Bike Frame Size Calculator

How inseam sets frame size

The oldest way to size a bike is by inseam. You multiply your inseam by a factor to get the seat tube length, which is the frame size most charts list. A road bike uses the largest factor because its frame stands tall and the rider reaches down to the bars. Mountain bikes use a smaller factor so you have room to stand over the top tube on rough ground. This tool uses 0.67 to 0.70 for road, 0.62 to 0.66 for mountain and 0.65 to 0.68 for hybrids.

To measure, stand barefoot against a wall with your feet about six inches apart. Hold a hardcover book level between your legs, pressed up firmly, and measure from the top of the book to the floor. That is your inseam. The default of 32 inches is 81.3 centimeters. Times the road range, that gives 54.5 to 56.9 centimeters, with a midpoint of 55.7. That lands near a 56 cm road frame, or a medium-large.

The height lookup gives a quick letter size as a cross check. A rider 70 inches tall usually lands in L on brand charts, while 65 to 68 inches is often M. If your inseam and height point to different sizes, your legs or torso are longer than average. In that case the top tube length matters more than seat tube length, and a shop fit is worth the money.

Treat the number as a starting point. Many modern frames use sloping top tubes and size by reach, so a 56 from one brand can ride like a 54 from another. Seat height, stem length and saddle setback fix small differences. If you ride on paved roads and trails, pair this with the bike tire size converter and the gear ratio calculator when you pick a build.

Where these numbers come from

The formulas and default values in this calculator are based on:

• common inseam-multiplier rules of thumb used by bike shops and fit guides. Note: multipliers are rules of thumb, and brand geometry charts override them

This is a planning estimate, not medical, legal or immigration advice. Fees, allowances, limits and entry rules change, so confirm the figures with your airline, cruise line, customs agency or the official government source before you book or travel, and follow their rules if they differ from this page.