See Description for Size Chart
Measure your pup with a measuring tape. If you do not have one, you can use a string and then measure the string with a flat ruler afterwards.
To measure the neck, use measuring tape or string around where the collar would normally sit. Check to make sure you can slip two fingers into the measured width to keep your pup comfortable.
To measure the chest, use the measuring tape or string around the circumference of your pup's chest. This is the widest part of his / her body, right behind the front legs. To ensure comfortable fit, make sure you can slip two fingers into the measured width.