Use a soft measuring tape and measure your dog while standing naturally.
Neck
Measure where a collar normally sits. Allow room for two fingers.
Chest
Measure around the widest part of the rib cage, just behind the front legs.
Back
Measure from the base of the neck to the base of the tail.
Sizes vary by product. Always use the chart shown on the specific product page. If your dog is between sizes, check the product-specific guidance before ordering.