Figuring out a calorie deficit comes down to two numbers: how many calories your body burns in a day, and how many fewer you need to eat to lose weight. A daily reduction of about 500 calories below your total burn rate produces roughly half a pound to one pound of weight loss per week. Getting to that number requires a bit of math, but the process is straightforward once you break it into steps.
Step 1: Estimate Your Basal Metabolic Rate
Your basal metabolic rate (BMR) is the number of calories your body burns just to stay alive: breathing, circulating blood, maintaining body temperature, keeping your brain running. Even if you stayed in bed all day, your body would still need this energy. BMR accounts for the largest share of your daily calorie burn, typically 60 to 75 percent of the total.
The most widely used formula is the Harris-Benedict equation. You’ll need your weight in kilograms (divide pounds by 2.2), your height in centimeters (multiply inches by 2.54), and your age in years.
- For men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age in years)
- For women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age in years)
For example, a 35-year-old woman who weighs 70 kg (154 lbs) and stands 165 cm (5’5″) would calculate: 447.593 + (9.247 × 70) + (3.098 × 165) − (4.330 × 35) = approximately 1,446 calories per day. That’s her baseline burn before any movement or activity gets factored in.
Step 2: Factor In Your Activity Level
Your BMR only captures what your body burns at rest. To get your total daily energy expenditure (TDEE), the full picture of calories burned in a day, you multiply your BMR by a physical activity level (PAL) multiplier. The Food and Agriculture Organization of the United Nations classifies these into three broad tiers:
- Sedentary or light activity (desk job, minimal exercise): multiply BMR by 1.4 to 1.69
- Moderately active (regular exercise, on-your-feet job): multiply BMR by 1.7 to 1.99
- Vigorously active (heavy daily training, physical labor): multiply BMR by 2.0 to 2.4
Most people with office jobs who exercise a few times a week fall somewhere between 1.5 and 1.7. Be honest here, because overestimating your activity level is one of the fastest ways to erase a deficit before you even start. Using the example above, if that same woman is lightly active (multiplier of 1.55), her TDEE would be roughly 1,446 × 1.55 = 2,241 calories per day. That’s her maintenance number, the amount she’d eat to stay at her current weight.
Step 3: Set Your Deficit Size
Once you have your TDEE, you subtract calories to create a deficit. A reduction of about 500 calories per day is the most common starting point, and it typically produces about half a pound to one pound of loss per week. For the woman in our example, that means eating around 1,741 calories daily.
You can also think of deficits as a percentage of your TDEE, which scales better across different body sizes. A 10 to 20 percent reduction is moderate and sustainable for most people. A 25 percent cut is more aggressive and works for a shorter push, but gets harder to maintain over time. Going much beyond that raises the risk of muscle loss, fatigue, and the kind of hunger that leads to binge eating.
Smaller bodies need smaller deficits. If your TDEE is only 1,800 calories, cutting 500 leaves you at 1,300, which can be difficult to sustain and may not provide enough nutrients. In that case, a 15 percent deficit (roughly 270 calories) paired with some added movement is a more realistic approach.
Why Your Deficit Shrinks Over Time
One of the most frustrating parts of fat loss is that your body fights back. When you eat less, your metabolism doesn’t stay fixed. It slows down in a process called adaptive thermogenesis. Research on overweight adults found that after just one week of calorie restriction, their daily energy expenditure dropped by an average of 178 calories beyond what weight loss alone would explain. That metabolic slowdown stayed remarkably consistent throughout the dieting period and even after it ended.
This means the deficit you calculated on day one gets smaller over weeks and months, even if you eat the same amount. A person whose metabolism adapted by an extra 100 calories per day ended up with a cumulative energy shortfall that was about 8,200 calories less than expected over six weeks, translating to roughly 2 kg (4.4 lbs) less weight loss than the math predicted.
The practical takeaway: recalculate your numbers every four to six weeks. As your weight drops and your metabolism adapts, your TDEE changes. Plugging in your new weight and adjusting keeps the deficit real.
Tracking Calories Accurately
The biggest variable in this entire process isn’t the formula you use. It’s how accurately you track what you eat. People consistently underestimate their calorie intake, and the gap is larger than most realize. In the general population, about 23 percent of people significantly underreport what they eat. Among people actively following a low-calorie diet, that number jumps to nearly 39 percent. People following carb-restrictive diets had the worst accuracy, with almost 44 percent underreporting their intake.
The most common sources of error are cooking oils and sauces (a tablespoon of olive oil is 120 calories), eyeballing portions instead of measuring them, forgetting to count drinks and snacks, and underestimating restaurant meals. A food scale removes most of the guesswork. Weighing food in grams and logging it in a tracking app is significantly more accurate than estimating cup measurements or using “medium banana” as a serving size.
You don’t necessarily need to weigh everything forever. A few weeks of careful tracking builds a mental library of what portions actually look like, and that calibration sticks with you even after you stop measuring daily.
Protecting Muscle During a Deficit
A calorie deficit doesn’t just burn fat. Without the right inputs, your body will also break down muscle for energy. The two most important tools for preventing this are protein intake and resistance training.
When you’re actively losing weight, aim for roughly 1 to 1.2 grams of protein per kilogram of body weight per day. For someone weighing 150 pounds (68 kg), that’s about 68 to 82 grams of protein daily. This is higher than what a sedentary person maintaining their weight needs, because your body has greater demand for amino acids when it’s in a deficit. Spreading your protein across three or four meals rather than loading it all into dinner gives your muscles a steadier supply throughout the day.
Resistance training sends the signal that your muscles are still needed. Even two to three sessions per week is enough to preserve lean mass during a cut. Without that stimulus, a larger percentage of the weight you lose will come from muscle rather than fat, which further lowers your metabolic rate and makes maintaining your results harder in the long run.
Putting It All Together
Here’s the full sequence in practice. Calculate your BMR using the formula above. Multiply by the activity factor that honestly reflects your lifestyle. Subtract 500 calories, or 15 to 20 percent of your TDEE, whichever leaves you at a sustainable daily intake. Track your food with a scale for at least the first few weeks. Eat enough protein. Lift something heavy a few times a week. And every month or so, rerun the numbers with your updated weight, because the target moves as you do.
If you’re losing half a pound to one pound per week, you’re in the right zone. If the scale hasn’t moved in two to three weeks despite consistent tracking, your actual intake is likely higher than you think, or your TDEE estimate needs adjusting downward. The math works, but only when the inputs are accurate.