To calculate a caloric deficit, you need two numbers: how many calories your body burns in a day and how many you eat. The difference between those two is your deficit. A daily deficit of about 500 calories leads to roughly one pound of weight loss per week, which falls within the one-to-two-pound weekly range recommended by the NIH for safe, sustainable progress.
The process has three steps: estimate your resting metabolism, adjust for your activity level, and then subtract calories to create your target intake. Here’s how to do each one.
Step 1: Estimate Your Resting Metabolism
Your body burns calories just to keep you alive. Breathing, circulating blood, regulating temperature, and repairing cells all require energy. This baseline burn is called your basal metabolic rate, or BMR, and it accounts for the largest share of the calories you use each day.
The most widely recommended formula for estimating BMR is the Mifflin-St Jeor equation. You’ll need your weight in kilograms, your height in centimeters, and your age in years. To convert, divide your weight in pounds by 2.2 and multiply your height in inches by 2.54.
- For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
As an example, a 35-year-old woman who weighs 170 pounds (77 kg) and stands 5’6″ (168 cm) would calculate: (10 × 77) + (6.25 × 168) − (5 × 35) − 161 = 770 + 1,050 − 175 − 161 = 1,484 calories per day. That’s the energy her body needs at complete rest.
Step 2: Factor In Your Activity Level
You don’t spend the day lying still, so your actual daily calorie burn is higher than your BMR. To get your total daily energy expenditure (TDEE), you multiply your BMR by a physical activity factor. The Food and Agriculture Organization of the United Nations classifies these into three tiers:
- Sedentary or light activity (desk job, minimal exercise): multiply by 1.4 to 1.69
- Active or moderately active (regular exercise, on-your-feet job): multiply by 1.7 to 1.99
- Vigorously active (intense daily training, physical labor): multiply by 2.0 to 2.4
Using the same example, if that woman walks regularly and exercises a few times a week, she’d fall around the low end of the active range. Multiplying her BMR of 1,484 by 1.7 gives a TDEE of roughly 2,523 calories per day. That’s her estimated maintenance level, the amount she’d eat to neither gain nor lose weight.
A small portion of your TDEE also comes from digesting food itself. This “thermic effect of food” accounts for about 10% of your total daily energy expenditure, so on a 2,000-calorie diet, roughly 200 calories go toward processing what you eat. The activity multipliers already account for this in a general way, so you don’t need to add it separately.
Step 3: Subtract Calories to Create Your Deficit
Once you have your TDEE, subtract calories to create a gap between what you burn and what you eat. The size of that gap determines how fast you lose weight.
A 500-calorie daily deficit works out to about 3,500 calories per week, which roughly corresponds to one pound of fat loss. A 1,000-calorie daily deficit doubles that pace to about two pounds per week. For most people, staying in the 500 to 750 range is more sustainable and easier to stick with long term.
Returning to the example: with a TDEE of 2,523, subtracting 500 gives a target intake of about 2,023 calories per day. That’s the number she’d aim to eat to lose approximately one pound per week.
Minimum Calorie Floors to Know
There’s a lower limit to how far you should cut. The Academy of Nutrition and Dietetics recommends that women stay at or above 1,200 to 1,500 calories per day, and men at or above 1,500 to 1,800 calories per day. These thresholds exist because dropping below them makes it very difficult to get adequate vitamins, minerals, and fiber from food alone. Diets below 800 calories per day are classified as very-low-calorie diets and should only be followed under direct medical supervision.
If your calculated target falls below these floors, use the floor as your intake and accept a slower rate of loss. Aggressive cuts increase the risk of muscle loss, nutrient deficiencies, and the kind of metabolic slowdown that makes weight regain more likely.
Why Your Deficit Shrinks Over Time
One important thing the simple math doesn’t capture: your body adapts. When you eat less than you burn, your metabolism slows by more than you’d expect from the weight loss alone. Research on this phenomenon, called adaptive thermogenesis, found that after just one week of caloric restriction, people’s daily energy expenditure dropped by an average of 178 calories beyond what the loss of body mass would predict. That extra slowdown stayed remarkably consistent throughout the study’s six-week period.
The practical impact is real. For every additional 100-calorie drop in daily metabolism from this adaptation, participants lost about 2 kg (4.4 pounds) less over six weeks than expected. This is one reason weight loss often stalls after the first few weeks even when you’re sticking to the plan. Your TDEE has quietly decreased, and the deficit you started with has narrowed.
The fix is straightforward: recalculate every few weeks using your current weight. As you get lighter, your BMR drops, your TDEE drops, and your target intake needs to adjust downward (or your activity needs to increase) to maintain the same deficit.
Protecting Muscle During a Deficit
When you eat fewer calories than you burn, your body pulls energy from both fat and muscle tissue. Losing muscle slows your metabolism further and changes your body composition in ways most people aren’t aiming for. Two strategies minimize muscle loss: resistance training and higher protein intake.
Research on athletes losing weight suggests that protein intakes of 1.6 to 2.4 grams per kilogram of body weight per day help preserve lean mass during a deficit. For a 170-pound person, that translates to roughly 123 to 185 grams of protein daily. Even if you’re not an athlete, aiming for the lower end of that range gives your body the raw material it needs to maintain muscle while you lose fat. Spreading protein across three or four meals rather than loading it into one also improves absorption.
Putting It All Together
Here’s the full process in sequence:
- Calculate your BMR using the Mifflin-St Jeor equation with your current weight, height, and age.
- Multiply by your activity factor (1.4 to 1.69 for sedentary, 1.7 to 1.99 for active, 2.0 to 2.4 for very active) to get your TDEE.
- Subtract 500 to 750 calories from your TDEE. This is your daily calorie target.
- Check against the calorie floor (1,200 to 1,500 for women, 1,500 to 1,800 for men). Don’t go below it.
- Recalculate every 4 to 6 weeks as your weight changes and your metabolism adjusts.
The NIH offers a free Body Weight Planner tool that runs a more sophisticated version of this math, accounting for metabolic adaptation over time. It lets you input a goal weight and time frame and generates personalized calorie and activity targets. It’s a useful cross-check against your manual calculation and often gives a more realistic projection of how long your goal will take.