Bike sizing
What Size Bike Should I Buy?
Start with your height and the manufacturer’s size chart.
Then check the actual geometry. A Medium from one brand is not necessarily the same size as a Medium from another.
The short version
Start here
Height + the manufacturer’s size chart for that exact model.
Compare
Reach, effective top tube and stack, taken from the geometry chart.
Confirm
Test ride, or compare with a bike that already fits you.
There is no single height-to-frame-size table that works for every brand. A 56 cm sticker tells you less than you might think. Sizing varies between brands, and it varies again between a race bike, an endurance bike and a trail bike wearing the same letter on the tube.
Why frame-size labels are confusing
S, M and L are clothing sizes wearing a bicycle. They differ between brands, and sometimes between models from the same brand. Centimetre labels are more precise, but a 56 cm endurance bike is still not a 56 cm race bike.
Geometry also changes between generations. Last year’s Medium can be a different length from this year’s. If you are buying used, look up the geometry for that model year, not the current chart, unless you have checked they match.
Do not buy a 56 cm bike simply because your current bike also says 56.
Used listings also mis-state size. If the seller is unsure, ask them to measure the effective top tube, or send the serial and a photo of the size sticker on the frame.
The geometry numbers worth understanding
You do not need a full geometry education. Four numbers cover most used-bike comparisons.
Reach
How long the frame feels horizontally. Particularly useful when comparing bikes, especially mountain bikes.
Stack
How tall the front of the frame is. Higher stack usually means a more upright starting position.
Effective top tube
A useful indication of seated bike length.
Standover
Useful for clearance on some bikes. Not enough to determine fit by itself.
Road and gravel bikes
On road and gravel bikes, position matters as well as leg length. Endurance and race geometries can feel different at nominally similar sizes — one puts you longer and lower, the other shorter and taller.
Stem length, spacer stack and saddle position fine-tune a sensible frame size. They cannot always rescue the wrong frame. If your hands feel miles from the bars, or your knees hit the wheel, look at another size rather than ordering a longer stem.
Gravel bikes add tyre clearance and often a slightly taller front end than a race road bike in the same labelled size. Compare the numbers for the actual model. Do not assume a 56 cm gravel bike replaces a 56 cm road bike you already like.
Mountain bikes
Modern mountain-bike fit relies heavily on reach and effective top tube, plus how you actually ride. Seat-tube length still matters for dropper-post insertion, but it is a poor way to compare trail bikes across brands.
A longer reach usually means a longer wheelbase and more stability at speed. A shorter reach is often easier to move around. That is a preference, not a rule. Use the manufacturer’s geometry chart for the bike you are buying.
Standover still matters if you want clearance when you dab a foot, but it is a poor primary size metric on modern long-and-low trail bikes. Reach and effective top tube will tell you more.
Between two sizes?
Height charts often put people on a boundary. That is normal. Do not treat “always size down” as a law.
Consider the smaller size if
- the manufacturer’s chart still places you in range
- you prefer a more compact, easier-to-move bike
- a test ride in the larger size feels stretched
Consider the larger size if
- the geometry supports a longer cockpit
- you have a longer torso or arms for your height
- the smaller size feels cramped even after saddle setup
When you are between sizes, compare the actual geometry rather than treating the size label as the answer. If you already have a bike that fits, compare reach and effective top tube with that bike rather than with a generic chart.
The bargain-bike trap
Do not persuade yourself that the wrong size will work because the price is good. A used road bike in the wrong size is still the wrong bike, however nice the groupset looks.
Sources & further reading
- REI — bike fit basics
- REI — mountain bike fit
- Trek — what size mountain bike — reach and effective top tube as comparison tools