Your calorie deficit is the gap between the calories your body burns each day and the calories you eat. To find it, you need two numbers: your total daily energy expenditure (how much you burn) and your actual intake (how much you eat). The difference between those two is your deficit.
Step 1: Estimate Your Resting Metabolic Rate
Your resting metabolic rate is the energy your body uses just to stay alive: breathing, pumping blood, maintaining body temperature. For most people, this accounts for the largest chunk of daily calorie burn. The most reliable formula for estimating it is the Mifflin-St Jeor equation, which uses your weight in kilograms, height in centimeters, and age in years:
- Men: (9.99 × weight) + (6.25 × height) − (4.92 × age) + 5
- Women: (9.99 × weight) + (6.25 × height) − (4.92 × age) − 161
This formula lands within 10% of a person’s actual measured metabolic rate about 70% of the time. That’s notably better than the older Harris-Benedict equation, which only hits that accuracy window in 39 to 64% of people and tends to overestimate. If you’ve been using an older calculator and your results haven’t matched reality, the formula behind it may be part of the problem.
For a 35-year-old woman who weighs 70 kg (about 154 lbs) and stands 165 cm (5’5″), the math works out to roughly 1,387 calories per day at rest. That’s before any movement is factored in.
Step 2: Factor In Your Activity Level
Your resting metabolic rate only covers the energy cost of existing. To get your total daily burn, you multiply it by an activity factor that reflects how you actually move throughout the day. The Food and Agriculture Organization of the United Nations defines three broad categories:
- Sedentary or lightly active (multiply by 1.4 to 1.69): You have a desk job, drive most places, and don’t exercise regularly. Most of your free time is spent sitting.
- Moderately active (multiply by 1.7 to 1.99): Your job involves some physical effort, or you have a desk job but consistently exercise at a moderate to vigorous level during the week.
- Vigorously active (multiply by 2.0 to 2.4): You do strenuous physical work or intense exercise for several hours daily. Values above 2.4 are difficult to sustain long-term.
Most people overestimate where they fall. If you work at a computer and hit the gym three times a week, you’re likely in the upper end of the sedentary range or the low end of moderate, not solidly “active.” Using the example above, that 35-year-old woman with a resting rate of 1,387 calories and a desk job who exercises a few times a week might multiply by 1.6, giving her an estimated total daily burn of about 2,219 calories.
Step 3: Track What You Actually Eat
This is where most people’s calculations fall apart. Research consistently shows that people underreport their calorie intake by 20 to 30% on average. Some methods are worse than others: detailed 24-hour food recalls underestimate by about 16%, while broader dietary questionnaires miss by 26 to 32%. These aren’t small rounding errors. If you’re actually eating 2,400 calories but logging 1,900, the 500-calorie deficit you think you have doesn’t exist.
The most common sources of underestimation are cooking oils, sauces, snacks eaten without thinking, and portion sizes that are larger than what you log. Weighing food with a kitchen scale, at least for a few weeks, closes this gap significantly. You don’t need to do it forever, but it calibrates your eye so your estimates improve.
Putting the Numbers Together
Once you have a reasonable estimate of your total daily burn and an honest picture of your intake, the math is simple. If your body burns roughly 2,200 calories a day and you eat 1,700, your deficit is 500 calories. That’s it.
A common starting point is a deficit of 500 calories per day, which produces about one pound of fat loss per week once your body settles into a steady rate of weight loss. But the old rule that 3,500 calories equals one pound of fat is an oversimplification. In the first few weeks of a deficit, weight drops faster because you’re losing stored carbohydrate (glycogen) and the water bound to it, not just fat. At four weeks, researchers have measured the actual energy content of weight lost at closer to 2,208 calories per pound, meaning early losses look more dramatic than the math would predict. By six months, that value climbs to about 2,986 calories per pound as the body shifts to burning a higher proportion of fat. The pace of weight loss slows even if your deficit stays the same.
Why Your Deficit Changes Over Time
Your body doesn’t passively accept a calorie deficit. Within the first week of eating less, your metabolism begins to slow down by more than what the loss of body weight alone would explain. This is called adaptive thermogenesis. When you cut calories, insulin secretion drops, glycogen stores deplete, and your body dials back thyroid hormone output and nervous system activity. These shifts reduce the number of calories you burn at rest, effectively shrinking your deficit without you changing anything.
This means the deficit you calculated on day one won’t be the same deficit your body experiences at week eight or month six. As you lose weight, your smaller body requires fewer calories, and metabolic adaptation narrows the gap further. Periodically recalculating your numbers, or simply paying attention to whether weight loss has stalled, helps you stay aware of where you actually stand.
Don’t Trust Your Wearable’s Numbers
If you’re relying on a smartwatch or fitness tracker to tell you how many calories you burned, be cautious. Research from Harvard’s School of Engineering found that wearable devices have estimated error rates of 30 to 80% for calorie burn. They’re useful for tracking relative effort (whether today was harder than yesterday), but the absolute calorie number on your wrist after a workout is often far off. Building your deficit strategy around those numbers can lead to eating back calories you never actually burned.
A Better Approach: Calculate, Then Verify
The most practical method combines calculation with real-world feedback. Start by estimating your maintenance calories using the Mifflin-St Jeor equation and an honest activity multiplier. Subtract 300 to 500 calories to set your target intake. Then weigh yourself daily at the same time (first thing in the morning, after using the bathroom) and track the weekly average. Daily weight fluctuates due to water, sodium, and digestion, but a weekly average smooths out the noise.
If your weekly average drops by about 0.5 to 1 pound over two to three weeks, your deficit is working as intended. If the scale isn’t moving, your actual deficit is smaller than you think, either because your calorie burn is lower than estimated or your intake is higher than you’re tracking. Adjust by reducing intake by 100 to 200 calories or adding activity, then observe for another two to three weeks.
This feedback loop matters more than getting the initial calculation perfect. No formula can account for your individual metabolism, your specific gut bacteria, or how efficiently your body extracts energy from food. The formula gets you in the ballpark. Your scale trend over weeks tells you whether you’re actually in a deficit.
Calorie Floors to Stay Above
Bigger deficits aren’t always better. Harvard Health recommends that women stay above 1,200 calories per day and men stay above 1,500 calories per day unless supervised by a professional. Dropping below these levels makes it very difficult to get adequate vitamins, minerals, and protein. It also accelerates the loss of lean muscle mass, which further slows your metabolism and makes maintaining weight loss harder in the long run.
If you’re using a body composition method like a DEXA scan or bioelectrical impedance to estimate your lean mass, the Katch-McArdle equation (370 + 21.6 × lean body mass in kg) can give you a more tailored resting metabolic rate. This is particularly useful if you carry significantly more or less muscle than average for your size, since muscle tissue is the primary driver of how many calories you burn at rest.