I'm encountering some confusion on frame size and any clarification would be greatly appreciated. I've always been under the impression that a stated frame size was the distance, usually in cm, from the top of the seat tube to the center of the BB opening. For example looking at any of the road bikes on the Trek site:


For all the bikes the "Frame Size Number" and the "Actual Frame Size" (see the chart at the bottom of the page for any of the bikes) are the same, even for the aero Madone.

I'd like to purchase a Dengfu FM098 frame for a bike I'm planning on building:


The available sizes are listed as "49/52/54/56/58/61cm", but upon going to the Geometry tab this chart is shown:

enter image description here

It would appear that the first column (which is oddly untitled) would correspond to the available size, but dimension "A" is different than the frame size on each line!

At first I figured this was a mistake or an oddity for this particular frame, however the Dengfu R02 (choosing the "Geometry" tab again):


and the Hongfu FM098:


Have the same frame size number vs actual frame size difference.

Can somebody clarify this? Is this standard in the industry for aero frames or frames with a curved or non-flat top tube? Is the idea that I should know what size I should be in a flat top tube frame, for example 52cm, then purchase the frame with the name "52cm" even though it is not actually 52cm? Please advise, thanks!

  • 3
    There have always been two or three different "standards" for how this is measured, and it got even more confusing when the traditional diamond frame with a horizontal top tube was tossed out in favor of a slant tube. Used to be you could select a frame sight-unseen and be able to assume that at least it would fit your leg length, but that's no longer true. Jan 26, 2017 at 2:27
  • Well, the harder part to get correct now is the top tube length rather than the standover. Your best bet is to find a similar frame and ride it, or if not, plug the frame measurements in along with your measurements into a bike sizing system (e.g. FitKit; most decent bike shops will have something of the sort).
    – Batman
    Jan 26, 2017 at 3:14
  • 2
    The size in the left column and also column B are the size the frame would be if it was a traditional diamond frame.
    – andy256
    Jan 26, 2017 at 5:05
  • The unlabeled leftmost column appears to the frame size for that model, but in mm rather than in cm. Because of sloping or curved top tubes rather than horizontal ones, seat tube length is no longer a reliable way to measure frames. Dimensions D and C are the stack and reach, which are independent of tube shape. Many manufacturers will now include stack and reach in their geometry tables. Here is a decade-old article by the guy who proposed these measures: slowtwitch.com/Bike_Fit/Choosing_a_Tri_Bike_via_Stack_and_Reach/…
    – R. Chung
    Jan 26, 2017 at 14:22
  • 1
    If you already have a bike that fits, you can measure its stack and reach with a tape measure and a plumb bob. Stick the rear wheel of the bike into a corner, then drop a plumb bob from the BB and the top of the head tube to the floor. Mark those spots with a piece of tape, make an X on them, then measure the heights of the BB and to the top of the head tube along the plumb line. The difference between them is the stack. The difference between the X's along the floor is the reach. Look at the chart and find the frame that's closest to your existing bike.
    – R. Chung
    Feb 4, 2017 at 1:01

2 Answers 2


The answer is the top tube is not straight but curves down at the seat stem.

The idea behind the difference is that somebody who need a 49cm size frame would have the seat adjusted to their leg length and somebody who as a slightly smaller leg length can't lower the seat to suit. So you have got 10 mm up and 10 mm down otherwise your seat would be set at the top of the seat tube all the time. Same on the next size if you bought a 49cm frame but your leg length is that bit longer you buy the next size up and lower the seat to the top of the seat tub.

You have a 49cm frame with +/-10mm, and the next size frame is a 51cm with plus and minus 10mm. Plus you can work out your crank length from 165 mm to 175 mm to suit your leg length.

So this mean you have some 40 mm to play with in various leg lengths. And the same with your cage length. Or top tube as some people say. If you've got short arms you use a small handle bar stem or long arms a longer handlebar stem.

I hope this may give some of you some better idea and thought to buying your frame. Measure your leg length and subtract 30mm from that and work from there.

I know this as a fact as I design cranks for a living. Hope you have something of a better idea now.



The column unlabeled on the left is that make and models advertised size. It is in mm some measure in cm (56cm, 58cm, 60cm) as in the USA. Measured in Centimeters (CM) or Millimeters (mm)The 1st unlabeled column is THEIR size (or what they call it) and usually corresponds to what they feel it most closely measures in terms of traditional frame sizing from years ago. Column 'A' is the actual seat tube and can be used as a reference point if you are between sizes and need to to figure if you should go up .5cm or down .5cm.and may help with extra data in helping determining crank length and seat post length needed (not positioning) along with 'D' (Stack) which would be similar to a traditional size number if the top tube was still horizontally level and because of this are notably very close to the first column numbers. One of the most important numbers is 'C' (Reach), as this will determine your stem length, comfort in terms or being stretched out and effect handling. I personally tend to consider 'B' as well since seat tube angles can vary from frame to frame and even though 'Reach' numbers can be the some on two different frames, with a different 'B' (Effective Top Tube Length) length they can end up feeling very different.

My best advice since you can not test ride this frame (unless you know someone that has one in your size locally) is to go to a sizing and fit website. One of the better ones that I know of is http://www.wrenchscience.com/ Create a login and go through that 'fit system' (its free) and it will ask you all kinds of question. You will need a tape measure among other things.

Yo may want to visit this information from a guy I trust that knows an awful lot about bicycles.

Articles about Bicycle Frames and Fitting

Revisionist Theory of Bicycle Sizing

If so inclined, here is some additional excellent reading on bike fit

The K.O.P.S. (Knee Over Pedal Spindle)method of bike fit

The Myth of K.O.P.S.

Debunking the Myths by Klein Bikes

How to Fit a Bicycle

The why they make frame measurement charts the way they do these day and how the industry has changed through history could easily take dozens of paragraphs. In short. When bike frames were primarily alloy tubes, they could be cut to whatever length imaginable. Typically varied 1CM or .34 inches and some manufacturers had half sizes that were differences of .5 cm or .197 inches. Top tube used to be almost always horizontally level and since there was no real standard, some manufacturers measured from the center of the crank to the top of the seat tube while others measured from the bottom of the bottom bracket to the top of the seat tube. The most widely adopted as anything close to a standard was measuring from the center of the bottom bracket to the center of the top tube.

Today more than 70% of bike frames manufactured are carbon fiber like the one you referenced. There are not as many frame sizes manufactured since CF molds are very expensive. Thus you do not see half sizes and some manufacturers even mold frame size offerings in 2cm increments. Nevertheless, there are far better ways to customize a fit and get it dialed in close than in days gone by. For instance there are every imaginable size of seat post, neck and crank length to help with bike fit and sizing.


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