In short:
- Bike size is determined by the rider’s height and inseam length.
- For a road bike, inseam x 0.665 gives the recommended frame size in centimetres.
- For a mountain bike, inseam x 0.226 gives the size in inches.
- Reach and stack, published in geometry charts, are more reliable than nominal S, M or L sizes.
Bike size chart by rider height
Choosing the right size using a bike size chart prevents most knee, back and hand pain. The table below gives benchmarks that work for most brands.
| Rider height | Road bike (cm) | Gravel | Mountain bike |
|---|---|---|---|
| 1.55 to 1.65 m (5 ft 1 in to 5 ft 5 in) | 47 to 50 (XS) | XS | S (15 in) |
| 1.65 to 1.75 m (5 ft 5 in to 5 ft 9 in) | 51 to 53 (S) | S | M (16 to 17 in) |
| 1.75 to 1.83 m (5 ft 9 in to 6 ft) | 54 to 56 (M) | M | L (18 to 19 in) |
| 1.83 to 1.90 m (6 ft to 6 ft 3 in) | 57 to 58 (L) | L | L or XL (19 to 20 in) |
| Over 1.90 m (6 ft 3 in) | 59 to 61 (XL) | XL | XL (21 in) |
These values are guidelines. Two people of the same height can have a 5 cm difference in inseam.
Calculating frame size from your inseam
The most common method multiplies the inseam by a coefficient. For a rider with an 84 cm inseam, the result is 84 x 0.665 = 55.9 cm, a size M road bike.
For a mountain bike, the coefficient is 0.226 and the result is in inches: 84 x 0.226 = 19 inches, a size L. The same reasoning applies to an electric mountain bike.
Stack and reach: the measurements that really matter
Manufacturers publish two key values. Stack is the vertical height of the frame, reach its horizontal length.
Comparing these two measurements across brands is far more reliable than the size on the label. It is the method to use when buying a bike online, where a test ride is not possible before delivery.
Frequently asked questions
What size bike do I need for my height?
For a road bike, riders from 1.65 to 1.75 m (5 ft 5 in to 5 ft 9 in) usually ride a size S (51 to 53 cm), from 1.75 to 1.83 m (5 ft 9 in to 6 ft) a size M (54 to 56 cm) and from 1.83 to 1.90 m (6 ft to 6 ft 3 in) a size L (57 to 58 cm). Multiplying your inseam by 0.665 gives a more precise estimate of frame size in centimetres.
How do you measure your inseam for a bike?
Stand barefoot against a wall with feet 15 cm apart, place a book between your legs and pull it up firmly. The distance from the top of the book to the floor is your inseam. Repeating the measurement two or three times reduces error.
What if you are between two bike sizes?
Between two sizes, the smaller one gives a more agile bike and a sportier position, the larger one a more stable bike and a more relaxed position. Comparing the reach of both sizes helps decide. A professional bike fit removes any doubt.