You do this calculation with the bathroom scale and your meal logbook whatever form that may take.
Don't adjust your diet, just start recording your calories and macronutrient breakdown.
Weigh yourself once a week on the same day under the same conditions. i.e. Friday morning after getting up and using the bathroom
After a couple of weeks, you're going to see one of three things.
- 0 movement, you're eating exactly as many calories as your body needs to remain in homeostasis.
- Gaining weight, you're taking in more than your body needs to maintain.
- Losing weight, you're taking in less than your body needs to maintain.
After that it's just math and your dietary preferences.
There are 454 grams in a pound.
Fat has 9 calories per gram while carbs and protein have 4 calories per gram.
If for example your goal is to add 1lb of weight per week and you're diet is maintaining net zero, you have to find a combination of protein fat and carbs that adds up to 454 grams per week because again, there are 454 grams in a pound whether that pound is sitting on your plate or as part of your body. This is in addition to your maintenance calories that you established earlier by tracking consumption and recording weight.
The reverse is also true, if you're gaining 1lb per week, you need to figure out which combination of protein, fat and carbs adds up to 454 grams and subtract that from your weekly diet.