To ensure dog boots don't hinder your dog's performance or comfort, they absolutely must fit properly.
The easiest way to measure is to place your dog's right front paw, standing with his full weight on the paw on a piece of paper.
Place the dog's heel against a line drawn on the paper, then mark a line at the front of the front toenail.
Measure the distance from the heel to the toenail as shown in chart. This will be the boot size you need.
Boot sizes vary by manufacturer, so be certain and select the appropriate size as listed in the boot's description.