How to use it
- Choose Adult or teen, or Kid.
- Enter your height in feet and inches (and your inseam if you know it).
- Pick the type of bike.
- Read the frame size and letter size.
How frame size is calculated
Frame size is the length of the seat tube, measured from the center of the crank. It's estimated from your inseam (floor to crotch, barefoot):
| Bike type | Frame size | Usually stated in |
|---|---|---|
| Road | inseam (cm) × 0.665 | cm |
| Mountain (MTB) | inseam (cm) × 0.226 | inches |
| Hybrid / trekking | inseam (cm) × 0.66 | cm |
| City | inseam (cm) × 0.66 | cm |
If you don't enter an inseam, it's estimated as height × 0.46. Example: 5 ft 10 in (178 cm) gives an estimated inseam of about 32 in (82 cm): a 54 cm road frame (M) or an 18.5 in mountain bike (M).
Measure your inseam
Stand barefoot with your back against a wall, feet about 6 in apart. Pull a hardcover book up between your legs, spine up, firmly as a saddle would sit. Mark the top of the book on the wall and measure from the floor to the mark. Measure twice and use the average.
Inseam gives a better result than height, because two people of the same height can have legs several inches different in length.
Letter sizes and modern frames
The letter size (XS to XXL) is a rough guide, as it differs between brands. Modern frames with sloping top tubes are often sold only in letter sizes, and fit depends as much on reach (how far forward the handlebars are) and stack (how high) as on seat tube length. Compare your result with the manufacturer's size chart, which usually lists rider height ranges.
Standover check: straddling the bike with feet flat, you want about 1–2 in of clearance over the top tube on a road bike and 2–4 in on a mountain bike.
Kids' bike sizes
Kids' bikes are sized by wheel diameter:
| Wheel size | Child's height | Typical age |
|---|---|---|
| 12 in | 33–41 in | about 2–4 |
| 14 in | 37–43 in | about 3–5 |
| 16 in | 41–47 in | about 4–6 |
| 20 in | 45–53 in | about 6–9 |
| 24 in | 51–59 in | about 8–12 |
| 26 in / 27.5 in (small adult frame) | 57 in and up | about 11 and up |
Age is only a pointer; height and inseam decide. Where ranges overlap, a confident rider gets more years from the bigger wheel, while a beginner is safer on the smaller, lighter bike. Seated, the child should reach the ground with the balls of the feet. Under about 34 in, a balance bike is the better choice.
Frequently asked questions
What size bike do I need for my height?
As a rough guide for road bikes: 5'4"–5'7" about 50–52 cm, 5'7"–5'10" about 52–54 cm, 5'10"–6'1" about 54–57 cm. Enter your height and inseam for a personal result.
What size mountain bike for a 5'10" rider?
Usually a medium, about 17–19 in. With an estimated inseam the calculator gives about 18.5 in.
Is it better to size up or down on a bike?
If you're between sizes, a smaller frame is easier to handle and adjust with a longer stem or seatpost; a larger one suits longer arms and torso. Test ride if you can.
What size bike for a 6-year-old?
Most 6-year-olds ride a 16 in or 20 in bike, depending on height: 16 in up to about 47 in tall, 20 in from about 45 in.
How do I measure my inseam for a bike?
Stand barefoot against a wall, pull a book up firmly between your legs, and measure from the floor to the top of the book.
Last updated: November 12, 2026