Measure from the center of the crank to the top of the seat tube

The standard way to measure a bicycle frame is to find the distance from the center of the crankset (where the pedals attach) straight up to the top of the seat tube (the vertical tube that holds the seat post). This measurement, in centimeters or inches, is what manufacturers list as frame size. Most bikes fall between 13 and 23 inches, though road bikes often use centimeters instead.

You will need a measuring tape, a level surface, and the bike itself. Place the bike upright on flat ground — a garage floor or driveway works well. Stand the bike so it does not lean. If you have a second person, have them hold it steady; if not, lean it gently against a wall or have someone support the seat.

Locate the center of the crankset by finding where the two crank arms meet. This is the axle that runs through the middle of the pedal assembly. Mark this point mentally or with a piece of tape. Then measure straight up along the seat tube — the tube that runs from the crankset toward the seat — until you reach the very top where the seat post enters. Write down this number.

Key Takeaways

  • Frame size is measured from the center of the crankset straight up to the top of the seat tube, and this number determines what size bike you need.
  • Road bikes are usually sized in centimeters, while mountain bikes and hybrids are often sized in inches, so check the manufacturer's label to know which unit to use.
  • A level surface and a straight measuring tape give you the most accurate reading; leaning bikes or uneven ground will throw off your measurement.
  • If the seat tube is cut at an angle at the top, measure to the highest point of that cut, not to where the seat post sits inside the tube.

Understand the difference between center-to-center and center-to-top measurements

Some manufacturers measure frame size from the center of the crankset to the center of the top tube instead of to the top of the seat tube. This is called center-to-center measurement and will give you a smaller number than center-to-top. Always check the manufacturer's specification sheet or the label on the frame itself to see which method they use.

Most modern bikes use center-to-top measurement, which is why that is the standard to start with. However, some older bikes and certain road bike brands still use center-to-center. If your measurement does not match the label on the frame, try measuring to the center of the top tube instead and see if that number lines up. The difference is usually 1 to 2 inches.

Account for angled or curved seat tubes

Not all seat tubes run straight up and down. Many modern bikes, especially mountain bikes, have seat tubes that angle backward or curve slightly. When this happens, measure to the highest point of the seat tube, not to where it would be if it were straight.

If the seat tube is cut at an angle at the top, measure to the edge of that cut that is highest off the ground. Do not measure to the seat post clamp or to where the seat post sits inside the tube — measure only to the frame itself. This ensures you are measuring the actual frame size the manufacturer intended.

Know what size range fits your height

Frame size and rider height are connected, but the relationship varies by bike type. Road bikes are typically sized smaller than mountain bikes, so a 56-centimeter road frame and a 20-inch mountain frame may both fit a rider who is 5 feet 10 inches tall.

As a general reference, a 5-foot-tall rider usually fits a 13- to 15-inch frame, a 5-foot-6-inch rider usually fits a 16- to 18-inch frame, and a 6-foot-tall rider usually fits a 19- to 21-inch frame. These are starting points only. Reach (the distance from the seat to the handlebars) and seat tube angle also affect fit, so always test-ride a bike before buying if you can, or check the manufacturer's sizing chart for the specific model.

Measure twice if the frame is damaged or modified

If the bike has a bent seat tube, a cracked frame, or a non-original seat post, take a second measurement to confirm your first one. Damage can shift the seat tube slightly and throw off your reading by half an inch or more.

If the frame has been modified — for example, if someone has cut down the seat tube or added a seat tube extension — the measurement you get will not match the original frame size. In this case, look for a label or sticker on the frame itself, usually near the bottom bracket or on the seat tube, that shows the original size. If no label exists and you cannot find documentation, contact the bike manufacturer with the model name and year if you have it.

Use your measurement to find replacement parts and compare bikes

Once you know your frame size, you can use it to find the right seat post diameter, the correct seat tube length for a new seat post, and the proper standover height (the distance from the ground to the top tube when you stand over the bike). You can also compare your current bike to other models to see whether they will fit you the same way.

Frame size alone does not tell you everything about fit — two bikes of the same frame size can have different reach and stack measurements — but it is the starting point for finding a bike that will work for your body and riding style. Keep your measurement handy when shopping for a new bike or ordering parts online.

Frequently Asked Questions

Should I measure the frame with or without the seat post?

Measure the frame itself, not the seat post. The seat post slides inside the seat tube and does not count toward frame size. Remove the seat post if it is in the way, or measure around it to the frame.

What if my measuring tape is not long enough?

Use a straight stick or a piece of string marked at one-inch or one-centimeter intervals. Lay the stick or string along the seat tube from the crank center to the top, then measure the stick or string with a shorter tape. A yardstick or meter stick works well for this.

Does frame size change if I adjust the seat height?

No. Frame size is fixed and does not change when you move the seat up or down. The seat post slides in and out of the seat tube, but the tube itself stays the same length. Seat height is a separate adjustment for comfort and pedaling efficiency.

Can I use frame size to tell if a used bike is the right size for me?

Frame size is a good starting point, but you should also check the reach and stack measurements if the seller has them, or test-ride the bike if possible. Two frames of the same size can feel very different depending on the top tube length and head tube angle.