To calculate a calorie deficit, you need two numbers: how many calories your body burns in a day and how many you eat. The difference between those two is your deficit. A common starting point is eating 500 fewer calories than you burn daily, though the right deficit for you depends on your size, activity level, and goals. Here’s how to work through each step.
Step 1: Estimate Your Resting Metabolism
Your body burns calories just to keep you alive. Breathing, circulating blood, maintaining body temperature, repairing cells. This baseline burn is called your basal metabolic rate (BMR), and it accounts for the largest chunk of your daily calorie expenditure. The most widely recommended formula for estimating it is the Mifflin-St Jeor equation.
For men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
For women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
If you use pounds and inches, convert first. Divide your weight in pounds by 2.2 to get kilograms, and multiply your height in inches by 2.54 to get centimeters. 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 = 727 + 1,047.5 – 175 – 161 = roughly 1,439 calories per day at rest.
If you know your body fat percentage (from a DEXA scan or other measurement), you can use the Katch-McArdle formula instead: 370 + (21.6 × lean body mass in kg). This version tends to be more accurate for people who are significantly overweight or very lean, since it directly accounts for how much muscle you carry. Research on women across different BMI categories found it was more accurate than other formulas for those with a BMI above 25.
Step 2: Factor In Your Activity Level
Your BMR only reflects what you’d burn lying in bed all day. To get your total daily energy expenditure (TDEE), you multiply your BMR by a physical activity level (PAL) factor that reflects how much you move throughout the day, not just formal exercise but also walking, standing, fidgeting, and household tasks.
- Sedentary or lightly active (desk job, minimal exercise): multiply BMR by 1.4 to 1.69
- Moderately active (regular exercise, active job): multiply BMR by 1.7 to 1.99
- Very active (intense daily training, physical labor): multiply BMR by 2.0 to 2.4
Using the example above, our 35-year-old woman with a BMR of 1,439 who exercises three times a week and otherwise works at a desk might use a multiplier of 1.55 (midrange sedentary-to-light). That gives her a TDEE of roughly 2,230 calories per day. That’s her maintenance number: the amount she’d eat to stay the same weight.
Most people overestimate their activity level. If you’re unsure, start with the lower end of the range. A desk job with an hour of exercise most days still lands in the light-to-moderate range for most people. The difference between sitting and standing is surprisingly small. Harvard Health data shows sitting burns about 80 calories per hour while standing burns about 88. Walking, by contrast, burns around 210 calories per hour, which is why step count matters more than whether you use a standing desk.
Step 3: Subtract Calories to Create Your Deficit
Once you have your TDEE, subtract calories from it. The size of the gap determines how quickly you lose weight.
A 500-calorie daily deficit is the most common recommendation. The old rule of thumb said this would produce exactly one pound of fat loss per week, based on the idea that a pound of fat contains about 3,500 calories. That math is a useful approximation but not perfectly accurate. When you lose weight, you lose a mix of fat, lean tissue, and water, and the ratio shifts over time. Your body also adapts, so the same deficit produces less weight loss as months go on.
A more practical way to think about it: a 500-calorie deficit will produce noticeable, steady weight loss for most people without feeling extreme. A 250-calorie deficit is slower but easier to maintain. A 750- to 1,000-calorie deficit is aggressive and generally only appropriate for people with a significant amount of weight to lose.
Regardless of your deficit size, Harvard Health Publishing notes that women should not eat below 1,200 calories per day and men should not go below 1,500 without medical supervision. If your TDEE is relatively low (common for shorter, older, or lighter individuals), a 500-calorie deficit might push you below those floors. In that case, a smaller deficit combined with more movement is the safer approach.
A Full Example
Here’s the complete calculation for a 40-year-old man who weighs 200 pounds (90.9 kg), stands 5’10” (177.8 cm), and exercises moderately three to four days a week.
BMR: (10 × 90.9) + (6.25 × 177.8) – (5 × 40) + 5 = 909 + 1,111.25 – 200 + 5 = 1,825 calories.
TDEE: 1,825 × 1.7 (low end of moderately active) = 3,103 calories.
With a 500-calorie deficit: 3,103 – 500 = 2,603 calories per day as a target intake.
With a 750-calorie deficit: 3,103 – 750 = 2,353 calories per day.
Both of those are well above the 1,500-calorie floor, so either option is reasonable to start with.
Why the Numbers Change Over Time
Your calorie deficit is not a set-it-and-forget-it calculation. As you lose weight, your BMR drops because there’s less of you to keep alive. A person who once burned 1,825 calories at rest might burn 1,650 after losing 20 pounds. If you keep eating the same amount, your deficit shrinks and weight loss slows or stalls.
On top of that normal recalculation, your body may temporarily lower its energy expenditure beyond what the math would predict. This is sometimes called adaptive thermogenesis, and it’s essentially your metabolism pushing back against the calorie gap. A systematic review in the British Journal of Nutrition found that this effect does occur in some people during active weight loss, but it tends to be small and often disappears once weight stabilizes. It’s not a permanent metabolic “break,” but it can make the last stretch of a weight loss phase feel slower.
The practical takeaway: recalculate every 10 to 15 pounds lost, or roughly every two to three months. Plug your new weight back into the formula and adjust your target intake accordingly.
Using a Percentage Instead of a Flat Number
Some people prefer setting their deficit as a percentage of TDEE rather than a fixed calorie number. A 15 to 20 percent deficit is considered moderate and sustainable. A 25 percent deficit is aggressive but manageable for most. Anything above 25 percent starts to increase the risk of muscle loss, energy crashes, and the kind of hunger that makes the plan hard to stick with.
Using our example of the man with a 3,103-calorie TDEE: a 20 percent deficit would be about 620 calories, giving him a target of roughly 2,480 per day. The percentage approach automatically scales to your body size, which makes it a useful alternative if you’re significantly larger or smaller than average.
What You Eat Inside the Deficit Matters
Not all calories behave the same way once you eat them. Your body uses energy to digest food, a process that accounts for roughly 10 percent of daily calorie burn. But the cost varies dramatically by nutrient type. Protein costs the most to digest, burning 15 to 30 percent of its calories during processing. Carbohydrates burn 5 to 10 percent. Fats burn 0 to 3 percent.
This means a higher-protein diet effectively increases your deficit slightly, even at the same calorie level, because more of those calories are “lost” during digestion. Protein also helps preserve muscle mass during weight loss, which keeps your resting metabolism higher. Aiming for 25 to 30 percent of your calories from protein is a practical target when eating in a deficit.
Tools That Do the Math for You
If formulas aren’t your thing, the NIH Body Weight Planner is one of the better free tools available. Developed by researchers at the National Institute of Diabetes and Digestive and Kidney Diseases, it uses a dynamic model that accounts for changes in metabolism, appetite, and calorie expenditure over time. You enter your current weight, goal weight, timeline, and activity level, and it generates a daily calorie target. Unlike static calculators, it adjusts for the metabolic slowdown that happens as you lose weight, so its predictions tend to be more realistic over months-long timelines.
Whatever method you use, treat the number you get as a starting point, not a precise prescription. Track your intake and weight for two to three weeks. If you’re losing 0.5 to 1 pound per week, the math is working. If nothing is changing, your TDEE estimate is probably too high, and you need to either reduce intake by another 100 to 200 calories or add more movement to your day.