Fat loss is an energy bookkeeping problem: burn more than you eat, and the body makes up the difference from storage. The mechanism is simple; the sustainable size of the deficit is not. Go too aggressive and hunger, muscle loss and rebound eating erase the progress within months.
Start from your BMR — the energy you burn at complete rest — estimated with the Mifflin-St Jeor equation, then multiply by an activity factor to get your TDEE. A deficit of 15–20% below TDEE is the evidence-backed sweet spot: roughly 0.5–0.75% of body weight lost per week while preserving muscle and sanity.
Protein is the lever that makes a diet survivable: 1.6–2.2 g per kg of body weight protects lean mass and keeps you full. Set that floor first with the fat intake calculator, then build the rest of your calories around it.
Finally, close the loop with activity: a brisk 45-minute walk is 180–250 kcal you never have to remove from your plate. Track it in the calories burned calculator and let the deficit come from both sides of the ledger.