How to use
- 1Measure your inseam barefoot: book between the legs, top edge to the floor.
- 2Enter your height and inseam in centimetres.
- 3Read the frame size in cm and inches, letter size and wheel size for each bike type.
- 4For children, use the height-based wheel size table.
The formulas behind the sizes
Frame size comes from inseam, not height, because leg length decides saddle height and standover. The common rules: road and gravel bikes inseam × 0.665 (seat tube in cm), city and hybrid bikes inseam × 0.66, mountain bikes inseam × 0.226 for the frame size in inches. With an 84 cm inseam that is a 56 cm road frame, a 55 cm city frame and a 19″ MTB. The letter size (XS–XXL) is taken from height, because brands increasingly size by S/M/L.
Wheel sizes
Road bikes use 700c (622 mm rim, called 28″ in Europe); small road and gravel frames often use 650b (27.5″) to keep the handling right. Modern mountain bikes are 29″ for most adults, with 27.5″ common on XS–S frames or as a playful option. Children’s bikes go by height: 12″ for about 85–100 cm, 14″ 95–110, 16″ 105–120, 20″ 115–135, 24″ 130–150 and 26″/27.5″ from about 145 cm.
Always test ride
These numbers are a starting point. Geometry differs between brands — reach, stack and top-tube length matter as much as seat-tube length, and an “M” from one maker may equal an “L” from another. Check the manufacturer’s size chart, test ride, and when you sit between two sizes, the smaller one is usually easier to adjust with a longer stem or seatpost. A professional bike fit is worthwhile for long rides.
Frequently asked questions
What size bike do I need for my height?
As a guide, road bikes are S for about 160–170 cm, M for 170–178 cm and L for 178–186 cm. Use your inseam for a better estimate: inseam × 0.665 gives the road frame size in cm.
How do I measure my inseam?
Stand barefoot against a wall, pull a hardback book firmly up between your legs as if it were a saddle, and measure from the top edge of the book to the floor.
What size mountain bike for a 180 cm rider?
Usually an L frame with 29″ wheels; with an 84–86 cm inseam the formula gives about 19–19.5″. Check the brand’s chart because MTB sizing varies widely.
What size bike for a 6-year-old?
Most 6-year-olds are around 115–120 cm tall, which fits a 16″ or a small 20″ wheel bike. The child should be able to touch the ground with the balls of both feet from the saddle.
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