My dog ate onions.
How much is toxic?

Onions, garlic, chives, and leeks damage dog red blood cells. The published toxicity threshold is about 5 grams of onion per kg of body weight in one sitting (garlic is several times more potent per gram). Signs appear DAYS later, so act on the amount, not the symptoms.

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1 · Dog's weight
2 · How much onion? (grams)
Rough guides: 1 medium onion = 150 g. Half onion = 75 g. 1 tbsp dried minced onion = 9 g (dried is ~3x potent, enter 3x). 1 garlic clove = 5 g (garlic is stronger; enter grams x 3). Onion powder in soups and baby food counts. Not sure? Guess HIGH.

How the threshold works

Published veterinary references put hematologic toxicity at roughly 5 g of onion per kg of body weight in one sitting (about 0.5% of body weight). Garlic contains more of the damaging compounds per gram, and dried or powdered forms are concentrated further. Cats are even more sensitive; any deliberate onion or garlic feeding to cats is unsafe.

Why "he seems fine" means nothing here

The damage is oxidative injury to red blood cells. Anemia signs (weakness, pale gums, fast breathing, red-brown urine) typically appear 1-5 days after ingestion. By the time signs show, the damage is done, which is why the amount eaten today is the whole decision.

What NOT to do

Do not wait for symptoms if the calculator shows over threshold. Do not forget cumulative exposure: small daily amounts of table scraps cooked with onion add up to the same injury.