What size bike do I need: measure your inseam, then choose between two sizes
Sizing is a fifteen-minute job with a tape measure and a hardback book. It ends with two candidate sizes, not one, and the second half of this guide is how to choose between them.
Height is the wrong number to start from
Almost every size chart asks for your height first. Height is a weak predictor of frame size, because it is two measurements added together — legs and torso — and a bicycle cares about them separately. Two riders who both stand 175 cm can have inseams several centimetres apart. One of them needs a frame the other cannot stand over.
The inseam is the number that decides. It sets how much seatpost you need to reach a correct saddle height, and whether you can stand over the frame. No part rescues a mistake here: a seatpost that will not go low enough is a permanent fault, and so is a top tube you cannot clear at a traffic light.
So: measure your inseam, use a chart to get two candidate sizes, then use reach and stack to pick one. Once the frame is right, saddle height and the rest of the setup is adjustment rather than compromise.
Measuring your inseam properly
You need a hardback book, a tape measure, a pencil and a wall you are allowed to mark. A second person helps but is not required. Do not use the inside leg from a pair of trousers: that is a garment measurement, cut short of the anatomical crotch height, and it will undersize you.
Stand barefoot against a bare wall
Heels, bottom and shoulders touching it, feet about 15 cm apart, weight even on both legs. Shoes add height and thick carpet swallows it, so work on a hard floor.
Put the book between your legs, spine up
Hold it level and slide it up firmly until it stops — firm enough to mimic sitting on a saddle, not so hard that you lift yourself off the floor. The spine should be flat against the wall and horizontal.
Mark the top of the spine on the wall
Keep the book level while you do it. If someone is helping, let them make the mark while you hold the book square.
Measure from the floor to the mark
Straight down, tape held against the wall so it cannot bow. Record it to the nearest half centimetre. That is your inseam.
Do it twice more
Step away and repeat the whole thing. Attempts vary by a centimetre or more depending on how hard you pull the book up. Average the three.
Write down your height while you are there
Barefoot, same wall, book flat on your head. You want both numbers, because a chart that disagrees with your inseam is telling you something about your proportions.
Inseam high for your height means long legs and a short torso: usually the smaller of two candidate frames, with the saddle high. Inseam low for your height means a long torso, and the larger frame with a shorter stem is more often right. That comparison explains most of the cases where a chart sends somebody badly wrong.
What the number on the frame means
Road frames are sold in centimetres. The number is the seat tube length, and it described a frame honestly when top tubes were horizontal. Modern frames slope, so the same rider fits a 56 from one brand and a 54 from another with no contradiction. Some brands measure the seat tube centre-to-centre and some centre-to-top, which moves the number again.
Mountain bikes, and most gravel and hybrid bikes, use S, M and L. That is more honest — it admits the label is a bucket rather than a measurement — but no more comparable: two mediums from different brands can be 20 mm apart in reach, which is the difference between a comfortable position and a stretched one.
This is real disagreement between manufacturers, not something you are failing to understand, and no standard is coming to settle it. The rule that follows: never cross-shop size labels. Cross-shop the geometry numbers underneath them.
Reach and stack, the numbers that compare
Modern geometry tables carry reach and stack, and they are why comparing frames across brands is possible at all. Both are measured from the centre of the bottom bracket to the top centre of the head tube: reach is the horizontal distance, stack the vertical one. Unlike seat tube length, they do not change meaning when the top tube slopes.
More stack means a taller front end and a more upright back. More reach means a longer, lower, more stretched position. An endurance road bike such as the Canyon Endurace 7 at €1,899 is a road frame with more stack and a little less reach than a race frame of the same nominal size. That is the whole difference, expressed in millimetres.
If you own a bike that fits, write down its reach and stack and treat them as your target. Ten millimetres of reach is about one step in stem length — stems are commonly sold from 70 to 130 mm in 10 mm increments — so a frame within 10 mm of your target is adjustable. One that is 30 mm out is the wrong frame.
A starting point, not an answer
The table below is the conventional mapping used across the industry. It tells you which two sizes to look at on a given brand, and nothing more. Take the size it gives you, look at the one either side, then decide with reach and stack.
| Height | Typical inseam | Road frame | MTB / hybrid |
|---|---|---|---|
| 145–155 cm | 65–72 cm | 47–50 cm | XS |
| 155–163 cm | 71–77 cm | 50–52 cm | XS–S |
| 163–170 cm | 76–82 cm | 52–54 cm | S |
| 170–178 cm | 81–87 cm | 54–56 cm | M |
| 178–185 cm | 86–92 cm | 56–58 cm | L |
| 185–193 cm | 91–97 cm | 58–60 cm | L–XL |
| 193–200 cm | 96–102 cm | 60–62 cm | XL |
Where your height and your inseam point at different rows, follow the inseam. That row reflects how much leg you have to fold into the bike, which is what the chart is a proxy for.
When you fall between two sizes
Most people do, and it is not a failure of measurement. The rule: take the smaller frame and its shorter reach for comfort, an upright back, easy handling at low speed, or if your inseam is long for your height. Take the larger frame and its longer reach for stability at speed, for long distances, or if your torso and arms are long.
The tie-breaker is what can be changed afterwards. A smaller frame is easy to grow into: a longer or higher stem and a setback seatpost stretch it out. A larger frame is hard to shrink, because a stem much under 70 mm makes the steering feel abrupt.
Cheap to change
- Stem length and angle — roughly €25–60, and the most effective single fit adjustment there is.
- Handlebar width, and on drop bars the reach and drop of the bar.
- Spacers already on the steerer. Once a steerer is cut you can lower the bars and never raise them again.
- The saddle, which is a comfort question rather than a sizing one.
- Tyres, within what the frame clears — 50 of the 53 bikes here take at least 35 mm.
Impossible to change
- Frame reach and stack. This is the bike.
- Standover height and wheelbase.
- How much seatpost the frame accepts. A post pulled past its minimum insertion mark is a structural failure waiting to happen, not a fit solution.
Whenever you move a seatpost or a stem, tighten the bolts to the torque printed on the part. Carbon posts, bars and steerers need a torque wrench rather than judgement: they fail by crushing, without warning, and the figure is always low. If you do not own one, a shop will do it in minutes.
Three road bikes within €600 of each other, and exactly the kind of purchase where the decision comes down to two adjacent sizes:
If you are still choosing the kind of bike rather than the size, start with the five questions that decide it , or hand it to the finder and let it rank the catalogue around your answers.
Prepared answers that start from the riding rather than the frame:
Questions readers ask
- Can I just use a size calculator online?
- Most of them are the table above with a form on top, and they are fine for producing two candidate sizes. They cannot tell you which of the two to take, because that depends on the specific frame and on what you want from the bike. Use one to start, then compare reach and stack.
- What if I am between sizes and cannot test-ride either?
- Take the smaller one. A small frame can be stretched with a longer stem and a setback seatpost for well under €100, and a large frame cannot be meaningfully shortened. This matters most with direct-to-door brands, where you are trading the test ride for the price.
- Do women-specific frames still exist?
- Fewer than they did. Most brands now offer one frame range with a wider spread of sizes, plus a fitted saddle and narrower bars, having concluded that the variation between individuals is larger than the average difference between groups. Where a brand offers both, judge them on reach and stack like any other frame.
- How much difference does one frame size really make?
- Typically 15 to 25 mm of reach and a similar amount of stack. That is beyond what a stem swap covers comfortably, which is why the size decision deserves fifteen minutes and the setup decisions can be made later, on the bike.
- Does wheel size affect what frame size I need?
- Not directly on an adult bike — sizing is set by reach, stack and standover, not by the wheel. It matters at the small end, where a 29 in wheel limits how low a frame can be built, which is why some brands fit smaller wheels to their XS mountain bikes.