A bag that says crude protein 26% and a can that says 10% are not 2.6 times apart. The bag is 10 percent water and the can is 78 percent, so on a dry matter basis the numbers read 29 percent and 45 percent, and the can wins. Guaranteed analysis is a set of legal floors, not a nutrition summary, and every comparison needs one conversion first.
What the 5 lines actually promise
Guaranteed analysis lists crude protein as a minimum, crude fat as a minimum, crude fiber as a maximum, moisture as a maximum, and ash as a maximum. A 26 percent protein claim means no scoop tested below 26, not that the food averages 26; most products run 1 to 3 points above the floor. Fiber at 4 percent max can be 1.5 percent in the bag, and moisture at 10 percent max can be 7. The floors are the manufacturer's legal promise, and they are also the number the regulator checks.
The dry matter conversion
Dry matter (DM) percent = 100 − moisture. Then divide the nutrient by DM and multiply by 100. Kibble at 26 percent protein and 10 percent moisture: DM = 90, protein DM = 26 ÷ 90 × 100 = 28.9 percent. Canned at 10 percent protein and 78 percent moisture: DM = 22, protein DM = 10 ÷ 22 × 100 = 45.5 percent. The can carries 16.6 points more protein per unit of dry matter, and it also costs more per calorie, which is a separate argument.
| Food | Moisture | Protein as fed | Dry matter | Protein in DM | kcal per 100 g |
|---|---|---|---|---|---|
| Adult chicken kibble | 10% | 26% | 90% | 28.9% | 380 |
| Light kibble | 10% | 30% | 90% | 33.3% | 320 |
| Kitten kibble | 10% | 34% | 90% | 37.8% | 410 |
| Canned pâté | 78% | 10% | 22% | 45.5% | 88 |
| Chunks in gravy | 82% | 8% | 18% | 44.4% | 70 |
Estimating metabolizable energy from the label
Many labels print a kcal ME line, and where they do, use it. Where they do not, estimate. First find nitrogen-free extract (NFE), the carbohydrate fraction: NFE = 100 − moisture − protein − fat − fiber − ash. For the adult kibble: 100 − 10 − 26 − 12 − 4 − 8 = 40. Then apply modified Atwater values: ME = 3.5 × protein + 8.5 × fat + 3.5 × NFE, all in grams per 100 g. That gives 3.5 × 26 + 8.5 × 12 + 3.5 × 40 = 91 + 102 + 140 = 333 kcal per 100 g.
Now check it against the printed 380. The estimate is 47 kcal low, 12 percent, because fiber is counted as carbohydrate at 3.5 kcal per gram when it really yields 0 to 2, and because ash and moisture rounding both shift the NFE. An estimate within 15 percent is good enough to portion food; a 30 percent gap means the label is inconsistent and the printed kcal line should be trusted over the arithmetic.
Protein per 100 kcal, the only fair comparison
Grams of protein per 100 kcal puts every food on one line. Kibble: 26 ÷ 333 × 100 = 7.8 g per 100 kcal. Canned: 10 ÷ 88 × 100 = 11.4 g per 100 kcal. Per day at 260 kcal that is 20 g of protein from the kibble and 30 g from the can, a 50 percent difference from two labels that looked 2.6 times apart in the other direction.
The same trick works for phosphorus, which matters in chronic kidney disease (CKD): mg per 100 kcal, not percent as fed, because a wet food at 0.2 percent phosphorus as fed can be 0.9 percent DM and still deliver fewer mg per calorie than a dry food at 0.7 percent.
Ash, fiber, and the numbers that hide
Ash at 8 percent max is the mineral residue, and it is a rough proxy for total minerals, not a measure of quality. A food at 5 percent ash and one at 9 percent can both be complete, but the higher figure usually tracks more bone or more mineral premix. Crude fiber at 4 percent max says almost nothing about fermentability: psyllium and cellulose both read as crude fiber and behave nothing alike in the gut.
Moisture at 78 percent max is also a pricing statement. The can is 22 percent food and 78 percent water, and a 400 g can holds 88 g of dry matter, the same as 98 g of a 10 percent moisture kibble. Judging a can by its price per 100 g rather than per 100 kcal will always make wet food look expensive. A line-by-line walk through a real label, including the parts that are optional, is in guaranteed analysis reading.
The statement that outranks the table
Look for the AAFCO nutritional adequacy statement, which says either formulated to meet the profile or animal feeding tests using AAFCO procedures. The feeding-test version rests on 6 months with 8 cats and is the stronger claim; the formulated version is arithmetic against a nutrient profile and can still fail in practice. Life stage matters too: a food for maintenance is not a food for growth, and a kitten on an adult formula at 28.9 percent protein DM is short of the 30 percent growth minimum.
Reading a label in 4 minutes
Example. The bag prints protein 26 percent min, fat 12 percent min, fiber 4 percent max, moisture 10 percent max, ash 8 percent max, and 380 kcal per 100 g, at 20 dollars for 4 kg. DM is 90 percent, so protein DM is 28.9 percent and fat DM is 13.3 percent. A 4.5 kg cat at 260 kcal a day eats 260 ÷ 380 × 100 = 68 g, which is 4 kg ÷ 0.068 kg = 58 days per bag, or 34 cents a day. Protein delivered: 68 × 0.26 = 17.7 g, above the 12 to 16 g most adult cats need at this weight.
Swap in the can at 88 kcal per 100 g: 260 ÷ 88 × 100 = 295 g a day, 2.3 cans of 130 g, and protein delivered is 29.5 g. Same calories, 12 g more protein, and about 4 times the water. Which one is right depends on the cat, not the label; the summer/winter calorie shift is in winter calorie needs.
When the label stops being enough
Call the vet if a cat on a 26 percent protein food loses weight over 2 months, if a senior cat's phosphorus matters and the label gives no mg figure, if a kitten is on an adult formula, or if a food change brings vomiting past 48 hours or diarrhea past 3 days. Bring the bag, the grams fed, and the cat's weight history; a 3-day food diary with weights ends most label arguments faster than any calculator.
Five steps that settle a comparison
- Write down all 5 guaranteed analysis lines plus moisture before comparing anything.
- Convert protein and fat to dry matter: divide by 100 minus moisture.
- Estimate ME with 3.5, 8.5, 3.5 if no kcal line is printed, and accept 15 percent error.
- Compare grams of protein and mg of phosphorus per 100 kcal, never per 100 g.
- Convert the daily kcal to grams, then to days per bag, and let the cost tool show the yearly figure.
Terms mentioned in this article
Frequently asked
Is 26 percent protein enough for an adult cat?
On a dry matter basis it is about 29 percent, which clears the 26 percent adult minimum and suits most healthy adult cats at 17 to 18 g of protein a day on 260 kcal. Senior cats, unwell cats, and growing kittens need more, and a cat losing weight on that food needs a check rather than a stronger bag.
Why does my estimate differ from the printed kcal?
The estimate treats all carbohydrate as 3.5 kcal per gram, while fiber yields 0 to 2, and ash plus moisture rounding shift the carbohydrate figure by a few points. A 10 to 15 percent gap is normal. If the gap passes 30 percent, one of the numbers is wrong and the printed kcal ME line is the one to portion from.
Does a higher protein percentage always mean better food?
No. Protein quality, amino acid balance, and the calorie base decide what the cat actually gets, and a 45 percent dry matter canned food can deliver less usable protein per day than a 29 percent kibble if the cat eats fewer calories of it. Compare grams per 100 kcal and check the AAFCO statement before the percentage.
Related guides
- Guaranteed Analysis: Protein, Ash, and the NFE Formula
- Cat Grass: 7-Day Wheatgrass and 10 cm Grazing
- How to Switch Cat Food: The 7-Day Schedule, With Grams
- Switching an 11-Year-Old Cat's Food: 7-Day Gram Plan
- Taurine in Cat Food: 250-500 mg, and Who Falls Short
- Carbs in Cat Food: Dry Matter, NFE, and the 10% Line