Calorie Deficit Calculator
Find out exactly how many calories to eat per day to reach your goal weight — and how long it'll take. Based on the Mifflin-St Jeor equation.
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How a calorie deficit actually works
Your body needs energy to function — breathing, circulation, digestion, movement. That's your Total Daily Energy Expenditure (TDEE). When you eat less than your TDEE, your body dips into stored energy (primarily fat) to make up the difference.
One kilogram of body fat stores approximately 7,700 calories. So a daily deficit of 500 calories adds up to 3,500 per week — roughly 0.45 kg (1 lb) of fat loss. It's not perfectly linear (water weight, hormones, and metabolic adaptation all play a role), but the math holds remarkably well over months.
How much deficit is too much?
Bigger isn't always better. Research consistently shows that deficits beyond 25% of TDEE increase muscle loss, tank energy levels, and trigger metabolic adaptation (your body burns fewer calories to conserve energy). You lose weight on the scale, but a bigger chunk of it is muscle — which makes long-term maintenance harder.
The sweet spot for most people is 15-25% below TDEE. For someone burning 2,500 cal/day, that's a 375-625 cal deficit, eating 1,875-2,125 cal/day. If you're significantly overweight (BMI 30+), you can sustain a slightly larger deficit because your body has more fat reserves to draw from.
Why tracking matters more than willpower
A 2022 study in the American Journal of Clinical Nutrition found that people underestimate their calorie intake by 30-50% on average. That means someone eating "1,800 calories" might actually be eating 2,500. No amount of willpower fixes a deficit that doesn't exist.
You don't need to weigh every gram of food forever. But tracking for 2-4 weeks builds awareness of portion sizes and calorie density. After that, most people develop solid intuition. CalShot makes this easy — snap a photo and get an estimate without touching a food scale.
This calculator provides estimates only and is not a substitute for professional dietary advice.