My dog ate grapes.
How many is too many?

Straight answer, from every major poison-control authority:

There is no safe number. Call now.

Unlike chocolate, grape and raisin toxicity has no established dose threshold. Kidney failure has been reported after small amounts, a few grapes in some dogs, while other dogs eat more with no effect. Sensitivity is unpredictable and there is no way to test for it in advance. That is why there is no calculator on this page: any ingestion is a call-the-hotline event.

Pet Poison Helpline · 855-764-7661 ASPCA Poison Control · 888-426-4435

Counts for all of these

Suspected culprit compound: tartaric acid, per recent veterinary toxicology work, which also varies unpredictably between grapes, matching the unpredictable sensitivity.

Source: ASPCA Animal Poison Control

What happens next (why calling early matters)

Kidney injury signs, vomiting, lethargy, low appetite, changes in urination, can take 12-24+ hours to appear. By then damage may be under way. Early decontamination and fluids, started before signs, are the whole game. Waiting to "see if he seems fine" is the classic mistake.

What NOT to do