📏 Bike Frame Size Calculator

A general starting-point frame size estimate for road bikes, based on inseam length. Bike fit varies by brand and geometry — always test ride or consult a bike shop's specific size chart before buying.

Inseam length (cm)

General estimate for road bikes only — mountain bikes, hybrids, and other styles size differently.

Inseam times 0.665

Road frame sizing has used the same single-factor rule for decades, and it is what this page computes:

frame size (cm) = inseam (cm) × 0.665
frame size (in) = frame size (cm) / 2.54

An 80 cm inseam gives 53.2 cm, or 20.9 in. Frames are not sold in tenths, so that figure points you at a 52 or a 54 and tells you which two to try. The factor is the classic seat-tube number, measured from the bottom bracket centre up the seat tube.

Measuring, and where the number stops helping

Inseam is not trouser size. Stand barefoot against a wall, feet about 15 cm apart, pull a hardback book up firmly into your crotch, and measure floor to the top edge of the book.
Seat tubes are measured differently by different brands - centre to top, centre to centre, or centre to the top tube. Two "54 cm" frames can differ by a centimetre.
Sloping and compact frames are sold as S/M/L, and their seat tubes are short by design. For those, compare stack and reach on the geometry chart instead.
Torso and arm length set the reach, and your inseam predicts neither.

Road frames only. Mountain, gravel and hybrid geometry sizes differently and typically runs smaller for the same rider. Ride the bike before buying, and use the manufacturer's own size chart as the deciding source.

Frequently asked questions

What bike frame size do I need for an 80 cm inseam?

About 53 cm (80 × 0.665 = 53.2), so a 52 or 54 cm road frame. Test ride both - the shorter one with a longer seatpost usually suits a rider with a long leg and short torso.

How do I measure my cycling inseam?

Barefoot against a wall, book pulled up into the crotch to mimic the saddle, floor to the top of the book. Measure twice; a 2 cm error moves the frame recommendation by more than a size.

Is it better to size up or size down?

Down, if you are between sizes. A shorter frame can be stretched with a longer stem and more seatpost, whereas a frame that is too long cannot be shortened far without wrecking the handling.