It’s important to get a frame that fits you, so you can control the bike properly. This page explains a common method for estimating which frame size a cyclist needs for day to day cycling on roads and paths. It then explains which part of a bike is normally measured when stating frame size, before considering the important, but often overlooked effect of top tube length.
What size frame do I need?
An adult’s frame size requirement is determined by his/her inseam (crotch to ground, without shoes). The method recommended by bike maker Dawes, is to measure the cyclist’s inseam and multiply that figure by 0.69 (for a bike with a horizontal top tube) or 0.64 (for a sloping top tube).
For a method using cyclist height rather than inseam, there are frame size tables at the Wiggle site. Scroll to the bottom of that page to see them. The relevant table for transport cycling is likely to be the one dealing with hybrids, even if the bike in question is a drop bar road bike. (The road bike table gives a rather large frame size, and appears to assume you want a stretched out riding position for competitive road cycling).
Of these two methods, the inseam method (in bold) is likely to be more accurate. In any case, this kind of calculation should only ever be a starting point; it’s always best to try for size before buying.
When deciding whether to buy a bike or a frame online, find a bike of the same size and try it for size.
How big is too big?
Which part of the bike do I measure?
The dimension usually given is the length of the seat tube and it can be measured either centre to centre (which can be written as ‘c-c’) or centre to top (c-t). Frame size may be stated in inches or centimetres.
Another important dimension is the length of the top tube. Since low speed manoeuvrability is important in utility cycling, it’s best to choose a frame long enough to avoid toe overlap. (Toe overlap is where the front of the cyclist’s shoe can touch the tyre or mudguard.) A longer top tube will give a longer wheelbase and reduce the likelihood of toe overlap, all else being equal.
Some frames designed for women have a shorter top tube for a given seat tube length. This is based on the theory that women have shorter bodies (and longer legs) than men of the same height.
Frames designed for drop bars often have shorter top tubes than frames designed for flat bars. This is to compensate for the fact that drop bars require the rider to reach forward more.
Sheldon Brown wrote in detail about frame sizing and top tube length at his site.
Effective top tube length
To allow sizing comparisons for frames without a horizontal top tube, some manufacturers refer to ‘effective top tube’, which is an imaginary horizontal line between the head tube and the seat post.
This page has explained how to estimate required frame size based on inseam, how to measure a frame, why it’s good to know top tube length and how to measure effective top tube. To learn more about bike frames, click one of the links below.