Bicycle size is measured by frame size, wheel diameter, or both, depending on the type of bike

Bicycle size is not one number. Road bikes and mountain bikes are sized by frame size—the length of the seat tube, measured in centimeters or inches. Kids' bikes and some cruisers are sized by wheel diameter—the width of the tire in inches. Some bikes list both. Knowing which measurement applies to your bike tells you whether you need a 56 cm frame or a 26-inch wheel, and whether a replacement part will fit.

The frame size matters most if you ride a road, mountain, or hybrid bike as an adult. The wheel size matters most if you are buying a children's bike or replacing a wheel. Many bikes have both numbers printed on them, but you may need to measure if the label is worn or missing.

Key Takeaways

  • Frame size for adult bikes is printed on the seat tube (the vertical tube running from the pedals to the seat) and measured in centimeters or inches.
  • Wheel size is printed on the tire sidewall and measured in inches, such as 26", 27.5", or 29" for mountain bikes.
  • You can measure frame size yourself with a tape measure from the center of the crank to the top of the seat tube.
  • Children's bikes are usually sized by wheel diameter rather than frame size, and the size is printed on the tire.

Finding the frame size on your bike

The frame size is printed on the seat tube, which is the vertical tube that runs from the pedals up to where the seat post slides in. Look for a label, sticker, or text stamped directly onto the tube. The number will be in centimeters (cm) for most road and mountain bikes, or in inches for some older or budget bikes. Common sizes are 48 cm, 52 cm, 56 cm, or 60 cm.

If the label is faded or missing, you can measure the frame size yourself. Stand the bike upright on a flat surface. Use a tape measure to measure from the center of the crank (the metal arm that holds the pedal) straight up to the top of the seat tube where it meets the top tube. Round to the nearest centimeter. This measurement is your frame size.

Finding the wheel size on your bike

The wheel size is printed on the tire sidewall in inches. Look at the side of the tire (not the tread) for a number like 26", 27.5", 29", or 700c. The first three are common on mountain bikes; 700c is standard on road bikes and many hybrids. Some older bikes may say 24" or 20".

If you cannot read the tire marking, you can measure the wheel diameter yourself. Lay the bike on its side on a flat surface. Use a tape measure to measure straight across the wheel from the outer edge of the tire on one side to the outer edge on the other side. This is the wheel diameter. Convert to the nearest standard size: if you measure about 66 cm, that is a 26-inch wheel; about 70 cm is a 27.5-inch wheel; about 73 cm is a 29-inch wheel.

Understanding frame size for different bike types

Road bikes are almost always sized by frame size in centimeters. A 56 cm road bike is considered medium for an average adult; smaller riders use 48–52 cm, and taller riders use 58–62 cm. The frame size directly affects how stretched out you sit and how the bike handles.

Mountain bikes are also sized by frame size, but the range is smaller. A 17-inch or 18-inch frame (measured in inches on many mountain bikes) is typical for an average adult. Mountain bike frames run from about 13 inches for smaller riders to 21 inches for taller riders.

Hybrid and cruiser bikes may be sized by frame size or wheel size, or both. Always check the label on your specific bike. Children's bikes are almost always sized by wheel diameter—a 20-inch kids' bike has 20-inch wheels, not a 20 cm frame.

Why both measurements matter when buying parts

When you replace a wheel, the new wheel must match the old wheel's diameter. A 26-inch wheel will not fit on a bike designed for 29-inch wheels. When you replace the frame or buy a new bike, the frame size determines whether the bike will fit your body and riding style.

Seat posts, handlebars, and other components are sized by diameter (measured in millimeters), not by frame size, so those measurements are separate. But knowing your frame size and wheel size will help you communicate with a bike shop and make sure replacement parts are compatible.

Measuring yourself to find the right size bike

Your height and inseam (the distance from your crotch to the floor when standing) are the best guides to frame size. Most bike shops have a sizing chart that matches your height and inseam to a frame size. As a rough guide, a rider who is 5'6" to 5'10" typically rides a 54–58 cm road bike or a 17–19 inch mountain bike, but this varies by brand and your arm length.

The best way to know if a frame size is right for you is to sit on the bike. When you straddle the frame with your feet flat on the ground, there should be 1 to 2 inches of clearance between your crotch and the top tube. When you sit on the seat with your hands on the handlebars, your elbows should have a slight bend, not be locked straight or cramped.

What to do if your bike has no size label

Measure the frame size as described above: from the center of the crank to the top of the seat tube. Write down the measurement. Then measure the wheel diameter on the tire or by measuring across the wheel. If the tire is worn and the sidewall marking is gone, measure the wheel and match it to the nearest standard size.

Once you have both numbers, you can search online for your bike's make and model to confirm the size, or bring the measurements to a local bike shop. The shop can also tell you what size replacement parts you need and whether parts from other brands will fit your frame.

Frequently Asked Questions

What does 700c mean on a bike tire?

700c is a wheel size used on road bikes and many hybrids. The 700 refers to the approximate outer diameter in millimeters (about 27.5 inches), and the "c" refers to the tire width category. It is the standard size for adult road bikes and is not interchangeable with 26-inch or 29-inch mountain bike wheels.

Can I put a larger wheel on my bike?

No. The frame, brakes, and forks are designed for a specific wheel diameter. Putting a 29-inch wheel on a bike designed for 26-inch wheels will cause the tire to rub the frame and brakes will not work correctly. Always match the wheel size to the original specification.

Is frame size the same as bike size?

Frame size is one part of bike size. When someone says "I ride a medium bike," they usually mean the frame size is medium for that brand. But "size" can also refer to wheel size on children's bikes. Always check the label on your bike to see which measurement is listed.

How do I know if a used bike will fit me?

Measure the frame size and compare it to a sizing chart for that brand, or sit on the bike. You should have 1 to 2 inches of clearance between your crotch and the top tube when you straddle the frame. If the frame is too small or too large, it will be uncomfortable and harder to control.

What if my bike is measured in inches instead of centimeters?

Some older bikes and budget bikes use inches for frame size. A 20-inch frame is roughly 51 cm, a 21-inch frame is roughly 53 cm, and a 22-inch frame is roughly 56 cm. You can convert by multiplying inches by 2.54, but the exact size depends on how the manufacturer measured the frame.