A calorie deficit is the difference between the calories your body burns in a day and the calories you actually eat, and calculating it starts with estimating your Total Daily Energy Expenditure (TDEE) using the Mifflin-St Jeor equation. TDEE is your resting metabolic rate multiplied by an activity factor between 1.2 (sedentary) and roughly 1.9 (extra active). Once you know that number, subtracting the calories you consume from it gives your deficit; a sustained daily deficit of about 500 kcal is widely used as a starting point for steady, sustainable fat loss. The Calorie Calculator handles this entire pipeline automatically by combining the Mifflin-St Jeor BMR with an activity multiplier and then adjusting the final number up or down depending on whether you want to lose, maintain, or gain weight.
Most people who search for "how to calculate calorie deficit" already understand the broad idea: eat less than you burn. The tricky part is turning that idea into a number that fits your actual body and your actual week. That is where two specific values have to be nailed down first. Your BMR, short for Basal Metabolic Rate, is the energy your body needs just to keep organs, breathing, and circulation running while you sit still. Your TDEE layers movement, exercise, and the thermic effect of food on top of that. The deficit is the gap between TDEE and intake — never between BMR and intake, since cutting below BMR for long stretches is associated with muscle loss and metabolic adaptation.
The Mifflin-St Jeor Equation Behind the Numbers
Among the handful of published BMR formulas, the Mifflin-St Jeor equation, introduced in 1990, is the one most modern tools default to because it tends to predict resting energy expenditure within about 10 percent for most adults. It separates the calculation by sex so that differences in lean mass and hormonal profile are accounted for at the formula level rather than patched on later.
| Sex | Mifflin-St Jeor BMR formula |
|---|---|
| Male | 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5 |
| Female | 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161 |
After BMR is computed, it is multiplied by an activity factor to estimate TDEE. These multipliers are standardized bands rather than precise measurements, which is why two people with identical stats can have meaningfully different real-world calorie needs depending on their job, commute, and training volume.
| Activity level | Multiplier applied to BMR | Typical weekly pattern |
|---|---|---|
| Sedentary | 1.2 | Desk work, little to no planned exercise |
| Lightly active | 1.375 | Light activity 1–3 days a week or daily walking |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extra active | 1.9 | Physical job plus daily training |
The exact figures that come out of this pipeline vary with each person's input values, so let a calculator do the arithmetic rather than working through the formula by hand. The Calorie Calculator applies Mifflin-St Jeor, picks the right activity multiplier, and shows your target calories for each goal in one screen.
What a Safe Deficit Looks Like
General clinical guidance published by major health bodies suggests that a daily deficit between roughly 300 and 750 kcal is a reasonable range for sustainable loss in adults without specific medical conditions, with larger deficits reserved for short phases under supervision. A useful rule of thumb is that one pound of stored fat contains about 3,500 kcal, so a 500-kcal daily deficit lines up with losing roughly one pound per week on average — though week-to-week water shifts can make the scale do strange things, especially early on.
Going below about 1,200 kcal per day for women or 1,500 kcal per day for men without medical oversight is generally discouraged because it becomes hard to hit protein, micronutrient, and fiber targets. Tracking app data and the lived experience of most dieters suggest that aggressive deficits look impressive on paper but produce more muscle loss, more hunger, and more rebound weight once the diet ends. A moderate deficit tends to win over months because it is easier to maintain.
Get Your Calorie Deficit Number
- Open the Calorie Calculator in your browser. Nothing is installed and nothing is uploaded, so the inputs stay on your device.
- Pick your sex, then type in your age in years, your weight in kilograms, and your height in centimeters. Stick with metric so the formula is used as published.
- Choose the activity level that most honestly matches your typical week. If you are between two levels, round down rather than up; most people overestimate how much they move.
- Read the maintenance calories shown on screen. That value is your TDEE — the daily burn the calculator just computed from your BMR and your activity multiplier.
- Use the goal toggle to switch between Lose, Maintain, and Gain weight. The number updates to your daily calorie target for that goal — the target for "Lose" is your maintenance calories minus the built-in deficit.
- Note the result and pair it with a quick protein target (around 1.6 to 2.2 g per kilogram of bodyweight is a common range) so the weight you lose is mostly fat rather than muscle.
Run the same flow again any time your weight or activity level changes meaningfully, since TDEE shifts as those numbers shift. There is no single "right" deficit for the entire journey; recalculating every few weeks keeps the math honest.
Picking the Right Activity Multiplier
The most error-prone step for most people is the activity selector. A "lightly active" rating covers a desk worker who takes a 20-minute walk most days, not a gym-goer doing four hard sessions a week — that person usually sits closer to "moderately active." A "very active" rating generally implies daily structured training on top of a non-physical job, while "extra active" is reserved for people who train intensely and also work on their feet most of the day. Picking one rung too high is a common reason deficit calculations stop producing results after the first few pounds.
If you train with a heart rate monitor or wear a fitness watch, compare its weekly active-kilocalorie estimate to a separate Target Heart Rate Calculator to sanity-check the intensity of your sessions. Effort matters as much as frequency when it comes to picking the right band.
Calculating the Deficit Number Yourself
If you want to see the formula in motion, a worked example helps. Take a hypothetical 30-year-old female, 165 cm tall, weighing 65 kg, who picks "lightly active." Plugging into the Mifflin-St Jeor equation for females:
BMR = 10 × 65 + 6.25 × 165 − 5 × 30 − 161
BMR = 650 + 1,031.25 − 150 − 161
BMR = 1,370.25 kcal/day
Apply the 1.375 activity factor for "lightly active":
TDEE = 1,370.25 × 1.375 = 1,884.1 kcal/day
A moderate deficit subtracts about 500 kcal from this TDEE, which works out to a daily target near 1,384 kcal. The exact target your tool recommends will depend on which deficit it applies, so verify on the Calorie Calculator rather than copying the figure above. For a related body-composition view that pairs well with this number, the BMI Calculator gives a quick category read, while a lean-mass-aware reading comes from the Body Fat Calculator.
Beyond the Formula: What the Number Doesn't Capture
Once you have a target, the day-to-day reality of hitting it depends on factors the formula cannot know about. Sleep deprivation tends to lower activity-based energy expenditure and raises hunger signals, which can quietly shrink your deficit. Higher-protein diets tend to preserve more lean mass during a cut, which keeps BMR higher than it would otherwise fall. Strength training has a similar protective effect, and consistent NEAT — the calories burned through walking, standing, and fidgeting — is one of the strongest predictors of how big someone's actual deficit ends up being.
For that reason, treat the calculator's output as a starting hypothesis rather than a verdict. Weigh yourself under consistent conditions, average the numbers across a week, and adjust in 100-to-200-kcal steps if the trend stalls for two to three weeks.
Putting the Daily Target Into Macros
Calorie targets become far easier to hit when they are translated into grams of protein, carbs, and fat. A common split is roughly 30 percent of calories from protein, 35 percent from carbohydrates, and 35 percent from fat, though athletes and people on higher-protein cuts often shift the first two numbers around. For step-by-step math from any calorie target into precise grams, the Macro Calculator applies the standard 4/4/9 kcal-per-gram factors and is built to chain cleanly off the deficit number you just produced.
When to Recalculate
Recalculate your TDEE and your goal calories on three triggers: when your weight has changed by more than a few percent from the value you entered, when your training volume shifts (for example, moving from off-season to a hard training block), or when your weekly average weight has stalled for two to three weeks at the same intake. The Mifflin-St Jeor equation itself does not change, but the inputs it gets fed do, and the only inputs that matter are the ones that currently reflect your life rather than your life from a few months ago.
A Short FAQ-Style Wrap-Up
Two practical notes are worth keeping in mind while you work through the steps above. First, the Mifflin-St Jeor equation is an estimate with a known error band of roughly plus or minus 10 percent for healthy adults, so two people with identical stats can have meaningfully different real TDEE. Second, "lose weight" on the scale and "lose fat" are not always the same thing, especially in the first two to three weeks when water, glycogen, and gut content can dominate the readout. Use the calculator as a compass rather than a contract, give each adjustment at least two weeks of consistent adherence, and let the trend on the scale — not any single morning — tell you whether to hold, tweak, or recalculate.
See also: How to Calculate Age Between Two Dates in Excel – Instant Online Tool.
For a deeper look, see How to Use the Chinese Gender Predictor for a Fun Baby Guess.