Cats hide extra weight under fur, and a few hundred grams that looks like nothing on a small animal is a large share of its body. A cat at 5.4 kg and one at 4.5 kg can look much the same from across the room, yet one is a fifth heavier than it should be. The percentage over ideal weight is the number that makes this visible, and it links to the body condition score that vets use. This guide shows how to work it out, gives a table for a 4 kg ideal cat, explains what the number does and does not tell you and describes the first steps for a cat that is over. It parallels our guide on how overweight a dog is. The decision about a plan is for your vet.
The formula
percent over ideal = (current weight − ideal weight) ÷ ideal weight × 100
The ideal weight is the one thing the formula needs and cannot compute. It is a judgement from your vet, based on the cat’s frame and body condition when it was last lean. A breed average is not a substitute; cats of the same breed vary a great deal in frame.
A worked cat
A neutered cat weighs 5.4 kg. Her vet has said that her ideal weight is about 4.5 kg.
| Step | Working | Result |
|---|---|---|
| Excess | 5.4 − 4.5 | 0.9 kg |
| Percent over | 0.9 ÷ 4.5 × 100 | 20% |
| Body condition (about 10% a point) | 20 ÷ 10 | about 2 points above 5 |
| Estimated score | 5 + 2 | about 7 of 9 |
The 10% a point is a rule of thumb from Laflamme’s body condition work, and it is approximate. The vet’s hands-on score is the authority.
A table for a 4 kg ideal
| Percent over | Weight | Excess | Approximate score (9-point) |
|---|---|---|---|
| 0% | 4.0 kg | 0 | 5 |
| 10% | 4.4 kg | 0.4 kg | 6 |
| 20% | 4.8 kg | 0.8 kg | 7 |
| 30% | 5.2 kg | 1.2 kg | 8 |
| 40% | 5.6 kg | 1.6 kg | 9 |
(Current = ideal × (1 + percent ÷ 100).) A cat at 4.4 kg looks only a little rounder than at 4.0 kg, but it is already 10% over. For a small animal, the numbers move quickly: half a kilogram is one eighth of the cat.
What the percentage does not tell you
The percentage assumes the ideal weight is right. If the ideal is 4.0 kg but the cat’s frame suggests 4.5 kg, a weight of 4.8 kg is 20% over by the first number and 6.7% over by the second. The percentage also does not distinguish fat from muscle, and a cat that has lost muscle with age can be a normal weight and still have too much fat. That is why the body condition score and the muscle condition score matter: they look and feel, rather than only weigh. WSAVA offers tools for both. Ask your vet to show you how to feel your cat’s ribs and waist.
| Check | What you do |
|---|---|
| Ribs | Feel along the side; they should be easy to feel under a thin layer |
| Waist | View from above; there should be a taper behind the ribs |
| Belly | View from the side; the belly should tuck up, not hang |
| Fat pad | A hanging pad under the belly is normal in small amounts, not large |
Why the first step is the vet
Cats that lose weight too fast, or stop eating for a spell, can become seriously ill, and an overweight cat is more at risk. So a weight-loss plan for a cat should be agreed with the vet, who can check that there is no medical cause for the weight, set a target and an expected pace and book weigh-ins. Do not stop feeding or cut the food by a large amount on your own.
Setting the pace and the date
The AAHA range of 1 to 2% of body weight a week is the general guide for a safe pace; for cats your vet decides. For the 5.4 kg cat with 0.9 kg to lose:
| Weekly rate | Weekly loss | Weeks to lose 0.9 kg | Goal date from 19 Oct 2026 |
|---|---|---|---|
| 1% | 54 g | 16.7 | 13 February 2027 |
| 2% | 108 g | 8.3 | 17 December 2026 |
The straight-line sums are in our guide to how to calculate a pet weight goal date. The plan may be slower than 1% if the vet thinks it wise.
Calories for an overweight cat
In our calculators, a cat’s weight-loss energy is 0.8 × the resting energy requirement of the ideal weight, and a vet may adjust it. For an ideal weight of 4.5 kg, RER = 70 × 4.5^0.75 = 216 kcal, so 0.8 × 216 = 173 kcal a day as a starting point. The maintenance amount for the same cat at 1.2 × RER of her ideal weight is 260 kcal.
| Plan | Working | kcal a day | Dry food at 3.8 kcal/g |
|---|---|---|---|
| Maintenance at current weight (5.4 kg) | 1.2 × 70 × 5.4^0.75 | 298 | 78 g |
| Maintenance at ideal weight (4.5 kg) | 1.2 × 70 × 4.5^0.75 | 260 | 68 g |
| Weight-loss start (ideal weight) | 0.8 × 70 × 4.5^0.75 | 173 | 46 g |
The weight-loss start is 42% below maintenance for the current weight, a large cut. This is the kind of number to discuss with the vet, who may use a different starting point or a diet made for weight loss, since a smaller portion of an ordinary food can leave a cat short of nutrients. Our Cat Calorie Calculator shows the figures, and indoor cat calorie needs explains the factors.
Tracking
Weigh the cat on a scale that reads to 10 to 50 g; see how to weigh a cat at home. Weigh every one to two weeks during a plan, as your vet advises, and record the weights. The Pet Weight Change Calculator turns two weights into a percentage and a weekly pace. If a cat loses faster than the agreed pace, call the vet.
A worked example
Sam’s cat is 5.4 kg. The vet’s ideal weight is 4.5 kg, and the plan is a slow loss.
| Step | Working | Result |
|---|---|---|
| Percent over | 0.9 ÷ 4.5 | 20% |
| Weekly loss at 1% | 5.4 × 0.01 | 54 g |
| Check at 4 weeks | 5.4 − 4 × 0.054 | about 5.18 kg |
| Check at 8 weeks | about 4.97 kg | |
| Check at 12 weeks | about 4.75 kg | |
| Check at 16 weeks | about 4.54 kg | |
| Goal | 16.7 weeks | 4.5 kg |
Sam weighs the cat every two weeks on a baby scale and sends the numbers to the vet’s nurse. At the 4-week check the cat is 5.2 kg, close to the expected 5.18. Sam keeps the food the same and the vet confirms the plan.
A useful pattern
For a cat that is a given percentage over, the weight to lose is that same percentage of the ideal weight, and the time at a fixed weekly rate depends only on the percentage, not on the size of the cat. A cat 20% over ideal needs to lose 0.2 of its ideal weight.
| Ideal weight | Current at 20% over | Weight to lose | Weeks at 1% a week |
|---|---|---|---|
| 3.5 kg | 4.2 kg | 0.7 kg | 16.7 |
| 4.0 kg | 4.8 kg | 0.8 kg | 16.7 |
| 4.5 kg | 5.4 kg | 0.9 kg | 16.7 |
| 5.0 kg | 6.0 kg | 1.0 kg | 16.7 |
| 6.0 kg | 7.2 kg | 1.2 kg | 16.7 |
Every one of these cats takes 16.7 weeks at 1% of its current weight a week, because 0.2 ÷ (1.2 × 0.01) is the same for all. At 2% a week, the time halves to 8.3 weeks. The pattern helps you judge a plan: if a plan for a cat 20% over promises a goal in a few weeks, it is faster than the general range, and you should ask the vet why.
Common mistakes
- Using a breed average as the ideal weight.
- Relying on the percentage without a hands-on check.
- Cutting food sharply.
- Using a coarse scale.
- Counting treats separately from the plan.
- Using the adult formula for a kitten.
- Stopping weigh-ins at the goal.
Quick summary
- Percent over ideal = (current − ideal) ÷ ideal × 100.
- A 5.4 kg cat with a 4.5 kg ideal is 20% over, roughly body condition 7 of 9 at about 10% a point.
- Your vet sets the ideal weight and the plan; fast loss can be dangerous for cats.
- A weight-loss start in our calculator is 0.8 × RER of the ideal weight (173 kcal here).
- Weigh every one to two weeks on a fine scale.
- Use the Cat Calorie Calculator.
Frequently asked questions
How do I know how overweight my cat is?
Subtract the ideal weight from the current weight, divide by the ideal weight and multiply by 100.
Who decides my cat's ideal weight?
Your vet, from the cat's frame and body condition. A breed average is a rough guide only.
How does percent over relate to body condition score?
On the 9-point scale, with 5 as ideal, each point above is roughly 10% over ideal weight, according to Laflamme (1997).
Is 10% over ideal a problem?
Your vet can say. Body condition, health and age all matter.
How fast should an overweight cat lose weight?
Slowly and under veterinary supervision. Cats that lose weight too fast can become seriously ill.
Can I use this for a kitten?
No. Kittens are still growing, so a fixed ideal weight does not apply.
Sources
- Laflamme D.P. (1997) Development and validation of a body condition score system for cats. Feline Practice 25:13-18
- AAHA (2014) Weight Management Guidelines for Dogs and Cats. Journal of the American Animal Hospital Association 50:1-11
- WSAVA Global Nutrition Guidelines: body condition score and muscle condition score tools
Related calculators
- Cat Calorie CalculatorWork out how many calories your cat needs each day, from its weight and life stage.
- 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.
Keep reading
- How Overweight Is My Dog? Percent Over IdealHow to work out how overweight a dog is: percent over ideal = (current − ideal) ÷ ideal × 100, the 10% per condition point rule, examples and next steps.
- 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.
- Indoor Cat Calorie NeedsHow many calories an indoor cat needs: RER × 1.0 if inactive versus 1.2 if neutered, a 3 to 6 kg table in kcal and grams, and how to adjust by weight trend.
- How to Read a Cat Food LabelHow to read a cat food label: guaranteed analysis, moisture, dry-matter and Atwater calorie estimates, a worked dry and wet comparison and the feeding chart.