If you are in a calorie surplus, your weight goes up. If you are in a calorie deficit, your weight goes down. MacroCodex uses that relationship to estimate your true maintenance calories over time: https://macrocodex.app/

This can be used to gain or lose weight. You simply need to eat above your maintenance calorie line to gain weight and below it to lose weight. No ads, no subscriptions, no payments, 16k users already.

If you're interested in how it works: https://macrocodex.app/knowledge/rethink/adaptive-tdee/