When measuring your cat's neck size, using a measuring tape is preferable however you can also use a piece of string and measure the string with a ruler. When measuring, ensure there is enough room for two fingers to slip between the measuring tape and your pet's neck so the harness will be snug but comfortable.

Size Neck (cm) Chest (cm) Size Guide
Kitten 21-24cm 27cm - 34cm Age 3-6 months 
S 25-28.5cm 38cm - 46cm Age 6 months+, average build cat or up to 6kgs in weight
M 28-34cm 46cm - 55cm Age 6 months+, average/large build cat or between 6-8kgs
L 34-40cm 55cm-64cm Large cat breeds e.g. Maine Coon