Figuring out 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. The difference between those two numbers is your deficit. Most people aiming for steady, sustainable fat loss target a deficit of 300 to 500 calories per day, which translates to roughly 1 to 2 pounds lost per week. Here’s how to calculate yours.
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, repairing cells. Even if you stayed in bed all day, your body would still burn this baseline amount. The most widely used formula for estimating BMR is the Mifflin-St Jeor equation, which requires your weight in kilograms, height in centimeters, and age in years.
For women: (10 × weight) + (6.25 × height) – (5 × age) – 161
For men: (10 × weight) + (6.25 × height) – (5 × age) + 5
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 = 1,418 calories per day. That’s her BMR, the starting point for everything else.
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 full number of calories you burn in a 24-hour period, you multiply your BMR by an activity factor. These multipliers account for everything from walking around the office to structured workouts:
- Sedentary (desk job, little or no exercise): BMR × 1.2
- Lightly active (light exercise 1 to 3 days per week): BMR × 1.375
- Moderately active (moderate exercise 3 to 5 days per week): BMR × 1.55
- Very active (hard exercise 6 to 7 days per week): BMR × 1.725
- Extremely active (intense training plus a physical job): BMR × 1.9
Using the example above, if that 35-year-old woman exercises moderately three to five days a week, her estimated TDEE would be 1,418 × 1.55 = roughly 2,198 calories per day. That’s the number she’d need to eat to maintain her current weight, and it’s the number she’ll subtract from to create a deficit.
Be honest with yourself about which category you fall into. Most people with office jobs who exercise a few times a week fall into the “lightly active” or “moderately active” range. Overestimating your activity level is one of the most common reasons calculated deficits don’t produce results.
Step 3: Subtract to Create Your Deficit
The CDC notes that people who lose weight at a gradual pace of 1 to 2 pounds per week are more likely to keep that weight off long-term. To hit that range, a daily deficit of 300 to 500 calories works for most people. You’ve probably heard the old rule that 3,500 calories equals one pound of fat, so cutting 500 a day should mean one pound lost per week. That math is a reasonable starting estimate, but it oversimplifies what actually happens in your body. When you lose weight, you lose a mix of fat, lean tissue, and water, and your calorie needs shift as you get lighter.
For our example, a 500-calorie deficit from a TDEE of 2,198 means a daily target of about 1,700 calories. A more moderate 300-calorie deficit would land at roughly 1,900 calories. If your calculated target drops below about 1,200 calories for women or 1,500 for men, the deficit is likely too aggressive. Eating too little makes it harder to get adequate nutrition, increases muscle loss, and tends to backfire through intense hunger and eventual overeating.
Why These Numbers Are Estimates
Every step in this process involves approximation. The Mifflin-St Jeor equation is the most accurate widely available formula, but it was developed from population averages. Your individual metabolism could be higher or lower based on genetics, muscle mass, hormonal status, and other factors the equation can’t capture. The activity multipliers are broad categories, not precise measurements of your specific day.
Technology doesn’t solve this problem as cleanly as you might hope. A Stanford study found that even the most accurate fitness trackers were off by an average of 27 percent when measuring calories burned, and the least accurate device missed by 93 percent. That means the “calories burned” number on your wristband or treadmill display is useful for comparing one workout to another, but shouldn’t be treated as a precise input for your deficit calculation.
The practical takeaway: treat your calculated deficit as a starting point, not a final answer. The real data comes from what happens on the scale and in the mirror over the following two to four weeks.
Adjusting Based on Real Results
Weigh yourself at the same time each day (morning, after using the bathroom, before eating) and look at your weekly average rather than any single reading. Daily weight fluctuates by 2 to 5 pounds based on water retention, sodium intake, and digestion. If your weekly average is dropping by roughly 0.5 to 1 percent of your body weight, your deficit is working. If it’s not budging after two to three weeks, you’re either eating more than you think or burning less than you estimated, and it’s time to adjust.
The first adjustment to make is tightening up your food tracking. Most people underestimate how much they eat. Using a food scale instead of eyeballing portions and logging everything, including cooking oils, sauces, and drinks, typically reveals a few hundred hidden calories. If your tracking is already precise and weight still isn’t moving, reduce your daily intake by another 100 to 200 calories and reassess.
Your Metabolism Adapts Over Time
As you lose weight, your body requires fewer calories to function. A lighter body simply costs less energy to maintain. But there’s an additional layer: your metabolism can slow down more than the weight loss alone would predict. This phenomenon, called adaptive thermogenesis, is your body’s response to sustained calorie restriction. Research published in Metabolism found that after just one week of dieting, participants’ daily energy expenditure dropped by an average of 178 calories beyond what their weight change would account for. That extra slowdown persisted throughout the dieting period and even after.
This means a deficit that produces steady weight loss in months one and two may stop working by month three or four, not because you’re doing anything wrong, but because the math has changed underneath you. Recalculate your TDEE every time you lose roughly 10 to 15 pounds, using your new weight in the formula. You may need to eat slightly less or move slightly more to maintain the same rate of loss.
Protecting Muscle During a Deficit
When you eat fewer calories than you burn, your body pulls energy from stored fat, but it can also break down muscle for fuel. Losing muscle is the opposite of what you want. Muscle tissue is metabolically active, meaning it helps you burn more calories at rest. Losing it makes future weight maintenance harder.
Two strategies protect against muscle loss. The first is protein intake. Nutritional guidelines from UCLA Health suggest that someone actively losing weight should aim for about 2.3 grams of protein per kilogram of body weight daily. For a 160-pound person, that comes to roughly 150 grams of protein per day. Spreading that across three or four meals tends to be more effective than loading it all into one sitting.
The second is resistance training. Lifting weights or doing bodyweight exercises two to four times per week signals your body to preserve muscle tissue even while you’re in a deficit. Cardio burns additional calories, but resistance training is what tells your body that your muscles are still needed.
A Quick-Reference Walkthrough
Putting it all together for a 40-year-old man who weighs 200 pounds (90.9 kg), stands 5’10” (177.8 cm), and exercises lightly three days a week:
- BMR: (10 × 90.9) + (6.25 × 177.8) – (5 × 40) + 5 = 1,825 calories
- TDEE: 1,825 × 1.375 = 2,509 calories
- Moderate deficit (500 cal): 2,509 – 500 = approximately 2,009 calories per day
- Conservative deficit (300 cal): 2,509 – 300 = approximately 2,209 calories per day
He’d start at one of those targets, track his weight for two to three weeks, and adjust based on actual results. If he’s losing about 1 pound per week, the number is working. If weight loss stalls after several weeks, he’d recalculate with his new weight or tighten up his tracking. The formula gives you the starting line. Consistent monitoring is what gets you to the finish.