A dry food printing 32% protein and a can printing 10% protein are not the same food described at different strengths. One is 10% water, the other 78%. Convert both to dry matter and the gap that matters appears: the dry food is usually 30 to 50% carbohydrate, the can 5 to 15%. Two arithmetic steps get you there, and both can be done in your head at the shelf.
The guaranteed analysis is a set of limits, not a measurement
Every number on the back panel is a minimum or a maximum the manufacturer guarantees, not a typical value. Protein 10% minimum means the batch tested at least 10%. Fiber 1.5% maximum means no more than that. Carbohydrate is not listed at all, because regulators do not require it, and ash is often omitted too. That is why the calculation below is an estimate with a known direction of error: using minimum protein and fat with maximum fiber and moisture pushes the carb figure upward, sometimes by 3 to 5 points. It is still the only way to compare two bags, and it is far better than reading the front of the package.
Step one: strip out the water
Dry matter (DM) is everything except water. Divide any nutrient by the dry matter fraction and multiply by 100.
A can with 78% moisture is 22% dry matter. Its 10% protein becomes 10 ÷ 22 × 100 = 45% protein on a dry matter basis. The same can next to a dry food with 32% protein on 90% dry matter (35.6% DM) suddenly looks like the higher-protein food. This single conversion resolves most arguments about wet versus dry, and it is the step people skip.
Step two: nitrogen-free extract
Carbohydrate by difference, also called nitrogen-free extract (NFE), is whatever is left after the other components are subtracted:
NFE (as fed) = 100 − protein − fat − fiber − ash − moisture
Then divide by the dry matter fraction. Worked example, the same can: 100 − 10 − 5 − 1.5 − 2.5 − 78 = 3.0% NFE as fed. On dry matter, 3.0 ÷ 22 × 100 = 13.6% carbohydrate.
A typical dry food: 100 − 32 − 15 − 3 − 7 − 10 = 33% NFE as fed, and 33 ÷ 90 × 100 = 36.7% carbohydrate on dry matter. The two foods differ by a factor of 2.7, and nothing on either label said so.
Sanity check with calories: 3.5 kcal per gram of protein, 8.5 per gram of fat, 3.5 per gram of NFE, the modified Atwater values. The dry food works out to about 355 kcal per 100 g of dry matter, or 3.2 kcal per gram of food, which is in the normal range for extruded kibble. If your numbers land far outside 3.0 to 4.5 kcal per gram of dry matter, a figure is missing from the panel.
Where commercial food actually sits
Dry food runs 30 to 50% carbohydrate on a dry matter basis, and most of the mid-priced bags sit near 40%. Wet food runs 5 to 15%, because a can needs less starch to hold its shape and often uses meat rather than cereal as the cheap bulk. Pate-style cans at the low end can be 2 to 5%. Freeze-dried raw foods land between 5 and 20% depending on how much vegetable matter is added. Prescription glycemic diets cluster at 5 to 10%, and they are not magic: they are simply formulated to a carb ceiling and a protein floor at the same time.
A cat has no dietary requirement for carbohydrate. It can run perfectly well on 5% DM and it can also live on 35% DM, which is what most cats in the world actually eat. The number matters at two points: when the cat is diabetic, and when the cat is overweight and the calories are coming mostly from starch.
The 10% line for a diabetic cat
Most veterinary internists set a ceiling of 10% carbohydrate on a dry matter basis for a cat with diabetes, and many aim at 5%. The reason is physiological: cats lack the hepatic enzymes to buffer a large glucose load, so a starch-heavy meal produces a wide post-meal spike that insulin cannot track. Feeding a food under 10% DM while keeping the same insulin dose is also the fastest way to cause hypoglycemia, which is why the switch has to be planned with the vet and the dose usually comes down 20 to 50% in the first weeks. A glucose curve at 7 to 14 days after the switch is standard, not optional. More on the disease side is in diabetes in cats.
Grain-free is not low-carb
Removing grain usually means adding potato, tapioca, pea, or lentil, and those are starch and fiber with a different label. A grain-free dry food commonly tests at 35 to 45% DM carbohydrate, identical to the grain-based bag beside it. The useful markers on a label are the protein and fat minimums on a dry matter basis and the calorie density in kcal/kg, not the word grain-free. How to read the rest of the panel, including the difference between minimums and typical values, is in reading cat food labels.
Where the common foods land
| Food type | Carbohydrate, % DM | Best suited to |
|---|---|---|
| Dry kibble, typical | 30-50% | Healthy, active, normal weight cats |
| Dry kibble, grain-free | 30-45% | No advantage for most cats; check the number, not the claim |
| Standard canned, gravy or chunks | 10-20% | Cats that need more water intake |
| Pate canned, meat first | 2-8% | Diabetic cats, weight loss, urinary support |
| Freeze-dried raw, rehydrated | 5-20% | Owners willing to rehydrate and pay 3-4x kibble price |
| Prescription glycemic diet | 5-10% | Diabetic cats under veterinary supervision |
| Home-cooked meat base | 0-5% | Only with a formulated recipe and a supplement plan |
Switching without a crisis
Change over 10 to 14 days, not overnight. Days 1 to 3, one quarter new food by weight; days 4 to 6, half; days 7 to 9, three quarters; day 10 onward, all new. Weigh the portions rather than scooping, because a wet food at 80 kcal per 100 g and a dry food at 380 kcal per 100 g are not interchangeable by volume. Expect stool to soften for 3 to 4 days and settle; diarrhea past day 5 means the pace was too fast. In a diabetic cat, test urine glucose and ketones daily for the first 2 weeks and keep the vet informed, since the insulin requirement can drop before the food transition is even finished. Why water content changes so much of this is covered in wet vs dry food.
Five steps at the shelf
- Find moisture, protein, fat, fiber, and ash; if ash is missing, assume 6 to 8% and note it.
- Convert protein and fat to dry matter before comparing anything.
- Run the NFE formula and divide by dry matter.
- Compare kcal/kg as well, since a denser food means smaller portions and less total starch per day.
- For a diabetic cat, aim under 10% DM and book the glucose curve before the first bowl.
Frequently asked
How do I calculate carbohydrate content from a cat food label?
Subtract protein, fat, fiber, ash, and moisture from 100 to get nitrogen-free extract as fed, then divide by the dry matter fraction and multiply by 100. A can with 78% moisture, 10% protein, 5% fat, 1.5% fiber, and 2.5% ash works out to 3% as fed, which is 13.6% carbohydrate on a dry matter basis.
Is grain-free cat food lower in carbohydrates?
Usually not. Grain-free recipes swap wheat and corn for potato, tapioca, pea, or lentil, which are starch and fiber under a different name, and most test at 30 to 45% carbohydrate on a dry matter basis. Judge a food by its dry matter protein, fat, and carbohydrate numbers, not by the grain-free claim on the front.
Can I switch a diabetic cat to low-carb food without changing insulin?
No, and doing so is the fastest route to hypoglycemia. A drop from 35% to under 10% carbohydrate on a dry matter basis can cut insulin needs by 20 to 50%, so the switch should be planned with the vet, the dose adjusted in advance, and a glucose curve run 7 to 14 days later.