How to use the bike frame size calculator
- Measure your inseam: stand barefoot against a wall, press a book up between your legs and measure from the floor to the top of the book.
- Choose inches or centimeters, enter the inseam and pick the bike type.
- Read the frame size in centimeters and inches. The table shows all three bike types side by side.
- Open Advanced options to change the example factors if a maker or shop gives you different ones.
Formula
Inseam in cm = inseam in inches x 2.54
Road frame (cm) = inseam in cm x road factor
Mountain frame (cm) = inseam in cm x mountain factor - amount subtracted, and hybrid is the same with its own factor and amount
The example factors are 0.67 for road, 0.66 less 10 cm for mountain and 0.685 less 12 cm for hybrid. They are starting values for you to edit, not figures from a maker.
An inseam of 32 inches
32 inches is 81.28 cm. With the example factors, the road frame is 81.28 x 0.67 = 54.5 cm (21.4 inches), the mountain frame is 81.28 x 0.66 - 10 = 43.6 cm (17.2 inches) and the hybrid frame is 81.28 x 0.685 - 12 = 43.7 cm (17.2 inches).
Those three numbers are not directly comparable, because road frames are measured along the seat tube and often sized larger, while mountain and hybrid frames are sized smaller to leave room over the top tube. Always compare the result with the chart of the bike you plan to buy.
What frame size means
Bike makers describe frame size in different ways. Some give the length of the seat tube in centimeters or inches, some give a letter such as S, M or L, and some give the reach and stack, which are measured from the bottom bracket to the head tube. A 54 cm frame from one maker may fit differently from a 54 cm frame from another, because seat tube angle, top tube length and sizing method all differ.
Inseam is a simple starting measurement because it relates to how much clearance you need between your body and the top tube when you stand over the bike. It does not capture your torso, arm length or flexibility, which decide how stretched out you feel when riding.
Why the factors are editable
Rules of thumb for converting inseam to frame size exist for each bike type, and shops quote different ones. The values here are examples that you can edit in the Advanced options, and the tool does not claim they are the rule. Fill in the factor and the amount your shop or maker recommends to see their result, or leave the examples for a rough idea.
Between sizes, riders often choose by riding style: smaller for a more upright, playful fit and larger for a longer, stretched fit. A test ride and a proper fitting are better than any formula. Once you have a bike, the tire size calculator helps with wheel dimensions, and the pace calculator works out speed and time on a ride.
Measuring well
Measure twice and use a hard flat book or a level, with shoes off and feet about the width of your hips. Do it with a friend if you can, because a tape pulled at an angle gives a wrong number. Children grow quickly, so check again every few months. If you are between a 20 inch and a 24 inch style frame or between a small and a medium, the standover clearance when you stand flat footed is the safest check.
Assumptions and limits
- The road, mountain and hybrid factors are example values and can be changed under Advanced options.
- The result is a seat tube style estimate in centimeters and inches. Letter sizes and reach and stack are not given.
- Inseam must be from 12 to 60 inches or 30 to 150 centimeters.
- Arm length, torso length, flexibility, riding style and bike geometry are not modeled.
Frequently asked questions
How do I measure my inseam for a bike?
Stand barefoot with your back to a wall and feet about hip width apart. Press a hardcover book level between your legs as high as is comfortable, then measure from the floor to the top of the book.
Are the factors in the calculator official?
No. They are example values that give a rough starting point, and you can change them under Advanced options. Use the size chart of the specific bike when you have one.
Why is the mountain bike frame smaller than the road bike frame for the same inseam?
Mountain bikes are built with more standover clearance, so riders usually choose a smaller frame, which the amount subtracted in the formula allows for. Road frame sizes run larger.
Does frame size matter for kids bikes?
Kids bikes are normally sold by wheel size rather than frame size. Use this tool for adult sized frames and check the maker's height or inseam chart for children.