Hi Dottie and thanks for starting this thread as it is certainly a subject that causes a lot of confusion in dog owners. I will try to make it a bit clearer.
The first thing to point out with nutrient levels is that, if your dog is well, then you don't really need to worry about them.
If not, nutrient levels can be a very important part of choosing the right diet, but without converting the numbers on the pack to their 'dry weight' values, comparing different foods (or at least different types of foods) is almost impossible.
The difficulties arise from the water that is present in the food, usually listed as 'moisture'. Let's look at an example:
Food 1: Dry food. 8% moisture, 25% protein.
Food 2: Wet food. 70% moisture, 15% protein.
To the untrained eye, the wet food looks like it's quite a lot lower in protein than the dry but, in reality, it contains almost twice as much! Here's why:
The dry food is only 8% moisture. That means, in 100g of food, 8g is water. This leaves 92g of 'dry matter', 25g of which is protein. As a percentage that is 25 / 92 x 100 which comes to 27%. This is the 'dry matter protein' value.
The wet food, on the other hand, is 70% moisture meaning that 100g of food only contains 30g of dry matter. Of that 30g, 15g is protein. 15 / 30 x 100 = a whopping 50% dry matter protein.
If you are comparing two foods with different moisture levels, dry matter is the only way to be sure that you're doing it right.
As Dottie kindly points out, we have already done the maths on all of the foods listed on the AADF site but if you're struggling with the calculations for any other foods or if there's anything else I can help with, please feel free to post here.