There's no single universal number — it depends on your goal weight-loss rate, but the general principle is creating a calorie deficit, meaning you burn more calories than you consume (through a mix of eating less and being active).
The basic math: One pound (0.45 kg) of body fat is roughly equivalent to 3,500 calories. So to lose about 1 pound per week — a commonly recommended, sustainable pace — you'd need a deficit of about 500 calories per day (500 × 7 = 3,500). For a slower, gentler pace of about 0.5 lb/week, aim for a 250-calorie daily deficit; for a more aggressive 2 lb/week (generally only advised under medical supervision), you'd need roughly a 1,000-calorie daily deficit.
Important nuance: This isn't just about "burning" calories through exercise — total daily calorie burn includes your Basal Metabolic Rate (BMR, the energy your body uses at rest for basic functions) plus calories burned through daily movement, digestion, and exercise. Total daily burn is often called Total Daily Energy Expenditure (TDEE). Your deficit can come from eating less, moving more, or — most sustainably — a combination of both.
Why there's no one-size-fits-all number: Your TDEE depends on age, sex, height, current weight, body composition, and activity level. Someone who is taller, more muscular, or more active will burn more calories at rest and through activity than someone smaller or more sedentary — so the same 500-calorie deficit looks different for different people. Online TDEE/BMR calculators (using formulas like Mifflin-St Jeor) can give a personalized starting estimate.
A few caveats: Very low-calorie diets or extreme deficits can backfire, causing muscle loss, nutrient deficiencies, metabolic slowdown, and rebound weight gain. Most health guidelines recommend women not go below ~1,200 calories/day and men not below ~1,500/day without medical supervision. For personalized, safe targets — especially if you have health conditions — consulting a doctor or registered dietitian is the most reliable approach.