8 inches < back length < 9.5 inches, chest girth < 11.8 inchesBefore purchasing, please be sure to measure your pet's back length and chest girth to confirm the size you need to buy. If you are between two sizes, please give priority to the back length. Small size: 8 inches < back length < 9.5 inches, chest girth < 11.8 inches. Medium size: 10 inches < back length < 12 inches, 11.8 inches < chest girth < 15.8 inches Recommended weight 2.5 pounds < small dogs and cats < 4.5 poundsIt is recommended to use small size when the weight is 2.5 pounds < small dogs and cats < 4.5 pounds, and medium size when the weight is 5 pounds < small dogs and cats <8 pounds. If you are between two sizes, please choose the next larger size. But please note that the weight is for reference only. When purchasing, please choose the size mainly based on the back length. Let your pet adaptLet your pet gradually adapt to the backpack before the first use. Let it wear the backpack for a short time at home, and gradually increase the wearing time so that it can get used to the feeling of being in the backpack. Two wearing methods, lighten the burdenThe hands-free pet backpack provides two wearing methods. Wearing it in the front makes it easy to interact with your beloved pet and cultivate a deeper connection between you. Wearing it in the back allows both of you to enjoy different scenery. At the same time, the thickened sponge pad helps to reduce the burden on your shoulders.