To calculate a calorie deficit, you need two numbers: the total calories your body burns each day and a target intake that falls below that number, typically by 500 to 1,000 calories. The gap between what you burn and what you eat is your deficit, and it’s what drives fat loss over time. Getting those numbers right takes a few steps, but none of them are complicated.
Step 1: Estimate Your Resting Metabolic Rate
Your resting metabolic rate (RMR) is the energy your body uses just to stay alive: breathing, circulating blood, maintaining body temperature. It accounts for the largest share of the calories you burn each day. The most widely recommended formula for estimating it is the Mifflin-St Jeor equation, which the Academy of Nutrition and Dietetics considers the most accurate option when you can’t measure RMR directly in a lab.
The math uses your weight in kilograms, height in centimeters, and age in years:
- Men: (9.99 × weight) + (6.25 × height) − (4.92 × age) + 5
- Women: (9.99 × weight) + (6.25 × height) − (4.92 × age) − 161
To convert pounds to kilograms, divide by 2.2. To convert inches to centimeters, multiply by 2.54. So a 35-year-old woman who weighs 170 pounds (77.3 kg) and stands 5’6″ (167.6 cm) would calculate: (9.99 × 77.3) + (6.25 × 167.6) − (4.92 × 35) − 161, which comes out to roughly 1,448 calories per day at rest.
Step 2: Factor In Your Activity Level
Your resting metabolic rate only covers the energy cost of lying still all day. To get your total daily energy expenditure (TDEE), the number that reflects everything you actually do, you multiply your RMR by an activity factor. These multipliers are standard estimates used across nutrition science:
- Sedentary (desk job, little exercise): 1.2
- Lightly active (light exercise 1–3 days/week): 1.375
- Moderately active (moderate exercise 3–5 days/week): 1.55
- Very active (hard exercise 6–7 days/week): 1.725
- Extremely active (physical job plus intense training): 1.9
Using the example above, that 35-year-old woman with an RMR of 1,448 who exercises moderately would multiply by 1.55, giving a TDEE of roughly 2,244 calories per day. That’s her maintenance number, the intake at which she’d neither gain nor lose weight. Worth noting: some researchers argue the sedentary multiplier of 1.2 may be too low for most people, with actual sedentary energy expenditure falling closer to 1.4 to 1.69. If you find you’re losing weight faster than expected, your true multiplier may be a bit higher than you estimated.
Step 3: Subtract to Create Your Deficit
Once you know your TDEE, you subtract calories to create the deficit. Harvard Health Publishing recommends cutting 500 to 1,000 calories per day to lose one to two pounds per week, a rate the CDC also considers safe and sustainable as of its 2025 guidelines. For our example, a 500-calorie daily deficit would mean eating around 1,744 calories per day.
There are floor numbers you shouldn’t go below. Women should generally stay above 1,200 calories per day and men above 1,500, unless working with a healthcare provider. Dipping below those thresholds makes it very difficult to get adequate nutrition and can accelerate the metabolic slowdown that makes weight loss harder over time.
If subtracting 500 calories would put you below those minimums, a smaller deficit of 250 to 300 calories still produces meaningful results. You can also widen the gap through exercise rather than cutting food further.
Why the 3,500-Calorie Rule Is Misleading
You’ve probably heard that cutting 3,500 calories equals one pound of fat lost. It’s a tidy number, and it’s been repeated for decades, but research has shown it overpromises. When researchers tested the rule against data from seven closely monitored weight loss studies, where participants lived in research facilities for up to three months, most people lost significantly less weight than the 3,500-calorie formula predicted.
The rule fails for two reasons. First, it assumes every person responds to the same calorie cut identically. In reality, men tend to lose weight faster than women on the same deficit, younger adults faster than older adults, and individuals within those groups still vary. Second, it treats your metabolism as fixed. As you lose even a pound or two, your body requires slightly fewer calories, so the same intake that created a deficit at 170 pounds creates a smaller deficit at 165. Your weight loss naturally slows unless you periodically recalculate.
The National Institutes of Health offers a free online Body Weight Simulator that models these real-world dynamics. It uses your height, current weight, sex, and goal weight to generate a more realistic timeline than the 3,500-calorie shortcut ever could.
How Your Body Adapts to a Deficit
Within the first week of eating in a deficit, your body starts adjusting. This process, called adaptive thermogenesis, means your total energy expenditure drops by more than the lost body mass alone would explain. One study measured this adaptation at an average of about 178 calories per day after just one week of calorie restriction. That’s a significant chunk of a 500-calorie deficit effectively being erased by your body’s attempt to conserve energy.
The practical consequence: for every 100 calories per day of extra metabolic slowdown a person experienced in that study, they lost about 2 kg (4.4 pounds) less over six weeks than expected. This adaptation was remarkably consistent within individuals, meaning some people are simply stronger adapters than others. If your weight loss stalls after a few weeks despite sticking to your numbers, this is likely why. Recalculating your TDEE at your new weight and adjusting your intake (or increasing activity) can restart progress.
Protecting Muscle While Losing Fat
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 most important signals are protein intake and resistance training.
For people actively losing weight, a protein intake of 1 to 1.2 grams per kilogram of body weight per day is a common recommendation. For a 170-pound person, that works out to roughly 77 to 93 grams of protein daily. Spreading that across meals rather than loading it into one sitting gives your body a more consistent supply for muscle repair.
Resistance training, even two to three sessions per week, tells your body that muscle tissue is actively needed. Without that signal, a larger share of the weight you lose will come from lean mass rather than fat, which lowers your metabolic rate even further and makes maintaining your results harder in the long run.
Putting It All Together
Here’s the full process in order. Calculate your RMR using the Mifflin-St Jeor equation. Multiply by your activity factor to get your TDEE. Subtract 500 calories (or a smaller amount if that would drop you below safe minimums) to set your daily intake target. Aim for 1 to 1.2 grams of protein per kilogram of body weight, and include some form of strength training to preserve muscle.
Every four to six weeks, recalculate. Your TDEE changes as your weight changes, and adaptive thermogenesis means the deficit that worked in week one may be smaller than you think by week six. Treat your calorie target as a living number, not a permanent setting. The people who sustain weight loss over months and years are the ones who keep adjusting.