Take your tape measure and put it around your cat's neck. Please make sure you are still able to put two to three fingers between the neck and the tape measure.
Make it easy and just measure a collar you already own and you know fits perfectly.
Don't have a tape measure? No problem. Just take a piece of rope or string and put it around your cat's neck. Again make sure you can still put two to three fingers between the rope and the neck. Now measure the rope/string with a ruler. Ready!