Water Content on Cat Food Labels: Dry Matter Math

A can that lists 11 percent protein and a bag that lists 33 percent look 3 times apart. Convert both to dry matter and the gap narrows to 50 percent against 36 percent, with the canned food ahead. Water is the reason: 78 percent of the can is water, so every nutrient number on it is diluted by a factor of about 4.5 before it reaches the label.

Two kinds of number share one label

As-fed values, also called original matter, describe the food as it sits in the bowl, water included. Dry matter values remove the water and describe the nutrients that remain. Guaranteed analysis prints as-fed numbers, with moisture as a maximum, protein and fat as minimums, and fiber and ash as maximums. Every comparison between a can and a bag has to happen in the same column, and the only column that works is dry matter.

Because moisture is a maximum, the dry matter you calculate is a minimum. A can labeled at 78 percent moisture or less may test at 76 percent, which moves dry matter from 22 to 24 percent and every dry matter nutrient down by 8 to 9 percent. That error is small next to the error of comparing as-fed to as-fed.

Wet food at 75 to 85 percent water, dry at 6 to 10

Canned pâté sits at 75 to 82 percent moisture, pouches and flaked foods at 80 to 85, mousse and soups at 85 to 90, and dry extruded kibble at 6 to 10. Freeze-dried and air-dried foods run 3 to 6 percent. A 400 g can at 78 percent water holds 312 g of water and 88 g of dry matter; a 400 g bag of kibble at 9 percent holds 36 g of water and 364 g of dry matter.

That single ratio explains most label confusion. It also explains why a cat on canned food produces 2 to 3 times the urine volume of a cat on kibble, which is a feature in urinary and kidney cases, not a problem.

The dry matter conversion in one line

Dry matter percent equals 100 minus moisture percent. Any nutrient on a dry matter basis equals its as-fed percent divided by the dry matter fraction, times 100. Two examples do the work. A pâté at 78 percent moisture and 11 percent protein: dry matter is 22 percent, and 11 divided by 22 is 0.5, so protein is 50 percent DM. A kibble at 9 percent moisture and 33 percent protein: dry matter is 91 percent, and 33 divided by 91 is 0.363, so protein is 36 percent DM.

Run the same division on fat, ash and fiber before you judge a formula. The pâté above at 5.5 percent fat as-fed is 25 percent fat DM, and 2 percent ash as-fed is 9.1 percent ash DM, which is above the 8 percent line that most formulators use as a screen. The kibble at 16 percent fat as-fed is 17.6 percent DM, 7 percent ash is 7.7 percent DM.

Why as-fed comparison misleads

Compare 11 percent to 33 percent and the kibble wins by 3 times. Compare 50 percent to 36 percent DM and the pâté wins by 14 points. Neither number tells you what the cat eats per day, because portion size closes the loop: a 4 kg cat eating 200 g of the 1.0 kcal/g pâté takes 22 g of protein, and the same cat eating 55 g of the 3.7 kcal/g kibble takes 18 g.

Carbohydrate is where the misreading gets expensive. Pet food labels do not print carbohydrate, so it is calculated by difference on a dry matter basis: 100 minus protein, fat, ash and fiber. The pâté works out to 13.6 percent DM carbohydrate and the kibble to 35.1 percent, which is the difference between a low-carb canned food and a starch-based dry food that both claim 30 percent protein on the front.

Water per 100 kcal is the number that matters

Cats need 40 to 60 mL of water per kilogram of body weight a day, so a 4 kg cat needs 160 to 240 mL and a 5 kg cat needs 200 to 300 mL. Food counts toward that total. A 200 g can at 78 percent moisture delivers 156 mL, a 60 g portion of kibble at 9 percent delivers 5.4 mL, and the gap between them, about 150 mL, has to come from the bowl.

Many cats on dry food drink 100 to 150 mL a day, which leaves a 4 kg cat 10 to 90 mL short depending on the day and the weather. Per 100 kcal the comparison is stark: canned food at 1.0 kcal/g carries 78 mL of water per 100 kcal, kibble at 3.8 kcal/g carries 2.4 mL. That is a 30-fold difference for the same energy, and it is why the moisture line deserves as much attention as the protein line (reading cat food labels).

Carbohydrate by difference, and where it breaks

The by-difference method has 3 weak points. Moisture is a maximum, so dry matter is a minimum and carbohydrate comes out high. Fiber is reported on an as-fed basis by some manufacturers, which double-counts it. And ash includes both minerals and any dirt or bone, so a high-ash formula shows an artificially low carbohydrate number.

Use the method to sort foods into bands rather than to argue over 1 or 2 points. Under 10 percent DM carbohydrate is low, 10 to 25 is moderate, and above 30 is a starch-based formula. The band matters more than the decimal, and the full method is in carb content in cat food.

Six formats side by side

Format	Moisture, %	Dry matter, %	Protein, as fed %	Protein, % DM	Water, mL per 100 kcal
Pâté, canned	78	22	11	50	78
Mousse or soup	86	14	8	57	101
Flaked in gravy	82	18	9	50	91
Extruded kibble	9	91	33	36	2.4
Air-dried	6	94	40	43	1.5
Freeze-dried	4	96	45	47	0.9

The last column is computed as moisture divided by energy density in kcal per gram. It is the fastest way to see why a bowl of kibble and a can of pâté are not interchangeable for a cat that needs more water, and why 2 pouches can do the work of 1 can plus a bowl.

Reading a label in 5 steps

Write down moisture, protein, fat, fiber and ash exactly as printed, and note which are maximums.
Subtract moisture from 100 to get dry matter, then divide each nutrient by that number and multiply by 100.
Calculate carbohydrate by difference on the dry matter basis, and sort the food into a band.
Divide moisture by kcal per gram to get water per 100 kcal, then multiply by the cat's daily energy need.
Compare the total with 40 to 60 mL/kg and add drinking water only for the shortfall (reading cat food labels).

One safety note that has nothing to do with water: a cat that stops eating for 24 hours, or a kitten for 12, needs a vet call rather than a new food. Common household toxins are listed in the toxicity checker.

Terms mentioned in this article

Frequently asked

Is 11 percent protein in canned food too low?

Not on a dry matter basis. At 78 percent moisture, 11 percent as-fed is 50 percent DM, which is higher than a 33 percent kibble at 9 percent moisture, at 36 percent DM. Judge canned food only after converting, and then check how many grams the cat actually eats, because a 200 g can supplies 22 g of protein.

How much water does a cat need per day?

Forty to sixty milliliters per kilogram of body weight, so 160 to 240 mL for a 4 kg cat, counting water in food. A 200 g can at 78 percent moisture supplies 156 mL, while 60 g of kibble at 9 percent supplies about 5 mL, which leaves most of the target to the bowl and to whatever the cat will actually drink.

Why is carbohydrate not printed on cat food labels?

Because it is not a required guaranteed analysis value. It is calculated by difference on a dry matter basis: 100 minus protein, fat, ash and fiber. The method overstates carbohydrate when moisture is a maximum and understates it when ash is high, so use it to sort foods into bands of under 10, 10 to 25, and over 30 percent DM.