Frame size comes from your inseam and the bike type
Multiply your inseam in centimeters by a factor set by the bike type. Road frames sit highest, city frames lowest, because each style puts you in a different riding position over the same legs.
| Bike type | Factor k | 75 cm inseam |
|---|---|---|
| Road | 0.70 | 52.5 cm |
| Hybrid | 0.685 | 51.4 cm |
| Mountain | 0.66 | 49.5 cm |
| City | 0.61 | 45.8 cm |
Divide the centimeter result by 2.54 for inches: a 52.5 cm road frame is about 20.7″.
Adult sizing by height (road bikes)
Manufacturers usually publish frames as XS to XL rather than exact centimeters. This maps height and inseam to the common labels for a road bike.
| Height | Inseam | Frame (cm) | Size |
|---|---|---|---|
| 150–160 cm | 70–75 cm | 49–53 | XS |
| 160–170 cm | 75–82 cm | 53–57 | S |
| 170–180 cm | 82–90 cm | 57–63 | M–L |
| 180–190 cm | 90–98 cm | 63–69 | L–XL |
| 190 cm+ | 98 cm+ | 69+ | XXL |
Mountain frames run roughly 3–4 cm smaller than a road frame for the same rider, and the size letters shift down with them.
The two checks the numbers miss
- Standover clearance. Straddle the top tube flat-footed. You want about 2.5 cm (1″) of gap on a road bike and 5 cm (2″) on a mountain bike. If the frame touches your crotch, it is too big whatever the chart says.
- Kids go by wheel, not frame. Children's bikes are sized by wheel diameter (12″ to 24″) against height, not frame length. Let the child sit on it: toes should reach the ground with the saddle at its lowest.
Common questions
Should I size a bike by my height or my inseam?
Use your inseam. Two people the same height can have very different leg lengths, and the frame follows the legs. Height only gives a rough starting point; the inseam sets the actual frame size.
How do I measure my inseam for a bike?
Stand with your back to a wall in socks, feet a few inches apart, and pull a book up between your legs until it is snug against your crotch. Measure from the floor to the top of the book. That distance, in centimeters, is your cycling inseam.
What if I fall between two frame sizes?
Pick the smaller frame for a lower, more aggressive road position, or the larger one for a more upright, comfortable ride. Test ride both if you can, since seat and stem adjustments only cover a small range.
