An accurate measurement is very important to selecting the right size Q-Collar. Referencing your shirt size and guessing are not recommended.
Use a soft measuring tape to measure the middle of the neck. Pull the tape snug so there is zero slack between the tape and neck. Measure three times and take the smallest measurement
Input your measurement from step 1, then find your Q-Collar size on the chart. If you are between sizes, choose the smaller size.