How much weight loss is serious in a dog or cat? The 5% rule
Nobody sees gradual weight loss on an animal they look at every day. The change hides in plain sight — a little less over the ribs each week, fur covering the rest — until a relative who hasn't visited in months says something. A scale sees it immediately, which is why weight is the most underrated number a home can track.
The rule vets actually use
The working convention in veterinary practice: a loss of about 5% of body weight over roughly a month is worth a call — not a panic, a call. It is the line where "probably nothing" stops being the safe assumption, for dogs and cats alike. Faster loss, or loss that continues month after month, moves up the priority list accordingly.
What 5% actually looks like — and why small animals fool scales
| 30 kg dog | 4 kg cat | |
|---|---|---|
| 5% of body weight | 1.5 kg — obvious on any scale | 200 g — inside the noise of a bathroom scale |
That right-hand column is the trap. A bathroom scale that drifts 100–200 g between readings can manufacture — or hide — a cat's entire 5%. For a small animal, weigh again before acting on any single reading, and treat two consistent readings as one data point.
How to weigh a pet properly at home
- Small animals — the hold-the-pet method: weigh yourself holding the pet, then yourself alone, and subtract. Same scale, same spot on the floor.
- Same conditions every time: same scale, same time of day, before food.
- Weekly, not daily. Day-to-day water and gut contents swamp the signal; week-to-week is where the trend lives.
- Write it down with the date. A number without a date can't make a trend.
If the trend is real, what might be behind it
That is genuinely your vet's question, not the internet's — but it's worth knowing the shape of it. In older cats, gradual loss with a normal or even ravenous appetite is a classic presentation of hyperthyroidism, and loss with increased drinking points toward kidney disease — both common, both manageable, both far better caught early. In dogs, dental pain, digestive disease, heart disease and worse all start as "just losing a bit of weight". The 5% call exists so these get found at the cheap, treatable stage.
When to call
- About 5% down over a month — confirmed by a second weighing for a small animal.
- Slower loss that continues for two or three months.
- Any loss alongside eating less, drinking more, or vomiting — mention them together.
Noticing is the job. Vetalio makes it stick.
Weight, breathing, appetite and medication — tracked at home, turned into trends and honest alerts, and printed on one page your veterinarian can read.
Join the waitlistVetalio is not a veterinary service. This guide is general information, not a diagnosis — when in doubt, call your vet.