How to Measure a Mountain Bike Frame

A single frame measurement determines a mountain bike's size.

Bike manufacturers size bikes based upon a single frame measurement. When a bike is described as an "18 in." bike, the reference is to the length of the seat tube--the part of the frame's triangle that extends beneath the seat. The horizontal tube it intersects with at the top is called the top tube. At the bottom of the seat tube is a round hole called the bottom bracket. Measuring a mountain bike frame is simple as long as you know where to measure to and from.

Difficulty:

Moderately Easy

Instructions

Things You'll Need

• Tape measure

1. Use your tape measure to measure from the center of the bottom bracket to the top of the seat tube. This measurement is the normal way to determine frame size.

2. Compare your measurement with the manufacturer's sizing system. If you suspect your measurement doesn't match the manufacturer's, it may be because the manufacturer of your frame uses a different measurement method. An alternative measurement is from the center of the bottom bracket to the center of the intersection of the seat tube and top tube.

3. Contact the manufacturer by phone or check the owner's manual if the measurement still appears to be incorrect. Some methods of frame measurement are peculiar to specific manufacturers.