That's the prime reason dogs and cats get fat. Calorie creep.
You can always read the feeding instructions on you bag or can of food. The caveat is that these guidelines are based on average caloric needs based on calculations.
I know my dog is not average and I'm sure you feel the same way. You can always start with these suggestions, but in my case, Miles need special calculations and formulas to estimate his caloric need. I want to get a close as possible.
I use a calculation for Resting Energy Requirement or RER and then multiply by an activity level factor. RER is simple enough. First I convert his weight in pounds to kilograms (kg). He weighs 75 pounds so I divide by 2.2 to get to 34 Kg. I multiply his weight in Kg by 30 and add 70.
For Miles the calculation goes 34 X 30 = 1020 + 70 = 1090. For a neutered dog of normal activity you multiply by a factor of 1.6 RER for a total of 1744 Kcals/ day. On the back of my Purina One bag it states that my formula contains 383 Kcals/ cup, so I divide 1744 by 383 and come up with 4.55 cups. The bag recommendation for a dog between 76 and 100 pounds is 4-5 cups by the way.
The problem with that is if I chose 4 cups because Miles is on the low end of the range, I'd be short changing him by around 200 calories and if I fed him 5 cups that would be 171 kcals too many. Over time that would show up in his body condition.
That's why all these estimates are estimates only. Even the RER calculation is based on average needs. After you come up with your estimate you need to get more involved with observing what is going on with your dog or cat. You have to get familiar with Body Condition Scoring. I did a post using Miles as the model for ideal. Read that and get familiar with this body condition scoring chart (for dog & for cat ) and how to use it. I'd do a check at least once a month and if things are changing for the worse cut back on the calories by at least 10% and recheck a month later. Read also: Weighty Matters – Dog Obesity Prevention
For cats use weight in Kg X 40 for RER and this handy chart for the activity level factor and this one for body condition. The danger of over feeding is a big problem for many of us. The closer we can get to ideal, the better for our pets and for us because we love them and we want them around as long as possible. But you already knew that.