How Protein is Measured

Protein is the one type of nutrient that has nitrogen added to the chemical composition. This study demondstrates how the protein content is calculated, which tells one nothing about the quality of protein sources, or what the amino acid makeup is, meaning whether it is, or isn't a complete protein. Along with this very antique Kjeldahl method developed in the late 1800's, there is the Dumas test that can extract ~2% more protein content from a product. There is a comparison of these two on this site.
There are new methods being explored, but they are not readily utilized as consumers are not demanding more proficient methods of measurement in pet foods. The latest accepted test is the NIR test, which tests for peptide bonds in protein that has been extracted from a source. Some of the more recent, cumbersome tests are described here.
Dry Matter Calculations:
Find the moisture percentage on the label (usually 70-80 for canned)
Subtract that from 100, (should be 20-30 for canned)
Now, move the decimal 2 places left (should be .20 to .30)
Use that number to divide into any other percentage on the label to get what is termed "Dry Matter Percentage" for that particular item. For instance, if your answer was .25, and the protein percentage was listed on the label as 10%, you would divide 10 by .25 and come up with 40, indicating that 40% of the dry matter in the food is actually protein content.

Many people have trouble understanding why I will pick a good dry food over a good canned food for weight reduction, so, to take this further, using top quality canned foods and top quality dry foods as an example with high muscle meat protein in both.

Canned Food Example = Fancy Feast varieties at 11% protein.

First you calculate the Dry matter percentages of the nutrients as shown above. Then you have the final percentage figures listed below, and what follows is the interpretation into caloric count of the food.

Canned food for 100 grams:
80% moisture = 80 grams of moisture leaving 20 grams of nutrition in 100 grams of canned food
50%, (dry matter calculation), of that 20 grams would be 10 grams of protein (40 calories)
22.7 %, (dry matter calculation), of the 20 grams would be 4.54 grams of fat. (40.86 calories)
That leaves about 5.46 grams left for carbohydrates and "ash" or 27.3% of the can's dry matter content. (21.84 calories)
Total amount of caloric content here is 102.7 calories.

Dry food for 100 grams:
10% moisture = 10 grams of moisture leaving 90 grams of nutrition in 100 grams of dry food.
44%, (dry matter calculation), of that 90 grams would be 39.6 grams of protein, (158.4 calories)
9.4%, (dry matter calculation), of that 90 grams would be 8.46 grams of fat, (76.14 calories)
That leaves about 41.94 grams of carbohydrates and "ash" content or 46.6 % of the dry matter content, (167.76 calories)
Total amount of caloric content here is 402.30 calories although Royal Canin is claiming 257 for this measure of their Indoor dry on the package.

According to the NRC, cats need an absolute minimum of 3.97 grams of protein per Kilogram of weight. If you start with a 16 pound cat you have about 7.25 Kilograms of cat to nurture before cutting back on calories. That requires approximately 30 grams of protein provided by the food you feed. This would require 2.8 cans (287.56 calories) of the cat food, or 3/4 of a cup of the dry food  (about 300 calories).
Other dry and canned foods are not equal to these calculations.  Most canned foods require more food to provide adequate protein in the diet, and opposite to that, most dry foods have higher caloric count due to fat replacing "carbs" or inappropriate ingredients, which happens with both canned and dry.
There would be about a 10 to15 calorie advantage to feeding the canned over the dry, however, we found that we didn't need more than half a cup of the dry food for our boy needing high protein. He started at 16 pounds before weight loss. The advantage to dry is that it can be left in small timed feeding portions. Everyone knows that "portion control" is what drops the weight as the stomach shrinks with time. While I have taken actual figures from the foods I recommend, there are few canned foods you can find that are as high as 11% protein on the guaranteed analysis, so that will easily take up the "15 calorie" difference to equate the provision of adequate protein. However, if you doubt what I say, you can follow the calculations above to come up with your own figures on this.