To determine 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’re eating. The gap between those two numbers is your deficit. A typical target is 500 calories per day below your TDEE, which supports a steady loss of 1 to 2 pounds per week.
Getting to that number involves a few steps, starting with your baseline metabolism and building up from there.
Step 1: Estimate Your Resting Metabolic Rate
Your resting metabolic rate (RMR) is the energy your body burns just to keep you alive: breathing, circulating blood, maintaining body temperature, running your brain. This alone accounts for the largest share of your daily calorie burn. Two well-known formulas can estimate it using your height, weight, age, and sex.
The Mifflin-St Jeor equation works like this:
- Men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age) – 161
The Harris-Benedict equation is older and follows a similar structure with slightly different coefficients. In a study of 362 healthy adults, both formulas predicted resting metabolic rate within 10% of the actual measured value for roughly 57% of participants. That’s decent but far from perfect, which is why any formula-based number is a starting point, not a final answer. The Mifflin-St Jeor equation tends to slightly underestimate for normal-weight individuals and people over 40, while Harris-Benedict tends to overestimate for adults under 30.
If you know your body fat percentage (from a DEXA scan, skinfold calipers, or a reasonably accurate smart scale), the Katch-McArdle formula can give you a more personalized estimate. First, calculate your lean body mass: total weight minus (total weight × body fat percentage). Then plug it in: 370 + (21.6 × lean body mass in kg). Because this formula uses lean mass instead of total weight, it’s more accurate for people who carry significantly more muscle or significantly more fat than average.
Step 2: Factor In Your Activity Level
Your resting metabolic rate only covers the calories you’d burn lying in bed all day. To get your total daily energy expenditure, you multiply your RMR by an activity factor that accounts for everything else you do, from walking to your car to structured exercise.
The standard multipliers are:
- Sedentary (desk job, little or no exercise): RMR × 1.2
- Lightly active (light exercise 1 to 3 days per week): RMR × 1.375
- Moderately active (moderate exercise 3 to 5 days per week): RMR × 1.55
- Very active (heavy exercise 6 to 7 days per week): RMR × 1.725
These multipliers bundle together three separate components. Digestion itself burns about 10 to 15% of total calories (protein-heavy meals burn more during digestion than carb- or fat-heavy meals). Physical activity, both intentional exercise and the everyday movement you don’t think about (fidgeting, standing, carrying groceries, pacing while on the phone), accounts for 15 to 50% of your total burn. That range is enormous, which is exactly why this step matters so much. Two people with identical heights, weights, and ages can differ by hundreds of calories per day based on how much they move outside the gym.
Most people overestimate their activity level. If you exercise three times a week but sit at a desk the rest of the time, “lightly active” is probably more honest than “moderately active.”
Step 3: Set Your Deficit Size
Once you have your estimated TDEE, subtract calories to create your deficit. A reduction of 500 calories per day is the most common recommendation, supporting a pace of about 1 to 2 pounds lost per week according to Mayo Clinic guidelines. A smaller deficit of 250 calories per day works well if you want to preserve more muscle or if your TDEE is already on the lower side.
There is an important floor to keep in mind. Typical calorie needs for sedentary adult women range from 1,600 to 2,000 per day, and for sedentary adult men, 2,200 to 2,600. Dropping below 1,200 calories for women or 1,500 for men is generally considered too aggressive for sustained dieting without medical supervision. Going too low increases the risk of nutrient deficiencies, muscle loss, hormonal disruption, and the kind of metabolic slowdown that stalls progress.
Why the “3,500 Calories per Pound” Rule Falls Short
You’ll see this number everywhere: cut 3,500 calories and lose one pound of fat. It’s a tidy rule, but researchers at the American Institute for Cancer Research have shown it doesn’t hold up in practice. When scientists tracked closely monitored participants living in research facilities for up to three months, most lost significantly less weight than the 3,500-calorie rule predicted. Weight loss also slowed as the weeks went on, something the rule completely ignores.
The reason is that your body adapts. As you lose weight, your RMR drops because there’s less tissue to maintain. Your body also becomes more efficient at movement, burning slightly fewer calories doing the same activities. On top of that, the same calorie cut produces different results in different people. Men tend to lose faster than women, younger adults faster than older adults, and individuals within those groups vary too.
The National Institutes of Health offers a free online Body Weight Planner (sometimes called the Body Weight Simulator) that models these adaptations. It takes your height, current weight, sex, age, and goal weight, then estimates a more realistic calorie target and timeline. It’s a better planning tool than simple arithmetic.
Track, Then Adjust
Every formula and calculator gives you an estimate. Your actual deficit only becomes clear when you compare your predicted intake to what happens on the scale over two to four weeks. Here’s a practical approach:
Start by eating at your calculated target for two full weeks. Weigh yourself daily at the same time (first thing in the morning, after using the bathroom) and track the weekly average rather than any single day’s number. Daily weight fluctuates by 2 to 4 pounds due to water retention, sodium intake, and digestive contents. The weekly average smooths all of that out.
If your average weight drops by 0.5 to 1 pound per week, your deficit is working. If you’re losing faster than 2 pounds per week consistently, your deficit is likely too aggressive and you risk losing muscle along with fat. If the scale isn’t budging, your actual TDEE is probably lower than the formula predicted, or your calorie tracking has some gaps. The most common culprits are cooking oils, sauces, drinks, and portion sizes that are slightly larger than estimated. Tighten up your tracking before cutting more calories.
Protecting Muscle During a Deficit
A calorie deficit doesn’t distinguish between fat and muscle. Your body will break down both for energy unless you give it reasons to keep the muscle. The two biggest levers are protein intake and resistance training.
For people who exercise regularly, protein needs rise to about 1.1 to 1.5 grams per kilogram of body weight per day. If you lift weights or train for endurance events, the range is 1.2 to 1.7 grams per kilogram. For a 165-pound person, that works out to roughly 75 to 125 grams of protein daily, depending on activity level. Spreading protein across three meals appears more effective for muscle maintenance than loading it all into one sitting.
Resistance training two to four times per week sends a strong signal to your body that muscle tissue is still needed. Without that signal, especially in a larger deficit, your body will increasingly tap muscle for fuel. This is also why extreme deficits backfire: the more aggressively you cut, the harder it becomes to maintain the training intensity that protects lean mass. A moderate deficit with adequate protein and consistent strength training preserves far more muscle than a crash diet ever could.
Putting It All Together: A Quick Example
Say you’re a 35-year-old woman who weighs 70 kg (about 154 pounds), stands 165 cm (5’5″), and exercises three times a week. Using the Mifflin-St Jeor equation: (10 × 70) + (6.25 × 165) – (5 × 35) – 161 = 1,396 calories at rest. Multiply by 1.375 (lightly active, since the rest of her week is sedentary) and her estimated TDEE is about 1,920 calories per day.
A 500-calorie deficit would put her at 1,420 calories, which is livable but leaves very little room for error. A more moderate 300-calorie deficit, targeting around 1,620 per day, gives her more flexibility with food choices and makes it easier to hit her protein targets (about 77 to 105 grams per day at her weight and activity level). She tracks her weekly weight average for three weeks, confirms she’s losing about half a pound per week, and adjusts from there if needed.
That iterative process, estimate, track, adjust, is really the core of determining a calorie deficit. The formulas get you in the ballpark. Your body’s actual response tells you where to go next.