To calculate a calorie deficit, you need two numbers: how many calories your body burns in a day, and a target intake that falls below that number. A reduction of about 500 calories per day from your total daily expenditure is the most common starting point, which typically produces about half a pound to one pound of weight loss per week. Here’s how to find your specific numbers.
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 running your organs. Even if you lay in bed all day, your body would still burn this many calories. BMR accounts for the largest share of your daily calorie burn, roughly 60 to 70 percent of the total.
The most widely used formula is the Mifflin-St Jeor 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: (10 × weight in kg) + (6.25 × height in cm) – (5 × age) + 5
For women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age) – 161
As an example, a 35-year-old woman who weighs 170 pounds (77 kg) and stands 5’6″ (168 cm) would calculate: (10 × 77) + (6.25 × 168) – (5 × 35) – 161 = 770 + 1,050 – 175 – 161 = 1,484 calories. That’s her BMR, the baseline her body needs before any physical activity is factored in.
Step 2: Factor In Your Activity Level
Your total daily energy expenditure (TDEE) includes everything beyond basic survival: walking, exercising, fidgeting, cooking, even digesting food. To estimate it, multiply your BMR by an activity factor.
- Sedentary or lightly active (desk job, little exercise): BMR × 1.4 to 1.69
- Moderately active (exercise 3 to 5 days per week): BMR × 1.7 to 1.99
- Very active (hard exercise 6 to 7 days, physical job): BMR × 2.0 to 2.4
Using the example above, if that woman walks a few times a week and exercises lightly, a multiplier of about 1.5 gives a TDEE of roughly 2,226 calories per day. That’s her estimated maintenance level, the amount she’d eat to neither gain nor lose weight.
One thing to keep in mind: non-exercise movement matters more than most people realize. Everyday activity like standing, pacing, taking the stairs, and doing household chores (called NEAT, or non-exercise activity thermogenesis) can vary by up to 2,000 calories a day between two people of similar size. If you have a job where you’re on your feet all day versus sitting at a computer, that difference alone can shift your TDEE significantly. Be honest when choosing your multiplier.
Step 3: Subtract to Create Your Deficit
Once you have your TDEE, subtract calories to create your deficit. A 500-calorie daily deficit is the standard recommendation and produces steady, sustainable fat loss. For someone with a TDEE of 2,226, that means a target intake of about 1,726 calories per day.
You can also think of your deficit as a percentage of your TDEE rather than a flat number. A 15 to 20 percent reduction works for most people. For a TDEE of 2,226, a 20 percent deficit would be about 445 calories, giving you a daily target of roughly 1,780. The percentage approach automatically scales to your body size, so a smaller person doesn’t end up with an unreasonably low calorie target.
One important floor to keep in mind: eating fewer than about 1,200 calories per day makes it very difficult to get adequate nutrition. Going that low can also backfire by slowing your metabolism and stalling weight loss as your body shifts into a conservation mode.
Why the Math Is an Estimate, Not a Guarantee
The old rule that cutting 3,500 calories equals one pound of fat loss is widely cited but increasingly considered outdated. As Dr. Fatima Cody Stanford, an obesity specialist at Harvard Medical School, puts it, the “calorie in, calorie out” model is an oversimplification. How your body burns calories depends on the type of food you eat, your individual metabolism, and even the composition of your gut bacteria.
Your body also actively resists sustained weight loss through a process called metabolic adaptation. As you lose weight, your resting metabolic rate drops, meaning you burn fewer calories doing the same activities. Research from the University of Alabama at Birmingham found that premenopausal women with overweight experienced measurable metabolic adaptation after losing about 16 percent of their body weight. The good news: this adaptation isn’t permanent. After a few weeks of weight stabilization (eating at maintenance), metabolic adaptation significantly decreases or disappears entirely, and further weight loss becomes possible again.
This is why weight loss rarely follows a perfectly straight line. Plateaus are a normal, physiological response, not a sign you’re doing something wrong.
What You Eat Affects How Many Calories You Actually Absorb
Not all calories cost the same amount of energy to digest. Your body uses calories just to break down and process food, a phenomenon called the thermic effect of food. Protein costs the most to digest, burning 15 to 30 percent of its calories during processing. Carbohydrates burn 5 to 10 percent, and fats burn just 0 to 3 percent.
This means that at the same calorie count, a higher-protein diet effectively creates a slightly larger deficit because more energy is lost to digestion. Protein also helps preserve muscle mass during weight loss, which matters because muscle tissue burns more calories at rest than fat does. Active individuals benefit from eating 1.2 to 1.5 grams of protein per kilogram of body weight daily. For a 170-pound person, that’s roughly 84 to 116 grams of protein per day.
Putting It All Together
Here’s the full calculation in sequence, using a concrete example. A 40-year-old man, 5’10” (178 cm), 200 pounds (91 kg), who exercises three times a week:
BMR: (10 × 91) + (6.25 × 178) – (5 × 40) + 5 = 910 + 1,112.5 – 200 + 5 = 1,827.5 calories.
TDEE: 1,827.5 × 1.7 (moderately active) = 3,107 calories.
With a 500-calorie deficit: target intake of about 2,607 calories per day.
With a 20 percent deficit: target intake of about 2,486 calories per day.
Either approach is reasonable. Start with one, track your weight over two to three weeks, and adjust. If you’re losing more than about two pounds per week, eat a bit more. If nothing is changing after three weeks, reduce by another 100 to 200 calories or increase your activity. The calculation gives you a starting point. Your actual results over time tell you whether to adjust up or down.