Bike frame size calculator

The frame size comes from the classic inseam formula: seat tube length = inseam × a factor for the bike type (about 0.665 for road, 0.65 for gravel, 0.685 for hybrid, and 0.226 for mountain bikes, which converts to inches). Your height then picks the manufacturer size label. When the two answers disagree the calculator says so, because that is exactly the case where a stock size chart sends people wrong.

How do I measure my inseam for a bike?

Stand barefoot against a wall with your feet about 15 cm apart. Put a hardcover book between your legs and pull it up firmly, like a saddle. Mark the top edge of the book on the wall and measure from the floor to the mark. That is your cycling inseam, and it is usually 2–4 cm longer than your trouser inseam.

Is height or inseam more important for frame size?

Inseam sets the seat tube length and saddle height, so it drives the frame size number. Height is a cross-check on reach — the top tube. If the two disagree, you have a long torso or long legs relative to your height, and you should pick by inseam, then adjust the stem and saddle position for reach.

Should I size up or size down if I am between sizes?

Size down for a racier, more responsive fit and if you have short arms or limited flexibility. Size up if you want a more upright, comfortable position and have a long torso. A smaller frame is easier to adapt with a longer stem and a taller seatpost than a large frame is to shrink.

Why are mountain bike frames measured in inches?

Mountain biking grew out of the US market, so frame sizes stayed in inches (15.5", 17.5", 19") while road frames kept the European convention of centimetres. Many brands now just use S/M/L labels for both.

Are these sizes the same at every brand?

No. A 'medium' varies by a centimetre or two between brands, and modern geometry charts use stack and reach rather than seat tube length. Use this calculator to find your starting size, then check the brand's own geometry chart before you buy.

Does the right frame size mean I have a good bike fit?

It is the first step, not the last. The frame gets you in range; saddle height, saddle setback, stem length and bar drop decide whether the position actually works. Pedalyze measures those angles from two photos of you on the bike.

• Road bike fit angles — The complete reference chart for a road position: knee, hip, torso, elbow and shoulder angles plus KOPS, with target ranges and what to change when a number is off.
• How to take bike fit photos — The photo is the measurement: camera height, 90° side-on framing, crank at 6 and 3 o'clock, clothing and lighting — everything that makes fit angles accurate.

• Stack and reach calculator — Carry frame stack and reach up through your stem and spacers, and compare where two bikes really put the handlebar.
• Saddle height calculator — Estimate your saddle height from inseam with the LeMond method, measured from the bottom bracket to the top of the saddle.