Bike frame size calculator
Height and inseam in, frame size out - road, mountain and hybrid at once, in centimetres, inches and letter sizes.
How to measure your inseam
Stand against a wall in socks, feet about 15 cm apart. Put a hardback book between your legs, spine up, snug into your crotch like a saddle, and keep it level. Measure from the top of the spine straight down to the floor - that's your cycling inseam. It's normally a couple of centimetres longer than your trouser inseam, so don't use your jeans size.
If you skip the inseam, the calculator estimates it from your height (people's inseams average around 47% of height) - fine for a first pass, but measure before you buy.
The formulas this tool uses
- Road frame (cm): inseam × 0.70 - the classic seat-tube heuristic used by bike fitters for decades.
- Mountain bike (inches): inseam (cm) × 0.226 - smaller relative to the rider for standover and control.
- Hybrid (cm): inseam × 0.685 - between the two, matching the more upright position.
These get you the right starting size. Brands cut geometry differently - a 56 in one marque rides like a 54 in another - so treat the result as your shortlist size and confirm against the specific bike's geometry chart.
Road bike size chart (by height)
| Height (cm) | Frame (cm) | Letter |
|---|---|---|
| 152-160 | 47-49 | XXS |
| 160-168 | 49-52 | XS |
| 168-175 | 52-54 | S |
| 175-180 | 54-56 | M |
| 180-185 | 56-58 | L |
| 185-190 | 58-60 | XL |
| 190-200 | 60-63 | XXL |
Mountain bike size chart (by height)
| Height (cm) | Frame (in) | Letter |
|---|---|---|
| 150-160 | 13-14 | XS |
| 160-170 | 15-16 | S |
| 170-178 | 17-18 | M |
| 178-188 | 19-20 | L |
| 188-200 | 21-22 | XL |
Between sizes?
- Default to the smaller frame. You can raise a seatpost and lengthen a stem; you can't shrink a frame.
- Long torso or arms? Consider the bigger size - reach matters as much as height.
- Modern letter sizing varies wildly. One brand's M is another's L. The geometry chart (stack and reach) is the truth; the letter is marketing.
FAQs
Is trouser inseam the same as cycling inseam?
No - cycling inseam is measured to the crotch under saddle pressure and is usually 2-5 cm longer than a trouser inseam. Use the book method above.
Does frame size matter more than saddle height?
They do different jobs. Saddle height sets your leg extension and is fully adjustable; frame size sets reach and handling and is fixed at purchase. Buy the right frame, then set the saddle.
What about kids' bikes?
Kids' bikes are sized by wheel diameter (16", 20", 24"), not frame size - match wheel size to the child's height and check standover clearance.