
A small dog costume is the size most often bought and most often returned, because small breeds vary more in shape relative to their weight than large ones do. A dachshund, a chihuahua and a pug can all weigh 5kg and need three different garments. Weight is the one measurement that tells you almost nothing.
Three measurements, taken with the dog standing. Neck circumference where a collar sits. Chest girth at the widest point behind the front legs — the number that fails most often. Back length from the base of the neck to the base of the tail. Buy to the largest of the three against the maker’s own chart, never by breed name.
Because it is where small breeds differ most from each other. Pugs and French bulldogs are barrel-chested with short backs; dachshunds are long-backed and narrow; chihuahuas are small in every dimension. A costume sized to back length alone will be tight across the ribs on a broad-chested dog, which restricts breathing rather than merely looking wrong.
Often, for two practical reasons. The same fabric weight is proportionally far heavier on a 3kg dog, and small dogs are picked up and handled more, so costumes get adjusted on them constantly. Watch the quiet signals — lip licking, yawning, turning the head away, a lowered crouch, freezing. Cornell’s bite-prevention guidance notes that by the time a dog growls, they have already tried to communicate their discomfort in subtler ways.
Back-draped and rider-style costumes that fasten under the belly, rather than four-leg bodysuits. Leg holes are where small-dog costumes bind, and a dog that cannot take a normal stride will stand still rather than walk. Anything covering the head is a poor choice for a breed already carrying breathing risk.
Pugs, French bulldogs, shih tzus and Boston terriers have the least respiratory reserve of any dogs and overheat quickly indoors under lights. Keep costumes off the chest and face entirely, keep sessions to a few minutes, and stop at any change in breathing noise. For these breeds a bandana is the sensible default.

See the full dog costume guide or browse Apparel & Accessories.
Return it rather than adapting it. Pinning, knotting or taping a costume to fit creates exactly the loose fabric that catches on furniture and the pressure points that rub. A costume that needs modification to stay on is a costume the dog will get tangled in the moment it moves properly.

Sometimes, but puppy cuts allow for growth and often fit loosely. Measure and compare.
Both. Thin-coated small breeds warm up indoors and chill outdoors. Judge by the room, not the season.
Two fingers underneath, comfortably.
No. That is the costume binding somewhere. Take it off and check the leg openings and chest.
Only the lightest, briefly. Very small dogs chill and tire quickly under any additional weight.
Size up, then check nothing hangs loose enough to catch or be stepped on.
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