To find your calorie deficit, you need two numbers: how many calories your body burns each day (your total daily energy expenditure, or TDEE) and how many calories you eat. The gap between them is your deficit. A common starting point is eating about 500 fewer calories than you burn, which typically produces about half a pound to one pound of weight loss per week. Here’s how to calculate both numbers and put them together.
Step 1: Estimate Your Basal Metabolic Rate
Your basal metabolic rate (BMR) is the number of calories your body burns just keeping you alive: breathing, circulating blood, maintaining body temperature, repairing cells. For most people, this accounts for the largest share of daily calorie burn. The most reliable formula for estimating it at home is the Mifflin-St Jeor equation, which research has shown comes closest to lab-measured values. About half of its estimates fall within 10% of what gold-standard lab testing produces, outperforming older formulas like Harris-Benedict, which tend to overestimate.
Here’s the Mifflin-St Jeor formula:
- 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
To convert your weight, divide pounds by 2.2. To convert your height, multiply inches by 2.54. For example, a 35-year-old woman who weighs 160 pounds (72.7 kg) and stands 5’6″ (167.6 cm) would calculate: (10 × 72.7) + (6.25 × 167.6) – (5 × 35) – 161 = roughly 1,420 calories per day as her BMR.
Step 2: Factor In Your Activity Level
BMR only covers what your body burns at rest. You also burn calories through movement throughout the day (fidgeting, walking to the kitchen, standing), digesting food, and intentional exercise. To estimate your full daily burn, multiply your BMR by an activity factor:
- Sedentary (desk job, little to no exercise): BMR × 1.2
- Lightly active (light exercise 1 to 3 days per week): BMR × 1.4
- Moderately active (exercise 3 to 5 days per week): BMR × 1.6
- Very active (hard exercise 6 to 7 days per week or physical job): BMR × 1.9
Using the example above, if that same woman exercises three times a week, her estimated TDEE would be roughly 1,420 × 1.6 = 2,272 calories per day. This is the number she’d need to eat below in order to lose weight.
Most people overestimate their activity level. If you’re unsure, start with the lower category. You can always adjust upward later based on real-world results.
Step 3: Choose Your Deficit Size
Once you have your TDEE, subtract calories to create your deficit. The size of the gap determines how fast you lose weight:
- 250-calorie deficit: roughly half a pound lost per week
- 500-calorie deficit: roughly half a pound to one pound per week
- 750-calorie deficit: roughly one to one and a half pounds per week
The old rule that 3,500 calories equals one pound of fat is a rough approximation, not a universal law. Real-world weight loss varies based on your starting weight, body composition, and how long you’ve been dieting. A 500-calorie daily deficit is the most commonly recommended starting point because it’s aggressive enough to produce visible results without being so restrictive that it’s hard to maintain.
There are hard floors you shouldn’t go below. Harvard Health recommends women eat no fewer than 1,200 calories per day and men no fewer than 1,500 calories per day. Dropping below these levels makes it very difficult to get adequate nutrition and can trigger your body to slow its metabolism in response.
Using an Online Calculator as a Shortcut
If doing the math yourself sounds tedious, dozens of free TDEE calculators will do it for you. You enter your age, sex, weight, height, and activity level, and they spit out your estimated daily burn. Most of these calculators use the same Mifflin-St Jeor formula under the hood. Popular options include the calculators at tdeecalculator.net, the National Institute of Health’s Body Weight Planner, and apps like MyFitnessPal. Just remember that any calculator gives you an estimate, not a precise number. Think of it as a starting point you’ll refine with real data.
Tracking What You Actually Eat
Knowing your TDEE only helps if you also know your intake. This is where most people run into trouble. Research on self-reported eating habits paints a striking picture: in one well-known study, participants underreported their actual food intake by an average of 47% and overreported their physical activity by 51%. People aren’t lying on purpose. Portions are just genuinely hard to eyeball, and calorie-dense foods like oils, nuts, and sauces add up faster than you’d expect.
A food scale is the single most effective tool for accurate tracking. Measuring peanut butter by the tablespoon versus weighing it in grams, for example, can create a difference of over 100 calories in a single serving. One comparison found that three tablespoons of peanut butter measured with a spoon came to 306 calories, but when the same scoops were weighed, the actual amount was 64 grams instead of the expected 48, totaling 408 calories. Over a full day of meals and snacks, these measurement gaps can easily erase a 500-calorie deficit without you realizing it.
Use a calorie-tracking app to log what you eat for at least one to two weeks. Weigh foods in grams when possible, especially calorie-dense items like cooking oils, cheese, nut butters, and grains. Liquids like milk, juice, and alcohol are easy to forget but can add hundreds of calories.
Protecting Muscle While Losing Fat
A calorie deficit doesn’t just burn fat. Your body will also break down some muscle tissue for energy unless you take steps to prevent it. The two most important factors are protein intake and resistance training.
Research comparing different protein levels during weight loss found that people eating about 1.0 gram of protein per kilogram of body weight per day lost the same amount of total weight as those eating less protein, but preserved significantly more muscle mass. For a 170-pound person (77 kg), that translates to roughly 77 grams of protein daily as a minimum target. Many sports nutrition guidelines suggest going even higher, to 1.2 to 1.6 grams per kilogram, especially if you’re strength training regularly.
Prioritize protein at each meal rather than loading it into one sitting. Chicken, fish, eggs, Greek yogurt, cottage cheese, tofu, and legumes are all practical sources.
Adjusting Your Numbers Over Time
Your calorie deficit isn’t a set-it-and-forget-it number. As you lose weight, your body burns fewer calories because there’s less of you to fuel. A person who loses 20 pounds will have a meaningfully lower TDEE than when they started, which means the original deficit shrinks and weight loss slows or stalls.
Recalculate your TDEE every 10 to 15 pounds of weight loss, or whenever your progress plateaus for more than two to three weeks. Plug your new weight back into the formula and adjust your calorie target accordingly. It’s also worth reassessing your activity multiplier honestly, since people sometimes become less active outside of workouts when they’re eating less (sitting more, moving more slowly, fidgeting less). This is a normal, unconscious response to eating fewer calories.
If your weight hasn’t budged in two weeks despite consistent tracking, you’re likely not in a deficit. Before cutting calories further, double-check portion accuracy and consider whether you’re accounting for everything: cooking oils, condiments, drinks, and weekend meals that tend to be less carefully tracked than weekday ones. Small, honest audits of your tracking habits often reveal the gap before any formula adjustment is needed.