Bike Size Calculator

Enter your height and your inseam, pick the type of bike you are shopping for, and we will give you the frame size in centimeters or inches plus the letter size band (XS to XL) to look for. Your inseam decides most of it, which surprises people.

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Here is the thing nobody tells you in the bike shop: your frame size follows your inseam, not your height. Two riders who are both 5 foot 10 can need frames two sizes apart because one carries the height in their legs and the other in their torso. Height gets you to the right aisle. Your inseam picks the bike.

The formulas below are the standard fitting conventions the industry has used for decades. They are honest starting points, not verdicts. Geometry varies between brands, and a 10 minute test ride settles what no formula can, so treat your result as the size band to start test riding in.

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How to measure your inseam

Do not use your pant size; pant inseams run 1 to 2 inches short of your actual cycling inseam. Instead, stand against a wall in socks with your feet about 6 inches apart. Take a hardcover book, pull it up between your legs spine-up until it presses firmly where a saddle would, and hold it level against the wall. Measure from the top of the book spine straight down to the floor. Do it twice; if the two numbers disagree by more than half an inch, do it a third time and take the middle value.

The formulas

road frame (cm) = inseam (cm) × 0.70
mountain frame (in) = inseam (cm) × 0.66 ÷ 2.54
hybrid frame (cm) = inseam (cm) × 0.685

Inseam is your floor-to-crotch measurement taken with the book method above, converted to centimeters if you measured in inches (1 inch = 2.54 cm). The frame size is the seat tube length, which is the number stamped on road and hybrid frames in centimeters and on mountain frames in inches. Your height is used only as a cross-check: if your inseam falls outside 42 to 50 percent of your height, one of the two measurements is probably off and the calculator will ask you to remeasure.

Road frame size bands

Road frames are labeled in centimeters, but many brands also use letter sizes. Here is how the numbers map to letters.

Frame size (cm)Letter size
47 to 49XS
50 to 52S
53 to 55M
56 to 58L
59 to 62XL

Mountain frame size bands

Mountain frames are labeled in inches, and increasingly just in letters. Hybrid frames use the road table above; they are sized in centimeters and most brands sell them by letter.

Frame size (inches)Letter size
13 to 14XS
15 to 16S
17 to 18M
19 to 20L
21 to 22XL

Worked example

Say you are 5 feet 10 inches tall with a 32 inch inseam, shopping for a road bike. Your inseam converts to 32 × 2.54 = 81.28 cm. Multiply by 0.70 and the road frame size is 56.9 cm, which lands in the L band (56 to 58 cm). Your height converts to 177.8 cm, and 81.28 is about 46 percent of that, comfortably inside the 42 to 50 percent sanity range, so the measurements agree. You would start test riding 56 and 57 cm road frames, or size L in brands that use letters.

Between two sizes? Reach and flexibility decide

If your result lands on a band boundary, the tiebreaker is not your legs, it is your upper body. A longer torso or good flexibility favors the larger frame, because you can use the extra reach. A shorter torso, limited flexibility, or a preference for a more upright ride favors the smaller frame. When genuinely torn, most fitters say take the smaller one: a stem swap and a raised seatpost can grow a small frame, but nothing shrinks a big one.

Kids bikes work completely differently

Kids bikes are sized by wheel diameter, not frame size, so none of the formulas above apply. The honest age ranges below overlap on purpose, because kids grow at wildly different rates. The real test: the child should clear the top tube standing flat-footed and reach the ground from the saddle.

Wheel sizeTypical age
12 inch2 to 4
14 inch3 to 5
16 inch4 to 6
20 inch5 to 8
24 inch8 to 12

Once a rider fits a 26 inch wheel or larger, they are into adult sizing and the inseam formulas above take over.

Sorting out the rest of your sizes? Try the shoe size converter or the hat size calculator.

Frequently asked questions

What size bike do I need for my height?

Height alone cannot tell you, because two people the same height can have very different leg lengths. Frame size follows your inseam: multiply inseam in cm by 0.70 for a road frame in cm, by 0.685 for a hybrid frame in cm, or by 0.66 and divide by 2.54 for a mountain frame in inches. A rider with a 32 inch inseam lands around a 57 cm road frame or a 21 inch mountain frame regardless of exact height.

How do I measure my inseam for a bike?

Stand against a wall in socks with your feet about 6 inches apart. Pull a hardcover book up between your legs, spine up, until it presses firmly where a saddle would, keep it level against the wall, and measure from the top of the book spine to the floor. That number, not your pant inseam, is your cycling inseam. Pant inseams run 1 to 2 inches short.

Should I size up or size down between bike sizes?

Let your torso and flexibility decide. If your torso is long for your height or you like a stretched, aggressive position, take the larger frame. If your torso is short, your flexibility is limited, or you want a more upright ride, take the smaller frame, since a smaller frame is easier to adjust upward with stem and seatpost changes than a big frame is to shrink.

Why are mountain bikes sized in inches and road bikes in centimeters?

Convention, mostly. Road cycling grew up in continental Europe and kept metric seat tube measurements, while mountain biking started in the United States and adopted inches. Many mountain bike brands now just use letter sizes (S, M, L), which is why we give you the letter band alongside the number.

How are kids bikes sized?

By wheel diameter, not frame size. Kids bikes run from 12 inch wheels for roughly ages 2 to 4, up through 24 inch wheels for roughly ages 8 to 12, and a child moves up when their inseam clears the standover height and they can reach the ground. The age ranges are honest approximations only; a tall 6 year old may skip a size entirely.

Is a bike size calculator accurate enough to buy a bike?

It gets you to the right size band, which is most of the battle, but geometry varies between brands and models, so the same rider can be a 56 in one brand and a 54 in another. Treat the result as your starting point, check the brand's own size chart, and test ride before you buy whenever you can. A 10 minute test ride settles what no formula can.

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