Most people lose weight steadily by eating 500 calories below what their body burns each day, which works out to roughly half a pound to one pound lost per week. But “what your body burns” is personal, shaped by your size, age, sex, and how much you move. The real answer starts with calculating your number, then subtracting from there.
How to Estimate Your Daily Calorie Burn
Your body burns calories just by keeping you alive: pumping blood, breathing, maintaining body temperature. This baseline burn is called your resting metabolic rate. The most accurate formula for estimating it, validated in a systematic review published in the Journal of the American Dietetic Association, is the Mifflin-St Jeor equation. It predicted resting metabolic rate within 10% of measured values in more people than any competing formula.
Here’s how it works:
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
That gives you what your body burns at rest. To get the full picture, you multiply by an activity factor that accounts for exercise and daily movement:
- Sedentary (desk job, little exercise): multiply by 1.2
- Lightly active (light exercise 1 to 3 days per week): multiply by 1.375
- Moderately active (moderate exercise 3 to 5 days per week): multiply by 1.55
- Very active (heavy exercise 6 to 7 days per week): multiply by 1.725
The result is your total daily energy expenditure, the number of calories you need to eat to maintain your current weight. For a 35-year-old woman who weighs 170 pounds (77 kg), stands 5’5″ (165 cm), and exercises a few times per week, that comes to roughly 2,100 calories per day. A 40-year-old man at 200 pounds (91 kg) and 5’10” (178 cm) with the same activity level would land around 2,600.
How Big Your Calorie Deficit Should Be
The old rule of thumb said 3,500 calories equals one pound of fat, so cutting 500 calories per day should mean losing one pound per week. That math is a useful starting point, but it oversimplifies what actually happens. When you lose weight, you lose a mix of fat, lean tissue, and water. And as your body gets smaller, it burns fewer calories, so the same 500-calorie deficit produces smaller losses over time.
A more realistic expectation: cutting 500 calories daily leads to about half a pound to one pound per week in the early months, with the rate gradually slowing. Clinical guidelines from the American College of Cardiology recommend aiming for 5% to 10% of your starting body weight over six months. For someone at 200 pounds, that means 10 to 20 pounds, a pace of roughly half a pound to just under a pound a week.
There’s an important floor to be aware of. Harvard Health advises that women should not eat fewer than 1,200 calories per day, and men should not go below 1,500, unless they’re being supervised by a medical professional. Dropping below these thresholds makes it difficult to get adequate nutrition and can trigger more aggressive metabolic pushback from your body.
Why Weight Loss Slows Down
Your body treats a calorie deficit as a signal that food is scarce, and it responds by burning less energy than you’d expect. This phenomenon, called adaptive thermogenesis, has been documented in the vast majority of weight loss studies. A systematic review in the British Journal of Nutrition found it in 27 of 33 studies examined. For typical dieters, the extra slowdown amounts to roughly 30 to 100 calories per day beyond what body size changes alone would predict. People who lose very large amounts of weight (100+ pounds) can see a much larger metabolic dip.
Hormones amplify this effect. Calorie restriction raises levels of ghrelin, a hormone that drives hunger, while simultaneously lowering leptin, the hormone that signals fullness. The combined effect makes you hungrier while your body is burning fewer calories. This isn’t a flaw in your willpower. It’s a predictable biological response that explains why the first few weeks of a diet feel much easier than month three or four.
The encouraging finding: adaptive thermogenesis appears to shrink or disappear once your weight stabilizes at a new level. If you reach a plateau, holding steady at your new weight for a few weeks can help your metabolism recalibrate before you push further.
A Practical Example
Say you’re a 38-year-old woman, 5’6″ and 180 pounds, lightly active. Using the Mifflin-St Jeor equation, your resting metabolic rate is about 1,480 calories. Multiply by 1.375 for light activity, and your maintenance level is roughly 2,030 calories per day. To lose about half a pound to a pound per week, you’d aim for around 1,530 calories daily.
After losing 15 pounds over a few months, your maintenance number drops because your body is smaller. Recalculating at 165 pounds gives you a new maintenance of about 1,930 calories, so that same 1,530 intake now creates only a 400-calorie deficit instead of 500. This is why recalculating every 10 to 15 pounds keeps your expectations realistic and helps you decide whether to adjust your intake or increase your activity.
Age and Metabolism
One common belief is that metabolism tanks in your 30s and 40s, making weight gain inevitable. A large-scale analysis published in Science found this isn’t true. Metabolic rate stays remarkably stable from age 20 through 60, regardless of sex. The decline doesn’t begin until around age 60, at which point it drops by about 0.7% per year. By age 90, total energy expenditure is roughly 26% lower than in middle age.
Weight gain in your 30s and 40s is real, but it’s driven by changes in activity levels, eating patterns, and muscle mass rather than an unavoidable metabolic slowdown. This is actually good news: it means the calorie math works the same whether you’re 28 or 52.
Protecting Muscle While Losing Fat
Not all weight loss is equal. Losing muscle along with fat lowers your metabolic rate faster, makes you weaker, and produces worse long-term outcomes. Protein intake is the single biggest dietary lever for preserving muscle during a calorie deficit.
The standard recommendation is 0.8 grams of protein per kilogram of body weight per day, but that’s the minimum for general health, not for someone actively losing weight. Research suggests aiming for 1.0 to 1.6 grams per kilogram to preserve lean mass. For a 180-pound person, that’s roughly 82 to 130 grams of protein daily. Spreading this across meals rather than loading it into one sitting improves absorption.
Resistance training is the other critical factor. Your body is less likely to break down muscle tissue it’s actively using. Even two to three sessions per week during a calorie deficit makes a meaningful difference in what kind of weight you lose.
Putting It All Together
Start by estimating your maintenance calories using the Mifflin-St Jeor equation and an honest activity multiplier. Subtract 500 calories from that number. Check that the result doesn’t fall below 1,200 (women) or 1,500 (men). Eat enough protein, around 1.0 to 1.6 grams per kilogram of body weight, and include some form of strength training.
Expect to lose half a pound to a pound per week in the early months, with the pace slowing as your body adapts. Recalculate your numbers after every 10 to 15 pounds lost. If you hit a plateau that lasts more than two to three weeks, your maintenance level has likely dropped, and you’ll need a modest further reduction in calories, an increase in activity, or both to keep progressing.