The question we hear most in the showroom is: “I am this tall — which size do I need?” The short answer is that children are sized by wheel diameter and adults by frame size. The long answer — the one that gets you the right bike first time — is below.
The quick answer
If you are in a hurry, the two tables below get you 90% of the way there. The rest of this article covers the remaining 10% — which is the difference between a bike that works and a bike that is genuinely comfortable.
| Wheel size | Approx. age | Child height | Inseam |
|---|---|---|---|
| 12 in | 2 – 4 years | 85 – 100 cm | 35 – 42 cm |
| 14 in | 3 – 5 years | 95 – 110 cm | 40 – 48 cm |
| 16 in | 4 – 6 years | 105 – 120 cm | 45 – 55 cm |
| 18 in | 5 – 7 years | 112 – 127 cm | 50 – 60 cm |
| 20 in | 6 – 9 years | 120 – 135 cm | 55 – 63 cm |
| 24 in | 8 – 12 years | 135 – 150 cm | 62 – 72 cm |
| 26 in | 11 years + | 145 – 165 cm | 70 – 78 cm |
| Rider height | Frame size | Label | Suggested wheel |
|---|---|---|---|
| 148 – 158 cm | 13 – 14 in | XS | 26 in |
| 158 – 168 cm | 15 – 16 in | S | 26 in |
| 168 – 178 cm | 17 – 18 in | M | 26 or 27.5 in |
| 178 – 185 cm | 19 – 20 in | L | 27.5 or 29 in |
| 185 – 195 cm | 21 – 22 in | XL | 29 in |
| 195 cm + | 23 in + | XXL | 29 in |
Why inseam beats total height
Two riders who are both 175 cm can differ by a full 8 cm in inseam — one has a long torso and short legs, the other the reverse. The bike interacts with your legs, not your torso, so inseam is the number that determines saddle height and the largest frame you can safely straddle.
How to measure properly (two minutes)
- 1.Stand barefoot with your back to a wall and your feet about 15 cm apart.
- 2.Place a book between your legs and press it gently upward, the way a saddle would sit.
- 3.Mark the wall at the top edge of the book.
- 4.Measure from the floor to the mark. That is your inseam.
- 5.Repeat twice and take the average — rushing it can be 2 cm out.
The formulas mechanics use
- Saddle height (bottom bracket centre to saddle top) = inseam × 0.883 — the LeMond formula, and the accepted standard.
- Mountain frame size in inches ≈ inseam (cm) × 0.226.
- Road frame size in centimetres ≈ inseam (cm) × 0.665.
Example: a rider 178 cm tall with an 81 cm inseam. Saddle height = 81 × 0.883 = 71.5 cm. Mountain frame = 81 × 0.226 = 18.3 in, so an 18 or 19 inch frame (size L).
The standover test
This is the most important practical check and it takes one second: stand over the bike with both feet flat on the ground either side of the frame. There must be clearance between you and the top tube. If the tube touches you, the frame is too big — and this is a safety issue, not a comfort one, the moment you have to stop suddenly.
| Bike type | Clearance needed |
|---|---|
| Road bike | 2 – 3 cm |
| Hybrid | 2 – 5 cm |
| Mountain bike | 5 – 10 cm |
| Kids bike | 3 – 5 cm |
If you fall between two sizes
- Go smaller if: you want control and agility, you ride rough ground, or you have a short torso and long legs.
- Go larger if: you cover long distances on tarmac, you have a long torso, or you prefer a stretched, relaxed position.
- When in doubt, go smaller. A slightly small bike can be corrected with a longer seat post and a longer stem; a slightly large bike cannot be fixed at all.
The five most common sizing mistakes
- 1.Buying a child a size up “to grow into”. They cannot reach the brakes or the ground, and the first fall puts them off riding for a year.
- 2.Sizing by age instead of height. Two seven-year-olds can differ by 15 cm — height is the measure.
- 3.Using wheel size to size an adult bike. A 29-inch wheel is sold on 15-inch and 21-inch frames alike — the wheel is not the bike size.
- 4.Ignoring saddle height after purchase. Most people ride far too low, which causes knee pain and costs roughly 20% of your pedalling power.
- 5.Treating frame sizes as comparable across brands. A 17-inch frame from one maker can equal an 18-inch from another — try the actual bike where you can.
I am 170 cm — what size do I need?
A 17-inch frame (size M) for a mountain or hybrid bike, or a 54–55 cm frame for a road bike. Either 26 or 27.5 inch wheels will suit you.
Is wheel size the same as bike size?
No — and this is the most common confusion. On kids bikes the two are effectively linked, but on adult bikes the wheel size (26 / 27.5 / 29) determines how the bike behaves, while the frame size in inches or centimetres determines whether it fits you.
How do I know the saddle is at the right height?
Sitting on the saddle with your heel on the pedal at its lowest point, your leg should be completely straight. Moving to the ball of your foot — the normal riding position — leaves a slight 25–35° bend in the knee, which is correct.
