I am not an expert. But this is what I’ve learned from experience with dog sweaters:
First of all, not all dogs need sweaters. The average healthy adult dog does not need a sweater. Adult dogs can use their own body heat to stay warm, unlike humans, who require clothing.
However, if you see your dog shivering in the cold, or if your dog indicates he or she is uncomfortable, you may want to consider a sweater. Puzzle shivers in the cold, and the way she tells me she is uncomfortable is by refusing to walk! On several occasions we were about 50 feet from home, and she stopped walking! I had to pick her up and carry her. Next time, I was wiser, and put a sweater on her.
You will want to get a sweater that fits your dog. This is very important. An ill-fitting sweater will be uncomfortable for your dog, and therefore useless and a waste of money. Measure your dog and shop wisely. Puzzle’s chest is 16 inches. She is size M. If your dog is low to the ground, exceptionally heavy, or very thin, you will have to custom-make your sweater. But that is the fun part!
Always supervise your dog while he or she is wearing a sweater, especially if it is the first time wearing one. Why? Your dog may decide to rip it off. So you don’t think this would be the worst thing? Think again: You don’t want your dog ingesting the yarn. This could be very serious. (If your dog eats yarn, or has yarn coming out of his or her anus, call a vet immediately! Don’t try to remove it yourself!) So be very careful when your dog wears his or her sweater for the first time especially. Do not leave your dog crated with a sweater on. In fact, I would suggest taking the sweater off when you come inside from your walk.
I would recommend wool. Why? It is water-repellent, so it will keep your dog dry in the wettest wintry conditions. Also, wool will break down in the digestive system if tiny fibers are ingested (again, call your vet immediately if your dog eats yarn!). I use a wool sweater for Puzzle in the rain. Puzzle is a Schnoodle. Her fur is not water-repellent like a Labrador Retriever’s coat is, nor is it double-coated like the Sheltie’s. If she gets wet, the water will soak through to her skin and tangle her little Schnoodle fur. She also has very thin fur. You can actually see her skin under her coat. I find a wool sweater keeps Puzzle dry; only the sweater gets wet. I take the sweater off immediately when we get inside. An alternative is a doggie raincoat, but I have never seen one that seemed suitable for Puzzle.
If you want to make a dog sweater yourself, great! A handmade sweater is the best kind because it will be custom fit for your dog. Google “dog sweater patterns” and you’ll come up with some, or click on “dog sweaters” in my tags (over in the right-hand column of this page) and you’ll come up with some of my photos of Puzzle’s sweaters and patterns. I usually don’t use a pattern. I make it up as I go along.
Happy knitting! And to your dog: Happy Wardrobe!