Dog Onion Toxicity Calculator (grams of onion per kg body weight)

Understanding the dog onion toxicity calculator — concept and principle

This calculator compares the amount of raw onion a dog has eaten with the dose range at which the Merck Veterinary Manual reports clinical signs, 15–30 g per kg of body weight. Use it to put a number on an accident while you call your vet.

Onion dose per kilogram: with the default inputs the result is 15 g/kg (in the 15–30 g/kg range).

How it is calculated

Dose (g/kg) = onion eaten (g) ÷ dog's weight (kg)

As % of body weight = dose ÷ 10.
Compared with 15 g/kg = dose ÷ 15.
Amount that reaches 15 g/kg = 15 × weight; 30 g/kg = 30 × weight.
Assessment: below 15 g/kg, 15–30 g/kg range, or above 30 g/kg.

The principle

Onions, garlic, leeks and chives (the Allium family) contain sulfur compounds that damage red blood cells. The damaged cells form Heinz bodies and are removed from the blood, causing anaemia. Merck reports clinical signs in dogs after eating 15–30 g of raw onion per kg, that is 1.5–3% of body weight. The damage builds over the first day and peaks at about 72 hours, with red-cell breakdown typically 3–5 days after eating, so a dog can look normal at first. Cats are more sensitive than dogs.

Worked example

Default inputs: 10 kg dog, 150 g of onion eaten.

Dose = 150 ÷ 10 = 15 g/kg.
As body weight = 15 ÷ 10 = 1.5%.
Compared with 15 g/kg = 15 ÷ 15 = 1×.
Amounts: 15 g/kg = 15 × 10 = 150 g; 30 g/kg = 300 g.
Assessment: 15–30 g/kg range.

Each point is the calculator re-run with only “Dog’s weight (kg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Dog’s weight (kg)	Onion per kg of body weight (g/kg)
2.5	60
5	30
7.5	20
10	15
12.5	12
15	10
20	7.5

How to read the result

Where does your result fall?

A result in or above the 15–30 g/kg range matches the doses linked with clinical signs and needs prompt veterinary advice. Below the range is not a guarantee of safety: concentrated forms, repeated small amounts and sensitive breeds can cause problems at lower totals. Watch for weakness, pale gums, fast breathing and red or brown urine in the following days.

Common mistakes

Using this calculator for onion powder, dried onion or garlic, which are much more concentrated.
Forgetting onion hidden in gravy, soup, baby food or leftovers.
Assuming cooking removes the toxin; it does not.
Relaxing because the dog seems fine on the first day.

Keep in mind: This is not a diagnosis. If your dog has eaten onion, garlic or similar foods, call your vet or an animal poison helpline now; do not wait for signs.

Frequently asked questions

How much onion is toxic to a dog?

Merck reports clinical signs after about 15–30 g of raw onion per kg of body weight.

Is cooked onion safe for dogs?

No. Cooking does not destroy the compounds that damage red blood cells.

When do symptoms of onion poisoning appear?

Red-cell damage peaks around 72 hours and breakdown typically shows 3–5 days after eating.

Sources: Merck Veterinary Manual, C. Hayes, "Garlic and Onion (Allium spp) Toxicosis in Animals"

Checked 9 Nov 2026. Merck Veterinary Manual, C. Hayes (ASPCA Animal Poison Control Center), “Garlic and Onion (Allium spp) Toxicosis in Animals”: clinical signs in dogs after 15–30 g/kg raw onion; cats after 5 g/kg raw onion or less than a teaspoon of cooked onion; Heinz bodies within 24 h, peak about 72 h, haemolysis 3–5 days after exposure.

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