Below is a guide for you to choose the correct size:
- Mayfair 7"-8.5" (18cm - 22cm)
- Tiny 9"-12" (23cm - 30cm)
- Small 12"-15" (30cm - 37.5cm)
- Medium 15"-19" (37.5cm - 48cm)
- Large 19" -22" (48cm - 56cm)
A collar shouldn't be tight around your pet's neck and therefore it will not be the exact measurement of their neck. For safety reasons - so it doesn't snag on something - it shouldn't be too loose either.
A snug fit is what you are aiming for and not a tight one or one that chokes.
Two finger rule
When the collar is on, you should be able to slide two fingers between a properly fitting collar and your pets neck. If there's extra room, you need a smaller size. If both fingers don't fit, the collar is too small.