Find the right bike size from your inseam or height
This bike size calculator turns your inseam, or your height if you have not measured it, into a suggested frame size for a road, mountain or hybrid bike, with the typical S/M/L label and a saddle height to start from. For children it picks the wheel size from height, because kids bikes are sold by wheel diameter (12, 14, 16, 20, 24 or 26 inches).
Frame sizing has no industry standard, so every answer here is a well-known rule of thumb, not a guarantee. Use it to narrow the choice, then check the maker’s own size chart and, if you can, test ride. It works in centimetres and inches, in your browser, with no sign-up.
How to use the Bike Size Calculator
- Pick the bike type: road, mountain, hybrid or kid’s bike.
- Choose centimetres or inches, then enter your inseam: stand barefoot against a wall, hold a book level between your legs and measure floor to the top of the book.
- No tape measure? Enter your height instead for a rougher estimate. For children, height is required.
- Read the suggested frame size, the range to try and the typical size label, then your starting saddle height.
- Compare with the brand’s size chart, which uses seat tube, reach and stack differently from this formula.
Bike Size Calculator features
Road, MTB and hybrid
Road frames in centimetres, mountain frames in inches, hybrids midway between the two.
Kids bikes by wheel size
Height bands for 12, 14, 16, 20, 24 and 26-inch wheels, with a balance bike note for very small children.
Saddle height
A starting saddle height from your inseam (× 0.883).
Inches or centimetres
Enter and read measurements in either unit.
Honest estimates
Flags when the inseam was estimated from height.
The formulas behind the sizes
- Road: frame size in cm = inseam × 0.665. An 81 cm inseam gives about 53.9 cm, a size M in most ranges. Other sources use 0.65 to 0.70, which is why we show a range of ±2 cm.
- Mountain: frame size in inches = inseam in cm × 0.226, so 81 cm gives 18.3 in. Mountain frames are smaller than road frames for the same legs, because the top tube is more sloped and the standover clearance larger.
- Hybrid / city: the midpoint of the road and mountain results, since hybrids sit between the two in geometry.
- Saddle height: inseam × 0.883, measured from the centre of the bottom bracket to the top of the saddle along the seat tube. Adjust by feel: a slight knee bend at the bottom of the pedal stroke.
Kids bike wheel sizes
Retailers commonly size children’s bikes by height. These bands are typical; brands overlap.
| Child height | Wheel size |
|---|---|
| 85–97 cm | 12 in |
| 98–104 cm | 14 in |
| 105–114 cm | 16 in |
| 115–129 cm | 20 in |
| 130–144 cm | 24 in |
| 145 cm and up | 26 in or adult XS |
Why a bike can fit differently from the number
Two riders with the same inseam can need different bikes because of torso and arm length, flexibility and riding style. Reach (how far you stretch to the bars) matters as much as frame size, and stems, seatposts and bar choices can fix small differences. A bike that is too large is harder to handle and unsafe to stand over; one slightly small can often be fixed with a longer stem and higher seatpost.
After sizing, set the right tyre pressure with the bike tyre pressure calculator and plan training with the cycling power zones calculator.
Bike Size Calculator FAQ
How do I know what size bike I need?
Measure your inseam, then multiply by 0.665 for a road frame in cm, or by 0.226 for a mountain bike frame in inches. Then check the maker’s size chart and test ride.
What size bike for my height?
Enter your height and the tool estimates your inseam (about 0.45 × height) and a frame size. Measuring your inseam is more accurate.
What size bike for a child?
Kids bikes are sized by wheel diameter, chosen mainly by height: for example 20-inch wheels for children about 115 to 129 cm tall.
Is a 54 cm road bike medium?
Usually yes: 53 to 55 cm is typically a medium, but brands label sizes differently.
Which brands does this fit?
The formulas are generic and not tied to any brand. Check the manufacturer’s geometry chart for the exact model before buying.
What if I am between sizes?
Many riders choose the smaller size and adjust stem and seatpost, but riders who want a relaxed position or have long torsos may prefer the larger one. Try both if you can.