You cannot calculate your muscle gain, short of having a biopsy. You can calculate your lean mass gain. Many people confuse this with muscle, which it is not – it is a combination of anything that is not fat. That means the contents of food in your gut, the water that your muscles may draw in, water you may be retaining in various parts of your body, and muscle. This is why I tell people not to get too caught in the numbers, because you can easily lose/gain 5 pounds of lean mass from food/water alone. It’s best to focus on consistent, long term gains and use reliable markers such as the ratio of your biceps circumference to your abs (both flexed). For example, let’s say you started with 16″ biceps and a 34″ waist, and ended up with 16″ biceps and a 32″ waist. Here is the ratio:

16:34 = 0.47

16:32 = 0.5

When that ratio goes up, it is a good thing.

On the other hand, if you started with 16″ biceps and a 34″ waist, and ended up with 15″ biceps and a 34″ waist, the second ratio would be 0.44 -going down, indicating muscle loss.

You always want this ratio to maintain or go up. That is a positive indicator.

As for computing lean mass, it’s like this:

200 pounds @ 10% body fat = 200 x 0.1 = 20 pounds of fat

The rest is lean, so 200 – 20 = 180 pounds lean

Now, let’s say we ended up at 205 pounds and 12% body fat.

210 pounds @ 12% body fat = 210 x 0.12 = 25 pounds of fat

The rest is lean, so 210 – 25 = 185 pounds of lean

In this case, you could comfortably say you were gaining lean mass. Whether that was water weight or quality muscle would not be known, but generally this would be a positive movement because you are not gaining more fat than muscle.