Your calorie deficit is the gap between how many calories your body burns each day and how many you eat. To find it, you need two numbers: your total daily energy expenditure (TDEE) and your actual intake. The difference between those two is your deficit, and a good starting target for steady fat loss is 500 calories per day below your TDEE.
Step 1: Estimate Your Resting Metabolic Rate
Your body burns calories just to stay alive. Breathing, circulating blood, maintaining body temperature, and repairing cells all cost energy. This baseline burn is called your resting metabolic rate (RMR), and it accounts for roughly 60 to 70 percent of everything you burn in a day. That means most of your calorie burn happens without you doing anything at all.
The most widely recommended formula for estimating RMR is the Mifflin-St Jeor equation. A systematic review in the Journal of the American Dietetic Association found it was the most reliable option, predicting resting metabolism within 10 percent of lab-measured values in more people than any other formula tested. Here’s how it works:
- Men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
For example, a 35-year-old woman who weighs 70 kg (154 lbs) and is 165 cm (5’5″) tall would calculate: (10 × 70) + (6.25 × 165) – (5 × 35) – 161 = 700 + 1,031 – 175 – 161 = 1,395 calories per day at rest. That’s before any movement is factored in.
Step 2: Factor In Your Activity Level
Your RMR only captures what your body burns lying still. To get your total daily expenditure, you multiply by an activity factor that accounts for exercise, walking, digestion, and all the small movements you make throughout the day. Digesting food alone uses about 10 percent of your daily energy.
Use the multiplier that best fits your typical week:
- Sedentary (desk job, no planned exercise): RMR × 1.2
- Lightly active (light exercise 1 to 3 days per week): RMR × 1.375
- Moderately active (moderate exercise 3 to 5 days per week): RMR × 1.55
- Very active (hard exercise 6 to 7 days per week): RMR × 1.725
- Extremely active (intense training plus a physical job): RMR × 1.9
Using the example above, if that woman walks regularly and does a few gym sessions per week, she’d multiply 1,395 by 1.55, giving a TDEE of roughly 2,160 calories per day. That’s her estimated maintenance level, the number where her weight stays roughly stable.
Most people overestimate their activity level. If you’re unsure, pick the lower option. You can always adjust upward later based on real results.
Step 3: Set Your Deficit Size
Once you have your TDEE, subtract calories to create your deficit. The CDC recommends aiming for a weight loss pace of 1 to 2 pounds per week, which generally means eating 500 to 1,000 fewer calories than your TDEE each day.
You may have heard the old rule that 3,500 calories equals one pound of fat, so cutting 500 per day should produce exactly one pound of loss per week. That’s a useful starting estimate, but it isn’t perfectly accurate for everyone. When you lose weight, you lose a mix of fat, lean tissue, and water, and your body adjusts its calorie burn downward as you get smaller. In practice, this means the math gets less precise over time.
For the woman in our example with a TDEE of 2,160, a 500-calorie deficit would put her daily intake target at around 1,660 calories. A more aggressive 750-calorie deficit would bring her to about 1,410. Harvard Health Publishing recommends that women not go below 1,200 calories per day, and men not below 1,500, without professional supervision. Eating below those floors puts you at risk of missing essential nutrients.
Step 4: Track and Verify With Real Results
Formulas give you an educated starting point, not a guarantee. The only way to confirm you’re actually in a deficit is to track what happens over time. Weigh yourself under consistent conditions (same time of day, same clothing) and look at the trend across two to three weeks rather than any single day. Daily weight can swing 2 to 4 pounds from water retention, sodium intake, and digestive contents.
If you’re losing roughly 0.5 to 1 percent of your body weight per week, your deficit is in a healthy range. If the scale hasn’t budged after two to three weeks of consistent tracking, your actual intake is likely closer to your TDEE than you think.
One common reason for stalled progress is underestimating food intake. FDA regulations allow food labels to be off by up to 20 percent above the declared calorie count, meaning a snack labeled at 200 calories could legally contain 240. Restaurant meals, cooking oils, and sauces are especially easy to misjudge. Measuring portions with a food scale, at least for the first few weeks, helps close this gap.
Why Your Deficit Stops Working Over Time
Your body doesn’t passively accept a calorie deficit. When you eat less than you burn for an extended period, your metabolism slows down more than your weight loss alone would predict. This phenomenon, called adaptive thermogenesis, means your body actively conserves energy by reducing its baseline burn. Research on overweight adults found that after just one week of calorie restriction, their energy expenditure dropped by an average of 178 calories per day beyond what would be expected from lost body mass. That metabolic slowdown tends to persist even after the diet ends.
This is why weight loss often stalls after several weeks on the same calorie target. Your TDEE has shifted downward, and what used to be a deficit is now closer to maintenance. Recalculating your numbers every 10 to 15 pounds of weight loss, or roughly every 6 to 8 weeks, keeps your estimates current.
Signs Your Deficit Is Too Aggressive
A moderate deficit feels sustainable. An excessive one announces itself through your body. Common warning signs include persistent fatigue, frequent headaches, nausea, constipation, and dehydration. You might also notice you’re irritable and preoccupied with food, a combination sometimes called feeling “hangry” that can spiral into disordered eating patterns over time.
Severely restricting calories also increases the risk of gallstones, particularly during rapid weight loss. And if your goal is to maintain or build muscle while losing fat, you need to keep your deficit small. Strength training requires energy, so a large deficit works against muscle growth. A modest deficit of 300 to 500 calories per day, combined with adequate protein and resistance training, gives you the best chance of losing fat while preserving muscle.
If you’re experiencing several of these symptoms and losing more than 2 pounds per week consistently, your deficit is likely too large. Adding 200 to 300 calories back to your daily intake can ease these effects while still keeping you on a downward trajectory.