Aspirin dose = 10 mg / Pound of weight
Here's one of several Vet site discussions of Plain Aspirin for dogs >>>
"Aspirin has the same problems in dogs that it has in people.
It can cause gastrointestinal upsets and ulcers if the GI signs are ignored. It can cause renal failure if overdosed. It causes an increase in clotting time. This usually isn't a serious side effect but it does occur.
Despite these shortcomings it has a lot of beneficial effects and it is inexpensive. The currently recommended dosage of aspirin varies a little from publication to publication but it is between 5 and 15mg/lb every 12 hours. Since there is a range I usually pick the middle of it and go for about
10mg/lb every 12 hours.
This works well and seems to be pretty safe. That works out to an aspirin tablet per 32 pounds of body weight twice a day. I have to admit that I rarely advise giving more than 2 aspirin twice a day despite the fact that some big dogs could obviously take more based on the per pound calculation.
It is extremely important to pay attention to your dog's eating habits when administering aspirin. Dogs have no way to tell you their stomach is upset so you must watch for signs of this. If your dog stops eating while you are giving aspirin it is important to stop giving the aspirin. If any signs of gastric ulcers develop (like blood tinged vomiting or depression) it is important to stop the aspirin." /s/ Mike Richards, DVM
Taken from:
http://www.vetinfo.com/daspirin.html
/s/ Phred