What Size Bike Do I Need? How to Find Your Size and Measure Your Inseam

Your bike size comes from your height and is fine-tuned with your inseam: at 5 ft 9 in (175 cm) you’re a medium on road (53-54), gravel, mountain (16-17”) and city bikes, and with a 32 in (81.3 cm) inseam your classic road frame size is 54.1 cm (81.3 × 0.665). If you’re close to the line between two sizes, your inseam-to-height ratio decides which way to go. And before you buy, compare the frame’s stack and reach, because one brand’s medium can fit like another brand’s small.

Bike size chart by height and bike type

This is the chart our bike size calculator uses. It’s a rounded consensus of several brands’ public size charts: it tells you which size to start looking at, not what to buy sight unseen. Heights are rounded to the nearest inch (the centimeters are the exact cutoffs). You’ll find it inch by inch in the bike size chart by height, and in more detail by type in the road and mountain charts.

The number in parentheses is the usual numeric size: centimeters of seat tube on road, gravel and city bikes, and inches on mountain bikes. Today almost every brand sells by letter, but the number still shows up in used-bike listings and on bikes from a few years back.

If your height falls within about half an inch (1.5 cm) of a cutoff, the calculator warns you that you’re between two sizes. That happens, for example, at 5 ft 9 in on a road bike: you’re a medium, but the large starts at 176 cm (5 ft 9.3 in).

How to measure your inseam at home

Your cycling inseam is the distance from the floor to your crotch, with your weight supported as if you were on a saddle. It’s not your pants inseam: that’s measured to the seam and comes out shorter. You need a wall, a hardcover book, a pencil and a tape measure.

1. Take off your shoes and stand on a hard floor. Carpet compresses and costs you length.
2. Stand with your back and heels against the wall, feet about 6-8 in (15-20 cm) apart, roughly the width of the pedals.
3. Pull the book up between your legs, spine up and level, and press it firmly into your crotch, the way a saddle would. It shouldn’t hurt, but it shouldn’t be loose either.
4. Mark the wall at the top edge of the book. If someone can help, even better: that way the book doesn’t tilt when you turn around.
5. Measure from the floor to the mark. That’s your inseam.
6. Repeat the measurement at least once more. If the two are close (within about 3/8 in, or 1 cm), use the average. If not, do a third.

A 3/4 in (2 cm) error in your inseam shifts your classic road frame size by 1.3 cm and your saddle height by almost 3/4 in. It’s worth measuring twice.

Bike size by inseam: the three formulas

With your inseam you can work out the classic numeric size, the one that measured the seat tube back when every frame had a horizontal top tube:

• Road and gravel: inseam in cm × 0.665, in centimeters (measured from the center of the bottom bracket to the top of the seat tube).
• Mountain: inseam in cm × 0.226, in inches.
• City and hybrid: inseam in cm × 0.66, in centimeters.

The last column is saddle height with 170 mm cranks, from the center of the bottom bracket to the top of the saddle. We explain it step by step in the guide to bike seat height, and you can calculate it with your crank length in the saddle height calculator.

On modern frames, with sloping top tubes, the size number no longer matches the real seat tube length. So use the formula to check that your size by height makes sense, not to hunt for a frame that measures exactly that.

Inseam-to-height ratio and what to do between two sizes

Two people of the same height can need different sizes. The key is your inseam-to-height ratio: divide your inseam by your height (both in the same units).

• Above 0.475: long legs and a short torso. You need little reach to the handlebar and plenty of seatpost (the tube that holds the saddle). Lean toward the smaller size.
• Between 0.445 and 0.475: average proportions. Stick with the size your height gives you.
• Below 0.445: short legs and a long torso. You need more reach and won’t use as much seatpost. Lean toward the larger size.

Three road riders who are 5 ft 9 in (175.3 cm), all on the line between M and L:

If you’re between two sizes and your ratio is average, flexibility and how you ride decide. The smaller size has less stack and less reach: the handlebar sits lower, but closer. The larger one has more reach (and a bit more stack): the handlebar sits farther away. Since raising the bar is easy, with spacers under the stem, and bringing it closer isn’t, the smaller size usually fits better if you have limited flexibility, back trouble or ride long, easy miles; with good flexibility, long arms or a race position, the larger one. On mountain bikes, the smaller size is more nimble on slow trails and the larger one more stable descending. Some models barely change reach between sizes and change stack instead: each brand’s chart, like Trek’s or Scott’s, gives its numbers.

Why every brand sizes differently: stack and reach

There’s no standard that says what a medium is. Every manufacturer splits its sizes its own way, and every model has its own geometry. An example: on Canyon’s chart, the Ultimate road bike in medium is for 5 ft 10 in to 6 ft 0 in (178-184 cm), while on a generic chart like ours 5 ft 10 in is already a large. For someone who’s 5 ft 9 in, the right size from that brand is the small. That’s why we’ve collected the official charts from Canyon, Trek, Specialized, Giant, Orbea and others in the bike size chart by brand.

So the two numbers that really matter are these:

• Stack: the vertical distance from the center of the bottom bracket to the top of the head tube (where the stem mounts). The more stack, the higher the handlebar and the more upright you ride.
• Reach: the horizontal distance between those same two points. The more reach, the farther away the handlebar and the more stretched out you ride.

Because they’re measured from the bottom bracket, they don’t depend on seat tube angle or on what the size is called. Two bikes from the same brand with the same letter can be very different: Canyon lists an effective stack (with handlebar and stem) of 642 mm for the Aeroad in medium and 675 mm for the Endurace in medium. That’s 33 mm (1.3 in) more handlebar height on the Endurace, built for comfort on long rides.

The surest way to get it right is to start from a bike that already fits you: look up its stack and reach on the manufacturer’s website and compare them with the bike you want in the geometry comparison tool. A few millimeters can be fixed with the stem (one that’s 10 mm shorter brings the bar about 10 mm closer) and the spacers under it; 15 mm (about 5/8 in) or more is a different size.

Common mistakes

• Measuring your inseam in shoes or on carpet. You’ll be off by an inch or more, and the error carries into your frame size and saddle height.
• Trusting only the letter. One brand’s medium can be another’s small. Check that model’s height chart and its stack and reach.
• Going a size up “to have room.” You can raise the saddle a lot, but a handlebar that’s too far away can’t be fixed without swapping parts, and the bike will be uncomfortable.
• Buying a size too small and raising the saddle past the seatpost’s minimum insertion mark. On top of feeling cramped up front, you can damage the frame. It happens a lot with used bikes: before you pay, check the size with the guide to buying a used bike.
• Copying the size of a friend who’s your height. If your inseams differ by 2 in, so do your ratios, and maybe your sizes.
• Forgetting the type of bike. At 5 ft 5 in you’re a small on a road bike but a medium on a mountain bike: the charts aren’t interchangeable.

Kids’ bike sizes

Kids’ bikes aren’t sized by frame but by wheel size in inches, and it depends mostly on the child’s height and age. It’s explained by age in kids’ bike sizes, and you can calculate it with the kids’ bike size calculator.

Once you have your size, save your height and inseam in My Bike: the other calculators will use them without you having to type them in again.

Updated October 19, 2026 ·Report an error on this page