How to Read a Bike Frame Size Chart (and What It Leaves Out)

What the chart maps, and why two charts disagree

A manufacturer chart takes a rider dimension, usually standing height and sometimes inside leg, and returns the label the maker prints on that frame. The label itself has no fixed meaning. On one model it refers to a seat tube length in centimetres; on another it is a letter in a run from the smallest to the largest frame the maker builds; on a third it is a number in a sequence with no stated unit at all. None of those is wrong, and none of them is comparable with the others.

Manufacturers are quite open about this when you look at how their own tools work. Canyon's sizing and geometry service pages run a size finder that asks for the rider's height, the rider's inside leg measurement and the rider's current saddle height, and then returns a recommendation for the specific model being viewed rather than a universal size. That structure is the honest version of a chart: a model-by-model recommendation built from more than one body measurement. If a chart you are reading asks only for height and claims to cover every bike, it is doing less work than that.

The one seat-height figure federal regulation does define

United States regulation for bicycles sold here is set out in 16 CFR part 1512, and it contains exactly one seat-height number with legal force. A sidewalk bicycle is defined as a bicycle with a seat height of no more than 635 mm (25.0 in), with the seat height measured with the seat adjusted to its highest position. Recumbent bicycles are excluded from that definition.

The regulation also defines how that measurement is taken, which is worth knowing because it is the same measurement your own saddle height uses: the dimension from the point on the seat surface intersected by the seat post centre line, or the centre of the seating area where there is no seat post, down to the ground plane, with the wheels aligned and in a plane normal to the ground. The purpose of the 635 mm boundary is to separate children's machines from adult bicycles for the purposes of the braking, strength and reflector requirements that follow. It is not a fit chart and it will not tell you which frame suits you. But it does mean one thing is unambiguous: if a bike cannot reach a saddle height above that figure, it was built as a sidewalk bicycle and is regulated as one.

Compare the geometry numbers, not the labels

Every manufacturer publishes a geometry table alongside the size chart, and the figures in it are defined the same way everywhere, which makes them the only part of the page that compares across brands. Canyon's geometry concept pages exist precisely to explain what those figures describe. Two of them carry most of the weight. The vertical distance from the bottom bracket to the top of the head tube describes how tall the front of the bike sits; the horizontal distance from the bottom bracket to the same point describes how long the bike is before any adjustment is made.

That pairing matters because it is adjustment-independent. Saddle height, saddle fore-and-aft position, stem length and bar rotation all move afterwards, and all of them can hide a frame that is the wrong length by making you sit badly on it. The frame's own vertical and horizontal front-end figures cannot be adjusted at all. If you have a bike that fits, those two numbers from its geometry table are the single most transferable piece of information you own when shopping for the next one.

Two adjustment limits are fixed by regulation, not by preference

Riders who land between two sizes usually plan to close the gap with the seat post and the stem. Both have hard stops written into 16 CFR part 1512, and both are checkable in a shop in under a minute. The seat post requirement is the one people meet first: the post has to carry a permanent mark or ring that clearly indicates the minimum insertion depth, which is the maximum seat-height adjustment, and that mark must be located no less than two seat-post diameters from the lowest point on the post shaft, with the post's strength maintained for at least a length of one shaft diameter below the mark. Bikes with integrated seat masts have to indicate safe installation some other way. Pulling the post past the mark is not a matter of judgement; it is outside the condition the frame and post were tested in.

The second stop sits at the front. Handlebar ends may be no more than 406 mm (16 in) above the seat surface, measured with the seat in its lowest position and the handlebar ends in their highest, with recumbents excluded. There is a third, smaller figure that catches aftermarket saddle accessories: no part of the seat, its supports, or anything attached to it may sit more than 125 mm (5.0 in) above the top of the seat surface at the point where the seat post axis intersects it. Together these three numbers tell you how much of a size gap is actually closeable before you are riding a bike outside its tested configuration.

When the chart returns two answers

Charts commonly place a rider in an overlap, and the overlap is real rather than a flaw: two frame lengths can both be ridden, with different compromises. The way to resolve it is to stop reading the chart and start reading the geometry table. Work out which of the two frames lets your known saddle height sit comfortably inside the seat post's marked insertion range, then look at the horizontal front-end figure to see which one puts the bars at a distance you already know you can hold.

Two further considerations decide most remaining cases. Standover clearance is set by seat tube length and the shape of the top tube, and it is the one dimension that cannot be recovered by any adjustment once the bike is yours. And the direction of your intended adjustment matters: a frame at the small end of your range leaves you raising the post and lengthening the stem, both of which have limits, while a frame at the large end leaves you dropping the post and shortening the stem, which runs out of room in a different way. Neither direction is universally preferable. Knowing which way you will be adjusting before you order is what stops a second purchase.

• Measure your height without shoes, against a wall, and your inside leg to the floor the same way.
• Read the chart for that specific model only, never a chart from a different brand.
• Pull the geometry table for both candidate sizes and compare the vertical and horizontal front-end figures.
• Check your intended saddle height against the seat post's minimum insertion mark on both frames.
• Check standover clearance last, because no adjustment recovers it.

What to confirm before you commit

A frame size chart earns its place as a filter that removes most of the catalogue in one step. It cannot do more than that, because the label it returns is defined by the maker who printed it and by nobody else. The parts of the question that are fixed sit in two places: the geometry table, which is comparable between brands because the measurements are defined the same way, and the federal regulation, which fixes a small number of limits on how far the adjustable parts may be moved.

If you take one habit from this, make it the habit of writing down the geometry figures of any bike you have ridden and liked. Those numbers transfer; a size label does not. And if a chart and a geometry table disagree about which size suits you, the geometry table is the one describing the object you are buying.

Before you buy

• Take height and inside leg measurements without shoes and write both down.
• Use only the chart published for the exact model you are considering.
• Open the geometry table and record seat tube length plus the vertical and horizontal front-end figures.
• Compare those figures against a bike you already ride comfortably.
• Confirm your saddle height sits inside the seat post's marked insertion range.
• Check handlebar height against the 406 mm (16 in) ceiling above the seat surface.
• Check standover clearance on the smaller candidate before ordering.

Questions readers ask

Can I use one brand's bike frame size chart to buy another brand?

No. The label a chart returns is defined by the manufacturer that printed it, so a letter or number from one brand carries no information about another. The transferable figures are in the geometry table, where the measurements are defined the same way everywhere.

Is there a legal definition of bicycle frame size in the United States?

Not of frame size. 16 CFR part 1512 defines seat height and uses it to separate a sidewalk bicycle, at no more than 635 mm (25.0 in) with the seat at its highest position, from an adult bicycle. Frame labelling is left entirely to the maker.

How far can I raise the seat post to make a small frame work?

Only to the permanent minimum insertion mark the regulation requires on the post, which must sit no less than two seat-post diameters from the lowest point of the shaft. Above that mark the post is outside the configuration it was tested in.

Is there a limit on how high I can set the handlebars?

Yes. Handlebar ends may be no more than 406 mm (16 in) above the seat surface, measured with the seat at its lowest position and the bar ends at their highest, with recumbent bicycles excluded.

Which matters more, the chart or the geometry table?

The geometry table, once you have used the chart to narrow the field. The chart tells you what the maker calls the frame; the table tells you the dimensions of the frame itself.

Sources

1. 16 CFR part 1512 — Requirements for bicycles (eCFR) — checked 2026-11-06
2. 16 CFR part 1512 full regulatory text (GovInfo) — checked 2026-11-06
3. Canyon — Sizing and geometry service pages — checked 2026-11-06
4. Canyon — Geometry concepts explained — checked 2026-11-06