Two numbers worth knowing
Your BMR (Basal Metabolic Rate) is the energy your body burns just to stay alive — breathing, circulating blood, keeping your organs running — if you did nothing but lie still all day. Your TDEE (Total Daily Energy Expenditure) is BMR plus everything else: walking, working, exercising, even fidgeting. TDEE is the number that determines whether you gain, maintain, or lose weight. Our calorie calculator estimates both using the Mifflin-St Jeor equation, one of the more accurate formulas available for the general population.
Why a "deficit" causes weight loss
The underlying principle is simple: if you consistently eat fewer calories than your TDEE, your body draws on stored energy — mostly fat, some muscle — to make up the difference. Eat more than your TDEE and the reverse happens. This is real and well-established. What's often left out is how much individual variation exists in appetite, hunger hormones, sleep, and stress, all of which affect how sustainable a given deficit actually is for a real person.
How big should the deficit be?
A moderate deficit — roughly 15-25% below TDEE — tends to be more sustainable than an aggressive one, because it's easier to stick with over months rather than days. Very large deficits often backfire: they increase hunger, reduce energy for exercise, and are harder to maintain, which is why so many aggressive diets end in regained weight.
What a deficit doesn't tell you
Calories in versus calories out is real physics, but it says nothing about food quality, muscle retention, or how you'll feel along the way. Two people eating the same deficit can have very different experiences depending on protein intake, sleep, training, and stress. That's part of why pairing a calorie target with a sensible macro split — enough protein especially — tends to preserve muscle better than calories alone.
A more realistic way to think about it
Treat your TDEE estimate as a starting point, not gospel — formulas like Mifflin-St Jeor are accurate on average but can be off by a meaningful margin for any one person. Track your actual weight trend over 2-3 weeks, and adjust your intake based on what's really happening, not just what the calculator predicted.
This article is for general information only and isn't medical or nutritional advice. See our full disclaimer.