BMBPET Sizing Guide
 
Adjustable Collar:

Measure your dog's neck so that the collar is snug but you can slip two fingers between the collar and the neck. 

Choose the size collar for that measurement. i.e. Measurement is 13", choose the 8-14" collar.

If the measurement is exactly between two collar sizes, such as 14", you have two options:
3/8" x 8-14" Adj
5/8" x 14-20" Adj

-The 3/8" wide x 8-14" would fit fine and you might choose this if the dog is full grown or if you like a more narrow collar.
-The 5/8" wide x 14-20" would also fit fine but with more room to grow or if you like a wider width collar.
Either would fit and it is your preference whether you want 3/8" or 5/8" width. If the dog is growing, customers go up to the next size most often.

Regular Collar:

Measure your dog's neck so that the collar is snug but you can slip two fingers between the collar and the neck. 

Choose the size collar for that measurement. i.e. Measurement is 11", choose the 1/2" wide x 9-12" collar.

There are four holes so for a 12" collar example the holes are placed at 9", 10", 11" and 12". If a dog's neck measures 11" you could choose 1/2 x 11" for the perfect fit.

If the measurment is exactly between two collar sizes, such as 12", choose 1/2" wide x 9-12" for the perfect fit, or, 3/4" wide x 12-15" for the perfect fit now with more room to grow.
1/2" x 9-12" 
3/4" x 12-15"