BMR, activity, and TDEE — three different numbers
Basal Metabolic Rate (BMR) is the energy your body burns at complete rest just to stay alive — breathing, circulation, cell repair, and organ function. It says nothing about how active you are; a marathon runner and someone who is bedbound can have a similar BMR if they're a similar size, age, and sex.
Total Daily Energy Expenditure (TDEE) adds your activity on top of that baseline: BMR multiplied by an activity factor that reflects how much you move in a typical week, from sedentary to very active. TDEE is the number that actually matters for weight management, because it's your true daily calorie burn — eat close to it and your weight holds roughly steady; eat consistently below it and you lose weight; eat consistently above it and you gain.
Both numbers are estimates, not measurements. The only way to measure energy expenditure directly is with lab equipment (indirect calorimetry) or the doubly labeled water method used in research — a formula gets you within a reasonable range for planning, not a lab-grade figure.
The Mifflin-St Jeor formula
The most widely used BMR formula today is Mifflin-St Jeor, published in the American Journal of Clinical Nutrition in 1990. It takes weight in kilograms, height in centimeters, and age in years, and uses one of two published constant sets depending on which reference equation fits — this is a formula input based on which published constants the equation calls for, not a judgment about identity.
Once you have BMR, multiply it by a conventional activity factor to get TDEE. These published multipliers range from 1.20 for a sedentary routine with little structured exercise up to 1.90 for very hard training or a highly physical job — most people land in the 1.375 (light, roughly 1–3 exercise days a week) to 1.55 (moderate, roughly 3–5 days a week) range.
Worked example: 30 years old, 175cm, 80kg, moderately active
Using the male reference equation: BMR = 10 × 80 + 6.25 × 175 − 5 × 30 + 5 = 800 + 1,093.75 − 150 + 5 = 1,748.75 kcal/day. That's the estimated calorie burn at complete rest.
Apply the moderate-activity factor (1.55) for someone exercising roughly 3–5 days a week: TDEE = 1,748.75 × 1.55 ≈ 2,710.6 kcal/day, which the calculator rounds to 2,710 kcal/day for display. Because activity level is the biggest source of uncertainty in this estimate, the TDEE Calculator shows a maintenance range of roughly ±10% around that number — about 2,440 to 2,980 kcal/day here — rather than presenting one figure as exact.
Worked example
Turning TDEE into a calorie deficit
A calorie deficit means eating consistently below TDEE. Rather than picking a raw number out of the air, it's easier to work in percentages below maintenance — a gentle deficit (around 10% below TDEE), a moderate deficit (around 15%), or a larger deficit (around 20%) are common starting points, each trading sustainability for speed.
Continuing the example above (TDEE ≈ 2,710 kcal/day): a gentle 10% deficit targets about 2,440 kcal/day, a 270-calorie daily shortfall. A moderate 15% deficit targets about 2,300 kcal/day, a 410-calorie shortfall. A larger 20% deficit targets about 2,170 kcal/day, a 540-calorie shortfall.
The planning approximation used to convert a calorie shortfall into an actual weight change is roughly 7,700 kcal per kilogram of body weight (about 3,500 kcal per pound) — a widely used estimate, not a precise conversion, since it doesn't fully capture how much of the loss is fat versus water versus lean tissue.
What that deficit means for weekly pace
Applying the conversion to the same example: the gentle 270-calorie deficit works out to about 0.25 kg (roughly 0.54 lb) per week. The moderate 410-calorie deficit is about 0.37 kg (roughly 0.82 lb) per week. The larger 540-calorie deficit is about 0.49 kg (roughly 1.08 lb) per week.
CDC general guidance describes roughly 0.5–1 kg (1–2 lb) per week as a more sustainable pace for many adults — faster loss can raise the risk of losing muscle along with fat, running short on nutrients, and regaining the weight afterward. All three scenarios in this example sit at or under that range; a deficit that computes to a pace well above it is usually a sign to ease off rather than push harder.
Why the pace changes as you lose weight
A smaller body burns fewer calories at rest, so BMR — and therefore TDEE — falls as weight drops. Eating the same fixed number of calories that produced a 15% deficit at 80kg will produce a smaller deficit once you weigh 75kg, because maintenance itself has moved down to meet you partway. This is the mechanism behind the common plateau: the plan didn't stop working, the target quietly moved.
A naive projection — total kilograms to lose, divided by weekly deficit — doesn't capture this. The Calorie Deficit Calculator instead runs a week-by-week simulation that recalculates BMR and TDEE from your projected weight at each step, which is why its timeline is usually a little longer than a simple division would suggest, and why recalculating your TDEE every few weeks (or after a meaningful weight change) keeps a fixed-calorie plan accurate.
A note on body composition
Mifflin-St Jeor uses total body weight and doesn't distinguish muscle from fat, so it can be less accurate for people with unusually high or low muscle mass. If you know your body-fat percentage — from a scale, calipers, or a scan — the Katch-McArdle formula estimates BMR from lean body mass instead, which can track more closely for that group. Body-fat measurements themselves carry meaningful error depending on the method, so treat a Katch-McArdle result as a comparison point alongside Mifflin-St Jeor, not an automatic replacement for it.
Common mistakes
- Overestimating activity level. "Moderately active" on these formulas means roughly 3–5 days a week of real structured exercise, not a generally busy lifestyle or a desk job with a daily walk — picking too high a factor inflates TDEE and can turn an intended deficit into maintenance or even a surplus.
- Treating TDEE as one exact number instead of a working estimate. The formula is a population-level approximation; individual metabolism can sit meaningfully above or below it, which is why the calculator shows a maintenance range and why the real-world test is whether your weight is actually moving the way the number predicts.
- Setting a large deficit from day one instead of starting gentle. A bigger percentage deficit is harder to sustain and makes it easier to fall short on protein, vitamins, and minerals — starting moderate and adjusting based on real results usually holds up better than starting aggressive.
- Never recalculating as weight changes. A fixed daily calorie target that produced a real deficit at a heavier starting weight will produce a smaller deficit — or none at all — once TDEE has fallen with the lost weight, which is the most common cause of an unexplained plateau.
- Eating below roughly 1,200 calories a day without medical supervision. That figure is a widely cited general caution threshold, not a personalized floor — very low intakes raise the risk of nutrient shortfalls and are generally something to plan with a professional, not a calculator alone.
- Comparing this calculator's number against a fitness tracker or app and assuming one of them is wrong. Different tools use different formulas, activity multipliers, and rounding, so it's normal for two reasonable estimates to differ by a couple hundred calories — what matters is picking one method and tracking real-world results against it consistently.