A number on a scale means little until you know what it was last time. Whether a 0.8 kg difference is good news, bad news or nothing depends on the pet’s size, the time between weighings and what the plan was. The way to read it is to turn two weights into a percentage and, if the weighings are some weeks apart, into a weekly pace. This guide shows the arithmetic, a table of examples that you can compare with, how to tell real change from the noise of scales and bowls and what to do with the answer. The Pet Weight Change Calculator does the sums.
The formulas
total change % = (current weight − start weight) ÷ start weight × 100 weekly pace % = total change % ÷ number of weeks change in kg per week = (current − start) ÷ weeks
A 20 kg dog now weighs 19.2 kg four weeks later. Change = (19.2 − 20) ÷ 20 × 100 = −4.0%. Weekly pace = −4.0 ÷ 4 = −1.0% a week, or −0.2 kg a week.
A negative number is loss and a positive number is gain.
What the pace means
AAHA’s 2014 Weight Management Guidelines for Dogs and Cats describe a safe rate of loss of about 1 to 2% of body weight a week. The calculator compares your pace with that range and calls it slow, within range or fast.
| Weekly change | Description (for loss) |
|---|---|
| Less than 1% a week | Slow |
| 1 to 2% a week | Within the AAHA range |
| More than 2% a week | Fast; check with your vet |
For gain, the same numbers tell you how quickly a pet is putting on weight. A puppy is meant to gain weight, so a positive pace is normal there, and your vet will tell you what is right for its age and breed.
A table of examples
| Start | Now | Weeks | Total change | Weekly pace | kg a week | Pace for loss |
|---|---|---|---|---|---|---|
| 20.0 kg | 19.2 kg | 4 | −4.0% | −1.00% | −0.20 | Within |
| 20.0 kg | 19.0 kg | 4 | −5.0% | −1.25% | −0.25 | Within |
| 20.0 kg | 18.8 kg | 4 | −6.0% | −1.50% | −0.30 | Within |
| 20.0 kg | 19.9 kg | 4 | −0.5% | −0.13% | −0.03 | Slow |
| 10.0 kg | 10.5 kg | 4 | +5.0% | +1.25% | +0.13 | Gain |
| 4.0 kg | 3.8 kg | 4 | −5.0% | −1.25% | −0.05 | Within, but see note |
| 30.0 kg | 28.5 kg | 8 | −5.0% | −0.63% | −0.19 | Slow |
The cat in the sixth row lost 200 g, which is 5% of its weight in four weeks. By the percentage it is within the range, but for a cat that is a large change in a short time, and cat weight loss should be supervised by a vet. Percentages are the same for any animal; what is safe is not.
Noise versus signal
Every weighing has error. Food and water in the gut, a recent toilet trip, the scale’s rounding and the way the pet stands all move the reading by a percent or two. The table shows how big a change has to be before it stands out from the noise.
| Pet weight | 1% is | 2% is | 5% is |
|---|---|---|---|
| 4 kg | 40 g | 80 g | 200 g |
| 10 kg | 100 g | 200 g | 500 g |
| 20 kg | 200 g | 400 g | 1.0 kg |
| 40 kg | 400 g | 800 g | 2.0 kg |
For a 4 kg cat, 40 g is a spoonful of food; a scale that reads to 100 g cannot show a 1% change. Use a scale that is fine enough for the pet, as our guide to weighing a dog at home explains, and use the same method each time. A single reading is rarely enough; a change you can see across three or more readings is real.
Reading a trend over many weeks
Keep a log and look at the line. A good log has the date, the weight, the method and any notes about food or health.
| Date | Weight (kg) | Change since last | Change since start | Notes |
|---|---|---|---|---|
| 1 Sep | 20.0 | Start | ||
| 15 Sep | 19.7 | −1.5% | −1.5% | |
| 29 Sep | 19.5 | −1.0% | −2.5% | |
| 13 Oct | 19.4 | −0.5% | −3.0% | Slower |
| 27 Oct | 19.4 | 0.0% | −3.0% | Plateau |
The pace was 0.75% a week at first, then fell. This is the common pattern of a diet: quick at first and slower later. If it stalls for two to three weeks with careful measuring, the troubleshooting steps in dog not losing weight are the next step.
Gaining weight
The same formulas show gain. A pet gaining 1% a week for ten weeks has added about 10%, which for a 20 kg dog is 2 kg. Slow gain is easy to miss on the scale and in the mirror, which is why a log is valuable. If the trend is up and the pet is an adult who should hold steady, check the food amount and treats and tell your vet; our guide to weight gain after neutering shows a common cause.
Growing puppies and kittens
Young animals should gain weight, and the right pace depends on breed and age. Use the calculator to see the weekly percentage, and ask your vet or breeder for the expected pattern. A puppy that gains much faster or slower than expected deserves a conversation. Our puppy feeding chart shows how food amounts change with weight.
Seniors and sick pets
An older pet that loses weight without a plan should be seen by a vet, since weight loss can be a sign of a problem. A log with weights and dates helps your vet. If your pet is unwell or on treatment, follow your vet’s schedule for weighing and ask what changes matter.
From the trend to a plan
If the pace is not what you want, the options are to adjust food by about 10%, increase gentle activity within your vet’s advice, or ask your vet to review the plan. Use the Pet Weight Goal Date Calculator to see how long it will take to reach a goal at a given pace, and our guide to how long it will take a dog to lose weight for the arithmetic.
A worked case
Sam’s 12 kg dog was 12.0 kg on 1 October, 11.8 on 15 October and 11.5 on 29 October. Total change after four weeks: (11.5 − 12.0) ÷ 12.0 × 100 = −4.2%. Weekly pace: −1.04%. That is within the 1 to 2% range, and the dog is on track. The 15 October reading was −1.7% in two weeks (−0.83% a week), a bit slower, and the second fortnight −2.5% (−1.25% a week). The point of the calculation is that Sam can see the plan working without being thrown by one reading.
A slow change is still a change
Small monthly changes add up. A weekly pace looks tiny until it is carried forward.
| Start | Weight 4 weeks later | Change | Weekly pace |
|---|---|---|---|
| 24.0 kg | 23.4 kg | −2.5% | −0.63% |
| 4.00 kg | 4.08 kg | +2.0% | +0.50% |
| 12.0 kg | 12.0 kg | 0% | 0% |
A cat gaining 0.5% a week gains about 26% in a year if the pace holds, though it rarely does. The first dog is losing below the 1 to 2% a week range in AAHA guidance, which is gentler than the ceiling and can suit a pet on a plan, though your vet decides the pace. The useful habit is to compare each weight with the one before and the one at the start, so a small drift is seen early, while a single odd reading is not mistaken for a trend.
Common mistakes
- Comparing readings from different scales.
- Judging a plan by a single weigh-in.
- Using kilograms of change instead of percentages for different-sized pets.
- Expecting a straight line.
- Applying the dog pace to cats without a vet.
- Weighing too often.
- Not logging.
Quick summary
- Change % = (current − start) ÷ start × 100; weekly pace = change % ÷ weeks.
- AAHA describes about 1 to 2% of body weight a week as a safe pace of loss.
- A 20 kg dog at 19.2 kg after 4 weeks has lost 4%, or 1% a week.
- Scales and bowls add 1 to 2% of noise, so look at the trend over several weighings.
- Cat weight loss should be gradual and supervised; puppies and kittens should gain.
- Use the Pet Weight Change Calculator.
Frequently asked questions
How do I calculate my pet's weight change?
Subtract the start weight from the current weight, divide by the start weight and multiply by 100. Divide by the weeks for the weekly pace.
What is a safe pace of weight loss?
AAHA's weight management guidelines describe about 1 to 2% of body weight a week. Your vet sets the plan for your pet.
What counts as a significant change?
A change of about 5% or more in a month, or a steady drift across several weigh-ins, deserves attention and a mention to your vet.
Why do weights bounce around?
Food, water, toileting and scales all add noise of a percent or two. Look at the trend, not a single reading.
Does the same maths work for cats?
Yes, but weight loss in cats should be gradual and supervised by a vet, since rapid loss is risky.
How often should I weigh?
Monthly for healthy adults, weekly for puppies and kittens, and every one to two weeks on a weight plan.
Sources
- AAHA (2014) Weight Management Guidelines for Dogs and Cats
- Pure arithmetic: percentage change = (current − start) ÷ start × 100
Related calculators
- Pet Weight Change CalculatorTrack a weight change over time: percent change, percent per week and whether the pace is inside 1–2% a week.
- Pet Weight Goal Date CalculatorEstimate the date a pet could reach its goal weight at a steady weekly loss, inside the 1–2% a week range.
- Dog Weight Loss CalculatorPlan a safe weight-loss target: daily calories and how many weeks it should take.
Keep reading
- How to Weigh a Dog at Home (Small to Giant)How to weigh a dog at home for any size: baby scale, hold-and-subtract, a vet or pet-shop scale, how big the error is for each and how to log a trend.
- How Long Will It Take My Dog to Lose Weight?How long dog weight loss takes at 1% and 2% of body weight a week: a weeks-to-goal table, the formula behind it, why progress slows and how to track it.
- Dog Not Losing Weight? A Troubleshooting GuideWhy a dog may not be losing weight on a diet: measuring errors, treats, scales, plateaus, medical causes and when to ask your vet to review the plan.
- How to Weigh a Cat at HomeHow to weigh a cat at home: baby scale, kitchen scale and box, carrier method or hold-and-subtract, how big the error is for a 3 to 6 kg cat and how to log weights.