Measure the length of your dog from the base of their tail to the tip of the nose, and measure height from the floor to the top of their head. Add two to four inches to both measurements for small dogs, and about four to six inches each for medium and large dogs. When making your size selection, consider whether your dog tends to curl up or sprawl out when they sleep. (Sprawlers need more space!) We suggest sizing up if your dog is in between sizes to ensure their comfort.