From what point to where do I start measuring? I suspect the bike I'm trying to measure is a 56cm because I usually ride a 54 and this is slightly too big, but I want to be sure.
do you mean measure a frame of unknown dimensions for its specified height, or how to measure based on comfort (ie how to know which size frame you want when shopping)?– mfgAug 25, 2010 at 19:43
The specified height of a frame. I already bought it unfortunately and am now very aware that it's not very comfortable. Now I'm going to try and sell it, but I figure I should specify the size of it.– AndrewAug 25, 2010 at 19:48
This guide will help you measure your bike correctly. ebicycles.com/article/how-do-i-measure-a-bike-frame.html– Mr.LukashJan 31, 2017 at 9:49
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Centerline of the bottom bracket hole to the top of the seat tube (where the seat post goes in).
2Unfortunately, that depends on brand. What brand and model of bike do you have? You said it is a 56cm? Where did you get that number.– zenbikeApr 18, 2012 at 8:02
This only works on bikes with level top tubes Jan 31, 2017 at 10:37
There are a few more tricks that you need to be aware of. The most important is that a lot of modern bike frames, particularly mountain bikes, have a sloping top tube. If you use the method described by @kkeilman you're size measurement will end up being too small. The trick in this instance is to get an assistant to hold a horizontal reference such as a yardstick, metre rule or broomstick aligned with a spirit level (or by eye) at the level of the top tube/head tube junction. You can then measure the size from the centre of the bottom bracket to this reference line, keeping the measuring tape in line with the seat tube.
My understanding is that most US mountain/road bikes are measured from the center of the bottom bracket shell to the top of the seat tube, European road bikes are center of the bottom bracket shell to the center of the intersection of the top tube and seat tube, and BMX bikes are from the center of the top tube intersections between the seat and head tube.
- US Road/MTB: Center to Top (C-T)
- Euro Road: Center to Center (C-C)
- BMX: Top Tube (TT)
This page backs that understanding up, with pictures.
1Some frames are also sized as where the intersection of the top tube and seat tube would have been if the top tube was horizontal. sheldonbrown.com/frame-sizing.html– armbOct 18, 2012 at 10:52