Understanding Trek MTB Sizing
Starting with model year 2020, all Trek mountain bikes will use alpha sizing instead of numeric sizing.
Why did we do this?
It’s easier to understand. “Medium” is intuitive. “17.5” is not. Most other manufacturers already use alpha sizing. Switching from numeric to alpha sizing simplifies the buying process for both consumers and retailers. If you ride a Medium in a different brand, you probably ride a Medium Trek as well.
Frame size used to be measured off the seat tube length. However, modern mountain bike geometry is based on reach, not seat tube length. The new alpha sizing reflects that truth. This will help fit riders by reach.
Switching from numeric to alpha sizing eliminates the confusion that results from having a “virtual” and “actual” seat tube measurement.
The new alpha sizing corresponds with the previous numeric sizing for main line and Women’s models:
XS – 13.5”/14”
S – 15.5”
M – 17.5”
ML – 18.5”
L – 19.5”
XL – 21.5”
XXL – 23”
Extended Size Range
Trek offers more sizing options to ensure the perfect fit. That includes an “XS” with increased standover, a “ML” (medium-large) perfect for those in-betweeners in the middle of the sizing bell curve, and an “XXL” for taller riders struggling to find a bike big enough for them. While not all sizes are available on all models, with a Trek you'll find a frame that fits better from the start without making cockpit accommodations.