Most people lose fat consistently by eating 500 fewer calories per day than their body burns, which works out to roughly one pound of fat loss per week. But the specific number you should eat depends on your size, age, sex, and how active you are. A moderately active 35-year-old woman might aim for around 1,600 calories a day, while a man of the same age and activity level might target closer to 2,000. The process for finding your number takes about two minutes of math.
Why a Calorie Deficit Is the Only Requirement
Fat loss comes down to one thing: eating less energy than your body uses. When you do that consistently, your body pulls the difference from stored fat. Research has confirmed that the negative energy balance alone is responsible for weight reduction, regardless of whether the deficit comes from eating less, exercising more, or both. No specific food, meal timing strategy, or supplement changes this fundamental equation.
How to Estimate Your Daily Calorie Needs
Your body burns a baseline number of calories just to keep you alive: breathing, circulating blood, maintaining body temperature. This is your basal metabolic rate, or BMR. The most accurate widely used formula for estimating it is the Mifflin-St Jeor equation, which predicted resting metabolic rate within 10% of the actual measured value in about 71% of people tested. That’s better than every other common formula.
Here’s the equation (weight in kilograms, height in centimeters):
- Women: (9.99 × weight) + (6.25 × height) – (4.92 × age) – 161
- Men: (9.99 × weight) + (6.25 × height) – (4.92 × age) + 5
To convert: divide your weight in pounds by 2.2 to get kilograms, and multiply your height in inches by 2.54 to get centimeters.
Your BMR only covers what your body burns at complete rest. To get your total daily energy expenditure, multiply your BMR by an activity factor:
- Sedentary (desk job, little exercise): BMR × 1.2
- Lightly active (exercise 1 to 3 days per week): BMR × 1.375
- Moderately active (exercise 3 to 5 days per week): BMR × 1.55
- Very active (hard exercise 6 to 7 days per week): BMR × 1.725
- Extremely active (physical labor or twice-daily training): BMR × 1.9
The result is your maintenance calories, the number that keeps your weight stable. A concrete example: a 40-year-old woman who weighs 170 pounds (77 kg), stands 5’5″ (165 cm), and exercises three times a week would calculate a BMR of about 1,430 calories, then multiply by 1.55 to get roughly 2,215 calories per day as her maintenance level.
How Much to Subtract for Fat Loss
A daily deficit of 500 calories is the standard starting point. Health guidelines recommend losing 5% to 10% of your body weight over the course of about six months, which lines up well with a 500-calorie daily deficit for most people. Using the example above, that woman would aim for about 1,700 calories per day.
You may have heard the old rule that 3,500 calories equals one pound of fat, so a 500-calorie daily deficit should produce exactly one pound of loss per week. That math is oversimplified. Researchers have shown the rule fails to account for the fact that your energy balance changes dynamically as you lose weight. In practice, you’ll likely lose a bit more than expected in the early weeks (partly from water) and a bit less than expected later on. A 500-calorie deficit remains a useful target, but treat the “one pound per week” prediction as a rough guide, not a guarantee.
Some people use a larger deficit of 750 or even 1,000 calories per day to lose fat faster. This can work, but it comes with tradeoffs that get more serious the deeper you cut. Whatever deficit you choose, there are hard floors you shouldn’t drop below: 1,200 calories per day for women and 1,500 calories per day for men. Going lower risks depriving your body of essential nutrients according to Harvard Health Publishing.
Why Your Body Fights Back
Within the first week of a calorie deficit, your body begins adapting. Insulin secretion drops, glycogen stores in the liver deplete, and you lose water. This is why the scale often drops quickly at first, then slows down. Much of that initial loss is not fat.
Over time, your body gets more aggressive about conserving energy. Your metabolic rate decreases by more than you’d expect from the weight you’ve lost alone. This process, called adaptive thermogenesis, involves shifts in thyroid hormones, reduced activity in the sympathetic nervous system (the “fight or flight” system that burns energy), and lower levels of leptin, the hormone that signals fullness. The practical result: a deficit that produced steady fat loss in month one may produce almost nothing by month three if you don’t adjust.
This is why recalculating every four to six weeks matters. As you lose weight, your body gets smaller and burns fewer calories. A 170-pound person burns more just existing than a 155-pound person. Plug your new weight back into the equation periodically, or simply reduce your intake by another 100 to 200 calories when progress stalls for two or more consecutive weeks.
Body Composition Changes Your Numbers
Two people who weigh the same can have very different calorie needs. Muscle tissue burns significantly more energy at rest than fat tissue does. Someone with more lean mass will have a higher metabolic rate and can eat more while still losing fat. Men typically burn more calories than women of the same age and weight because they carry more muscle and less body fat on average.
This also explains why strength training matters during fat loss. When you cut calories without resistance exercise, you lose a mix of fat and muscle. Losing muscle further lowers your metabolic rate, making it harder to keep losing fat and easier to regain weight later. Keeping protein intake high (a common target is 0.7 to 1 gram per pound of body weight) and lifting weights helps preserve muscle so that more of the weight you lose comes from fat.
Signs Your Deficit Is Too Aggressive
Feeling mildly hungry at certain points in the day is normal during fat loss. Feeling constantly exhausted, foggy, or irritable is not. If you notice persistent fatigue, frequent headaches, nausea, constipation, feeling cold all the time, or difficulty concentrating, your calorie intake is likely too low. These are signs your body is aggressively slowing its metabolism to compensate for the energy gap.
The psychological side matters too. A deficit so steep that it makes you fixate on food, feel anxious about eating, or binge on weekends is working against you. A moderate deficit you can sustain for months will always outperform an extreme one you abandon after three weeks.
Putting It All Together
Calculate your BMR using the Mifflin-St Jeor equation, multiply by your activity factor, and subtract 500. That’s your starting calorie target. Track your weight over two to three weeks (weighing at the same time each day, then averaging the week) to see if you’re trending downward. If you are, keep going. If not, drop another 100 to 200 calories or add more movement to your day. Remember that the formula gives you an estimate. Even the best equation is off by more than 10% for about a third of people, so real-world tracking and adjustment is what turns an estimate into a plan that works.