According to Texas A&M University, it is too cold for a dog if the temperature is below 45 F during the day. However, it can be difficult to determine what temperature is too cold for a dog.
Certain factors such as how thick a dog's fur is and its weight can both play a role in determining what temperature is too cold for a dog. For example, a dog with thick fur and decent body weight can sustain colder temperatures than a smaller dog or a dog with short fur. According to Dog Time, dogs with low cold tolerance are best kept inside during cold weather and should be properly dressed in a jacket or sweater when outside.