactly what the c-c measurement was, it measured 55.9cm.

Modern sloping top tubes follow different rules and distances in a seat or top tube can be described as "actual" or "effective".

Dave Moulton has a take on the British way he was taught that I always thought actually demystifies this a bit.

Dave Moulton's Blog - Dave Moulton's Bike Blog - Measuring Frames

Yes.

Same thing with tires. Not all 700X 25 tires are the same size.

The Italian way, as described by Doug, is what makes the most sense to me. I guess it is my engineering background. I like to know the reference point and it should be reasonably stable! Hard to know what you are getting without the ctc or ctt or what ever reference, not to mention the inaccuracies as a result of people who don't know how to measure.

Dave's rule only works as an over simplification guide for a casual inquiry. I'm sure he did it as a way to easily explain C - T measurements without getting into the weeds of where a non-existent point is really located using a nice round number. It's like saying pie (π) is a single number of 3 (instead of 3.1416 to infinity). Where he has indicated the line of measurement is on a frame is not where most British (or 70's Americans) measured to. It is about where a Masi is measured. In fact everybody in the UK seemed to use a slightly different point. For example the 22" Hetchins I had made for my wife in 1973 is more like 21 3/4". The fixture we used at Ellis Briggs in Yorkshire where I learned had preset holes for each 1/2" so we never had to measure the frames themselves to establish seat tube length. It certainly wasn't where Moulton drew his line.

Where I decided the center to top measuring point should be way back when I did touring or sport touring frames for recreational cyclists (because back then that is how everyone that wasn't a racer understood frame size) was to the crease at the top of the top tube right where it touches the seat tube. This was a fixed and exact spot on every frame. In the picture below, my point of reference is the red line for center to top and green line is for center to center. The difference in distance between the two lines is around 13mm for a 1" top tube and 15mm on a 28.6 top tube.

Doug Fattic, Love your thorough, but always clear and understandable responses! Thank you for being a frequent contributor here.