The short answer: sooner than you think. The long answer depends on three things, and none of them is the temperature alone.
What actually matters
Size. Small dogs lose heat faster because they have a lot of surface area relative to their body mass. A Chihuahua is cold at temperatures at which a Bernese Mountain Dog is only just waking up.
Coat. Undercoat insulates. Dogs without one — sighthounds, dachshunds, boxers, many terriers and almost every dog with a short clip — are standing outside in their shirtsleeves at five degrees.
Age and health. Puppies, senior dogs and dogs with joint problems need warmth not as comfort but as protection. Cold makes stiff joints stiffer.
The honest rule of thumb
- Above 10 degrees: no dog needs a coat. Really, none.
- 5 to 10 degrees: small, slim and short-haired dogs on longer walks, older dogs generally.
- 0 to 5 degrees: anything without an undercoat should be dressed as soon as it is more than a quick toilet round.
- Below 0: now wet and wind count too. A wet dog loses many times more heat.
How to tell your dog is cold
Shivering is the late sign, not the first one. Before that come: a lifted paw, a tucked tail, an arched back, and that hesitant stop at the door that looks like stubbornness and isn’t.
If any of that sounds familiar, take a look at our coats and jackets. No dog needs five of them. One that fits is enough.