Unlike chocolate, grape and raisin poisoning cannot be worked out from a dose chart. Here is what is known, what is not, and why the advice is always to call a vet.
People search for a grape toxicity calculator because a chocolate one exists. Chocolate works as a calculation because the toxic compound, theobromine, has a known concentration in each type of chocolate and a known dose at which signs appear.
Grapes do not work that way. Some dogs have eaten large amounts with no effect; others have developed kidney failure after a small handful. Reported cases have not shown a reliable relationship between the amount eaten and the outcome, and there is no way to tell in advance which dog will react. Any tool that told you a certain number of grapes was safe for your dog's weight would be inventing that answer.
For decades the toxic principle was unknown. In 2021, veterinary toxicologists at the ASPCA Animal Poison Control Center reported that tartaric acid, which grapes contain in varying amounts, is the likely cause, after noticing that dogs were also poisoned by homemade play dough made with cream of tartar.
That finding helps explain the unpredictability: tartaric acid content varies between grape varieties, growing conditions and ripeness, so two handfuls of grapes may carry very different amounts. It also means the risk applies to every form — fresh grapes, raisins, sultanas, currants, and foods that contain them such as fruit cake, hot cross buns, trail mix and cereal bars.
If you get there soon enough, usually within an hour or two, a vet will normally give an injection to make your dog vomit and may follow it with activated charcoal. Many vets then recommend intravenous fluids for 48 hours and blood tests to check kidney values, because kidney injury can develop over one to three days before it shows.
Do not make your dog vomit at home unless your vet tells you to and tells you how. Salt water is dangerous, and home methods can cause more harm than the grapes.
In the US, the ASPCA Animal Poison Control Center (888-426-4435) and the Pet Poison Helpline (855-764-7661) can advise you and your vet; both may charge a consultation fee.
Vomiting or diarrhoea, often within hours, sometimes with pieces of grape visible
Lethargy, weakness and loss of appetite
Abdominal pain — a hunched posture or reluctance to be touched
Drinking and urinating more at first, then producing much less urine or none — the sign of kidney failure
A dog that seems well for the first day is not necessarily in the clear. Keep your vet informed and do not cancel follow-up blood tests because your dog looks normal.
Grapes are not the only household food where the honest answer is "no amount is known to be safe". Xylitol, a sweetener in some sugar-free gum, peanut butter and baked goods, causes a dangerous drop in blood sugar and liver injury in dogs at very small doses. Onions, garlic, leeks and chives damage red blood cells, and the effect is cumulative. Macadamia nuts cause weakness and tremors.
For chocolate, where a dose really can be estimated, use our dog chocolate toxicity calculator — and still call your vet.
Most grape and raisin poisonings are accidents with food left within reach: fruit bowls on low tables, lunch boxes in bags on the floor, and children sharing snacks. Keep grapes and raisin-containing foods out of reach, tell visitors and children, and do not use grapes as training treats.
Nobody can say. Some dogs react to a few grapes and others to none after eating many, and reported cases show no reliable dose. That is why the advice for any amount is to call a vet straight away.
Call your vet. Many dogs will be fine after one grape, but because the risk cannot be predicted, most vets would rather hear about it and decide with you whether to make your dog vomit.
Raisins, sultanas and currants are dried grapes, so the same compounds are concentrated into a smaller, easier-to-eat package. Treat them at least as seriously as fresh grapes.
Vomiting often starts within a few hours. Kidney injury can take 24 to 72 hours to show, which is why vets often recommend fluids and blood tests over two to three days even if the dog seems well.