To lose weight, you need to eat fewer calories than your body burns each day. For most people, that means eating roughly 500 calories below their total daily energy expenditure, which produces about one pound of weight loss per week. But the exact number depends on your size, age, sex, and how active you are, so there’s no single answer that works for everyone.
How to Estimate Your Daily Calorie Burn
Your body burns calories around the clock just to keep you alive: pumping blood, breathing, maintaining body temperature, repairing cells. This baseline burn is called your resting metabolic rate, and it accounts for roughly 70% of the calories you use each day. The remaining 30% comes from physical activity and digesting food.
The most accurate way to estimate your resting metabolic rate without lab equipment is the Mifflin-St Jeor equation, which a comparative study of predictive formulas found to be accurate within 10% of measured values for most people. Here’s how it works:
- For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
- For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
To convert pounds to kilograms, divide by 2.2. To convert inches to centimeters, multiply by 2.54. Once you have that number, multiply it by an activity factor to get your total daily energy expenditure:
- Sedentary (desk job, little exercise): × 1.2
- Lightly active (light exercise 1–3 days/week): × 1.375
- Moderately active (moderate exercise 3–5 days/week): × 1.55
- Active (hard exercise 6–7 days/week): × 1.725
A quick example: a 40-year-old woman who weighs 170 pounds (77 kg), stands 5’5″ (165 cm), and has a desk job would calculate (10 × 77) + (6.25 × 165) − (5 × 40) − 161 = 1,440 calories at rest. Multiplied by 1.2 for sedentary activity, her total daily burn is roughly 1,730 calories. Eating around 1,230 calories a day would create a 500-calorie deficit, but that falls close to the safety floor, so a smaller deficit or more activity would be a better approach for her.
The Calorie Deficit That Drives Weight Loss
You’ve probably heard that cutting 3,500 calories equals one pound of fat loss. That rule has been repeated for decades, but when researchers tested it against tightly controlled studies, most participants lost significantly less weight than the formula predicted. Weight loss also slowed as the weeks went on, something the old rule doesn’t account for.
The reason is straightforward: as you lose weight, your body needs fewer calories. The deficit that helped you lose the first five pounds shrinks unless you adjust your intake or increase activity. On top of that, the same calorie cut produces faster weight loss in men than women and in younger adults than older adults. Individual variation is real, so treat any calculation as a starting estimate, not a guarantee.
The CDC recommends losing one to two pounds per week as a pace that people are more likely to sustain long term. For most people, a daily deficit of 500 to 750 calories lands in that range. The NIH offers a free online Body Weight Planner that models these diminishing returns and gives more realistic timelines than the 3,500-calorie rule.
Minimum Calorie Floors
Cutting calories too aggressively backfires. Harvard Health recommends that women eat no fewer than 1,200 calories per day and men no fewer than 1,500 unless supervised by a healthcare provider. Dropping below those thresholds makes it extremely difficult to get the vitamins, minerals, and fiber your body needs, and very low calorie diets raise the risk of gallstones, muscle loss, and nutrient deficiencies.
If your calculated deficit would put you below these floors, close the gap with activity rather than further food restriction. Even small increases in daily movement add up significantly.
Why Daily Movement Matters More Than Workouts
Formal exercise (running, lifting, cycling) accounts for only about 5% of the average person’s total daily calorie burn. The bigger contributor on the activity side is what researchers call non-exercise activity thermogenesis: walking to the car, cooking, fidgeting, taking the stairs, standing instead of sitting. That category can account for 15% or more of your daily expenditure.
This means that someone who does a 30-minute gym session but sits for the remaining 15 waking hours may burn fewer total calories than someone who skips the gym but stays on their feet most of the day. Both matter, but if you’re looking for the easier lever to pull, increasing your everyday movement (parking farther away, walking during phone calls, taking short breaks from your desk) often has a bigger cumulative effect than adding a workout you might skip.
What You Eat Changes How Many Calories You Keep
Not all calories cost the same amount of energy to digest. Your body burns 15–30% of the calories in protein just breaking it down and absorbing it. Carbohydrates cost 5–10%, and fats cost 0–3%. So 200 calories of chicken breast leaves your body with noticeably fewer net calories than 200 calories of butter, even though the label reads the same.
Protein also helps preserve muscle during a calorie deficit, which matters because muscle tissue burns more calories at rest than fat does. The baseline recommendation is 0.8 grams of protein per kilogram of body weight per day, but research suggests that bumping that to 1.0–1.6 grams per kilogram supports muscle retention and strength, especially as you get older. For a 180-pound person, that’s roughly 82 to 130 grams of protein daily. Going above 2 grams per kilogram isn’t recommended for most people.
Your Metabolism Won’t Tank, but It Will Adjust
A common fear is that dieting “destroys” your metabolism. The reality is less dramatic. When you lose weight, your body gets smaller, your organs get slightly smaller, and you naturally burn fewer calories. That’s basic physics, not sabotage. Some researchers have investigated whether the body also slows down beyond what size alone would predict. When University of Alabama at Birmingham researchers gave participants a month after weight loss for their bodies to stabilize, true metabolic adaptation averaged only a few dozen calories per day, not the hundreds that headlines sometimes suggest.
The practical takeaway: your calorie needs will drop as you lose weight, and you’ll need to recalculate every 10–15 pounds or so. But the slowdown is mostly explained by being a smaller person, not by your metabolism breaking. Periodic recalculation keeps your deficit on track.
Putting It All Together
Start by running the Mifflin-St Jeor equation with your current stats and an honest activity multiplier. Subtract 500 calories from that number. Check that the result stays above 1,200 (women) or 1,500 (men). If it doesn’t, aim for the floor and add more daily movement to widen the gap. Prioritize protein at each meal to preserve muscle and take advantage of its higher digestion cost. Recalculate every few weeks as your weight changes, and treat early results as a calibration period rather than a permanent prediction. Weight loss that looks slow on paper, one to two pounds a week, adds up to 50–100 pounds in a year.