Why your pet's weight is the health signal worth tracking

If you only track one number about your pet, track their weight. It's cheap to measure, it needs no equipment beyond a scale, and it moves before most other signs do. Weight change is one of the earliest measurable signals that something is wrong — often weeks or months before an owner notices a behavioural change.

The catch is that it's only useful as a trend. A single number tells you almost nothing.

This is general guidance, not veterinary advice. Any weight change you're unsure about is worth a phone call to your vet.

Why the trend matters more than the number

"Is 18 kg healthy for a Border Collie?" is close to unanswerable. Healthy ranges vary by breed, frame, age and sex, and the ranges themselves are wide.

"Has this Border Collie lost 2 kg over four months without a change in diet?" is very answerable. That's a red flag, and you don't need to know the breed standard to see it.

This is the core idea: your pet is their own baseline. Once you have a handful of entries, deviation from their own normal is far more informative than comparison to a chart.

The percentage rule of thumb

Because pets vary so much in size, changes are best read as percentages of body weight rather than absolute amounts.

A commonly used threshold in small-animal practice is that an unexplained loss of around 5% of body weight is worth investigating, and 10% is significant.

Look at the cat row. 225 grams. That is genuinely invisible by eye and by feel — you will not notice it while stroking your cat, and you will not notice it in a photo. It is trivially visible on a chart. This is the entire argument for writing the numbers down.

The word doing the work in that threshold is unexplained. Weight coming off during a deliberate, vet-supervised diet is the plan working. The concern is weight moving when nothing about the pet's routine has changed.

How to weigh consistently

Inconsistent weighing produces noise that looks like signal. Four rules:

Same scale every time. Two scales can disagree by several hundred grams, which for a cat is the entire threshold you're watching for. If you weigh at the clinic, use the clinic scale each time.

Same time of day, ideally before a meal. A large dog can swing over a kilogram across a day depending on food and water.

For small pets, weigh yourself, then yourself holding the pet, and subtract. Household bathroom scales are imprecise at low weights. For cats and small dogs, a kitchen scale under a carrier is more accurate — just remember to subtract the carrier.

Record it immediately. A number you plan to write down later is a number you won't write down.

How often

• Puppies and kittens: weekly. They should be gaining steadily, and a plateau is worth asking about.
• Healthy adults: monthly, or at minimum at every vet visit.
• Seniors (roughly 7+ for dogs, 10+ for cats): monthly at least. Gradual weight loss is common in older animals and easy to write off as "just ageing" when it's often treatable.
• Any pet on a weight management plan or with a chronic condition: whatever interval your vet asks for, usually weekly or fortnightly.

Pair the number with a body condition check

Weight alone misses changes in body composition — an animal can lose muscle and gain fat at a stable weight. A 30-second hands-on check catches that:

• Ribs. You should feel them easily under a thin layer of fat, without pressing hard. Ribs you can see clearly at rest suggest underweight; ribs you can't find suggest overweight.
• Waist from above. There should be a visible narrowing behind the ribs.
• Profile from the side. The belly should tuck up toward the hind legs rather than running straight or sagging.

Do this at the same time you weigh. Two data points, thirty seconds.

When to call the vet

Contact your vet if you see:

• Unexplained loss of 5% or more of body weight
• Steady loss across three or more consecutive measurements
• Rapid gain without a change in food, especially with a swollen abdomen
• Weight loss alongside increased thirst, increased appetite, vomiting, or lethargy
• Any loss at all in a senior pet, however small
• A young animal that has stopped gaining

Weight loss with a normal or increased appetite is particularly worth acting on quickly. In cats it's a classic presentation for hyperthyroidism and diabetes; in dogs it can point to malabsorption or other conditions. Both are far more manageable when caught early.

Make it a two-second habit

The failure mode isn't weighing wrong, it's not weighing at all — or weighing, noting it on a scrap of paper, and losing the scrap.

Petnote's weight tracker exists for exactly this: log a weight with a date, and the chart draws the trend for you. A slow decline that's invisible month to month is obvious as a line. The Free plan charts the last three months; Pro keeps the complete history and exports it to CSV — useful when a new vet wants to see two years of data rather than hear you describe it.

The short version

• Track the trend, not a single number — your pet is their own baseline.
• Roughly 5% unexplained loss warrants a call; 10% is significant.
• Same scale, same time of day, recorded straight away.
• Monthly for healthy adults, weekly for the very young and for seniors.
• Pair the number with a hands-on body condition check.

Start tracking free — one pet, a full health record and an Emergency Mode QR tag, no card required.