Finding your 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 deficit of 500 calories per day produces roughly one pound of weight loss per week, while a 1,000-calorie daily deficit targets about two pounds per week. Getting to those numbers requires a few steps, but none of them are complicated once you understand what goes into the calculation.
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, and keeping your organs running. Even if you lay in bed all day, your body would still burn this amount. For most people, BMR accounts for 60 to 70 percent of total daily calorie burn.
The most widely recommended formula for estimating BMR is the Mifflin-St Jeor equation. You’ll need your weight in kilograms, height in centimeters, and age in years:
- Men: (10 × weight) + (6.25 × height) – (5 × age) + 5
- Women: (10 × weight) + (6.25 × height) – (5 × 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 160 pounds (72.7 kg) and stands 5’6″ (167.6 cm) would calculate: (10 × 72.7) + (6.25 × 167.6) – (5 × 35) – 161, which equals about 1,417 calories per day.
If you carry a lot of muscle relative to your body fat, a formula called the Katch-McArdle may be more accurate. It uses lean body mass instead of total weight, because lean tissue is far more metabolically active than fat. The equation is simple: 370 + (21.6 × lean body mass in kilograms). The catch is that you need to know your body fat percentage first, which usually requires a DEXA scan or a skilled trainer with calipers.
Step 2: Factor In Your Activity Level
BMR only captures what your body burns at rest. To get your total daily energy expenditure (TDEE), the number that represents everything you burn in a full day including movement, exercise, and digestion, you multiply your BMR by an activity factor:
- Sedentary (desk job, little exercise): BMR × 1.2
- Lightly active (light exercise 1-3 days/week): BMR × 1.375
- Moderately active (moderate exercise 3-5 days/week): BMR × 1.55
- Active (hard exercise 6-7 days/week): BMR × 1.725
- Very active (intense daily training or physical job): BMR × 1.9
Using the example above, that 35-year-old woman with a BMR of 1,417 who exercises moderately three to five days a week would multiply by 1.55, giving a TDEE of roughly 2,196 calories. That’s her maintenance number, the amount she’d eat to stay at the same weight.
Most people overestimate their activity level. If you work out three times a week but sit at a desk the rest of the time, “lightly active” is probably more honest than “moderately active.” Starting with a conservative estimate and adjusting later is better than inflating the number and wondering why the math isn’t working.
Step 3: Subtract to Create Your Deficit
Once you have your TDEE, subtract 500 to 1,000 calories to create your deficit. Harvard Health Publishing recommends this range as a safe target for losing one to two pounds per week. For our example, a 500-calorie deficit would mean eating around 1,696 calories daily.
There’s a floor you shouldn’t go below. Women should not eat fewer than 1,200 calories per day, and men should stay above 1,500, unless working with a healthcare provider. Going lower than these thresholds makes it very difficult to get adequate vitamins, minerals, and protein, and it increases the risk of muscle loss, fatigue, and nutrient deficiencies. If your TDEE is low enough that a 500-calorie cut would drop you below these floors, aim for a smaller deficit and accept a slower rate of loss.
Why These Numbers Are Estimates
Every formula-based calculation is an approximation. Laboratory testing called indirect calorimetry is the gold standard for measuring resting energy expenditure, but it’s not routinely available outside of clinical settings. When researchers compare predictive equations against actual lab measurements, the results are humbling: only about 30 to 34 percent of estimates land within 10 percent of the measured value. Individual variation can swing hundreds of calories in either direction.
Wearable devices aren’t much better for calorie tracking. Research from Harvard’s School of Engineering found that the “calories burned” figures on smartwatches carry estimated error rates of 30 to 80 percent. These devices are useful for tracking relative effort (did you move more today than yesterday?) but should not be treated as precise calorie measurements.
This doesn’t mean the formulas are useless. They give you a solid starting point. The real accuracy comes from what you do next.
Adjusting Based on Real Results
The most reliable way to find your true calorie deficit is to treat your initial calculation as a hypothesis and then test it. Pick your target calorie intake based on the math above, eat consistently at that level for two to three weeks, and track your weight. Weigh yourself at the same time each day (morning, after using the bathroom, before eating) and look at the weekly average rather than any single day. Daily weight can fluctuate by several pounds due to water retention, sodium intake, and digestion.
If your weekly average is dropping by about one pound, your deficit is close to 500 calories. If it’s not moving, you’re either eating more than you think, burning less than you estimated, or both. Adjust by reducing intake by 100 to 200 calories and reassess over another two weeks.
This iterative approach matters because your body adapts to a deficit over time. A phenomenon called adaptive thermogenesis causes your energy expenditure to drop more than you’d expect from the weight loss alone. As you lose weight, your body adjusts hormone levels related to insulin, thyroid function, and appetite signaling, all of which slow your metabolism beyond what a simple recalculation would predict. This is why weight loss often stalls after several weeks even when you haven’t changed your eating habits.
Tracking Your Intake Accurately
A calorie deficit only works if you actually know how much you’re eating. Most people underestimate their intake, sometimes significantly. A food tracking app helps close that gap, but the quality of the app matters.
The most important feature is the food database. You want an app with a large, verified database that includes both packaged foods and generic whole foods from your region, not just U.S. brand names. Look for flexible logging options like barcode scanning, photo logging, or voice input, anything that reduces friction so you’ll actually use it consistently. Some newer apps include adaptive algorithms that recalculate your calorie target weekly based on your actual rate of progress, which automates the adjustment process described above.
A kitchen food scale is the single most impactful tool for accuracy. Eyeballing portions is unreliable, especially for calorie-dense foods like oils, nuts, cheese, and grains. A tablespoon of peanut butter can easily be 50 percent more than you think if you’re scooping generously. Weighing food in grams removes the guesswork.
A Practical Example Start to Finish
Here’s how this looks for a real scenario. A 40-year-old man weighs 200 pounds (90.9 kg), stands 5’10” (177.8 cm), and works a desk job but goes to the gym three times a week.
His BMR using Mifflin-St Jeor: (10 × 90.9) + (6.25 × 177.8) – (5 × 40) + 5 = 909 + 1,111 – 200 + 5 = 1,825 calories. Using the “lightly active” multiplier of 1.375 (conservative for someone with a sedentary job who exercises a few times a week), his estimated TDEE is about 2,509 calories.
To lose one pound per week, he’d subtract 500 calories and aim for roughly 2,009 calories daily. He tracks his food for three weeks, weighs himself each morning, and reviews his weekly averages. If the scale moves down about a pound per week, he’s found his deficit. If not, he drops to 1,850 and reassesses. Every six to eight weeks, or after losing 10 or more pounds, he recalculates his BMR with his updated weight, because a lighter body burns fewer calories at rest.