At minimum, many state regulations require a pet food to guarantee the minimum percentages of crude protein and crude fat, and the maximum percentages of crude fiber and moisture. The "crude" term refers to the specific method of testing the product, not to the quality of the nutrient itself.
Some manufacturers include guarantees for other nutrients as well. The maximum percentage of ash (the mineral component) is often guaranteed, especially on cat foods. Cat foods commonly bear guarantees for taurine and magnesium as well. For dog foods, minimum percentage levels of calcium, phosphorus, sodium, and linoleic acid are found on some products.
Guarantees are declared on an "as fed" or "as is" basis, that is, the amounts present in the product as it is found in the can or bag. This doesn't have much bearing when the guarantees of two products of similar moisture content are compared (for example, a dry dog food versus another dry dog food). However, when comparing the guaranteed analyses between dry and canned products, one will note that the levels of crude protein and most other nutrients are much lower for the canned product. This can be explained by looking at the relative moisture contents. Canned foods typically contain 75-78% moisture, whereas dry foods contain only 10-12% moisture. To make meaningful comparisons of nutrient levels between a canned and dry product, they should be expressed on the same moisture basis.
The most accurate means of doing this is to convert the guarantees for both products to a moisture-free or dry matter basis. The percentage of dry matter of the product is equal to 100% minus the percentage of moisture guaranteed on the label. A dry food is approximately 88-90% dry matter, while a canned food is only about 22-25% dry matter. To convert a nutrient guarantee to a dry matter basis, the percent guarantee should be divided by the percentage of the dry matter, then multiplied by 100. For example, a canned food guarantees 8% crude protein and 75% moisture (or 25% dry matter), while a dry food contains 27% crude protein and 10% moisture (or 90% dry matter). Which has more protein, the dry or canned? Calculating the dry matter protein of both, the canned contains 32% crude protein on a dry matter basis (8/25 X 100 = 32), while the dry has only 30% on a dry matter basis (27/90 X 100 = 30). Thus, although it looks like the dry has a lot more protein, when the water is counted out, the canned actually has a little more. An easier way is to remember that the amount of dry matter in the dry food is about four times the amount in a canned product. To compare guarantees between a dry and canned food, multiply the guarantees for the canned food times four first.
It is especially important to look at the moisture guarantee for canned foods, even when comparing a canned food with another canned. Under AAFCO regulations, the maximum percentage moisture content for a pet food is 78%, except for products labeled as a "stew," "in sauce," "in gravy," or similar terms. The extra water gives the product the qualities needed to have the appropriate texture and fluidity. Some of these exempted products have been found to contain as much as 87.5% moisture. This doesn't sound like much difference until the dry matter contents are compared. For example, a product with a guarantee of 87.5% moisture contains 12.5% dry matter, only half as much as a product with a 75% moisture guarantee (25% dry matter).