Ivermectin in dogs is not a simple answer - are you talking about controlling heartworm? Controlling ascaridal worms? Mites and lice? Mange?
The general dosage we use for heartworm control is to use the INJECTABLE (1%) given orally at one-tenth ml per 100 lb. One-TENTH. Some people will use one ml per 100 lb but that's waaaaaaay too high and will cause side effects in vulnerable dogs.
If you are using POUR-ON (.5%) for mites or mange, the dosage is one-tenth ml per kg, poured on the topline. So a 25-lb dog would get about one ml. This dosage should never be given to a sensitive breed and it, again, is way overkill for heartworm. It's for treating mites and generalized mange. This dose will also work pretty well on worms (intestinal) but it's kind of like hitting a nail with a bomb. You can control worms with much gentler medications and it's a good idea to do that if you can. Ivermectin is not a gentle medication and I'd always rather use a gut-only medication (pyrantel or fenbendazole) than something that's absorbed throughout the body.
Obviously, it's important not to mix up the two, since they are not the same strength.
If I can be preachy, it's not a good idea to overdose on ivermectin even if it doesn't kill the dog (or even hurt the dog in the short term). Ivermectin is linked with the later formation of autoimmune disease, and treating a dog with Cushings or polyarthritis makes treating worms look like a cakewalk. I would reserve the big doses of ivermectin ONLY for mites and bad mange, where the dog actually needs that strong a treatment and will be hurt more by the mange than by the ivermectin. I don't even treat the tiny puppy mange spots; they'll go away on their own just as fast as if you treat them.