Here is how to measure your pup's neck
Mini - 11" Square (For neck sizes 7" to 10" - diagonal fold 14")
Small - 14" Square (For neck sizes 11" to 13" - diagonal fold 19.5")
Medium - 17" Square (For neck sizes 14" to 17" - diagonal fold 24")
Large - 20" Square (For neck sizes 17" to 20" - diagonal fold 28")
XLarge - 23" Square (For neck sizes 19" to 24" - diagonal fold 32")
Size Large at 20" is the classic size bandana and is also suitable for humans!