How this works
Daily calorie needs come in two layers. Your Basal Metabolic Rate (BMR) is what you burn at rest — keeping organs running and body temperature steady. Multiply that by an activity factor to get your Total Daily Energy Expenditure (TDEE) — your real daily burn including movement, exercise, and digestion. From there, eat at TDEE to maintain weight, ~500 below for steady fat loss, or ~300 above for lean muscle gain. The calculator handles all three steps in one go.
The BMR formula used here is Mifflin-St Jeor, published in 1990. It was developed by measuring resting metabolism in 498 healthy adults using indirect calorimetry, then fitting a regression. It replaced the older Harris-Benedict equation (1919) — which overestimates BMR by about 5% in modern populations because the underlying study sample was leaner and more active than today's average. A 2005 meta-analysis in the Journal of the American Dietetic Association compared four BMR equations against measured values and found Mifflin-St Jeor was within 10% of the true value for 82% of non-obese adults — the best performance of any non-DEXA method. The Katch-McArdle formula (which uses lean mass instead of total weight) is more accurate if you know your body fat percentage, but Mifflin-St Jeor is the right default when you don't.
A caveat worth keeping in mind: any formula-based estimate can be off by ±200–400 kcal/day for an individual, even though it's accurate on average. Genetics affect basal metabolism by 5–10%. People who chronically diet often have a "metabolic adaptation" of 100–300 kcal below predicted. Non-exercise activity (NEAT — fidgeting, posture, walking around) varies by up to 800 kcal/day between sedentary office workers. Use the calculator for a starting target, then track for 2–3 weeks: if your weight isn't moving the way the math says it should, your actual TDEE is just different from the prediction, and you adjust by 100–200 kcal/day until it lines up.
The formula
Activity factors: 1.2 sedentary (desk job, no exercise), 1.375 light (1–3 sessions/week), 1.55 moderate (3–5 sessions), 1.725 active (6–7 sessions), 1.9 very active (twice-daily training or hard physical job). The 500/300 cut and gain targets are starting points — adjust by 100–200 kcal after a few weeks based on actual weight change.
Example calculation
- A 30-year-old man, 175 cm, 75 kg, with a desk job and 4 weekly gym sessions chooses "moderate" activity.
- BMR ≈ 1,699 kcal/day. TDEE = 1,699 × 1.55 ≈ 2,633 kcal/day to maintain weight.
- For fat loss: 2,633 − 500 ≈ 2,133 kcal/day. For lean gain: 2,633 + 300 ≈ 2,933 kcal/day.
Frequently asked questions
Why are the deficit and surplus different sizes (500 vs 300)?
A 500 kcal deficit gives ~0.5 kg (~1 lb) of weight loss per week, the upper end of what most experts consider sustainable without losing muscle. A 300 kcal surplus is enough to support ~0.25–0.5 kg of muscle gain per week — much more than that and the extra calories tend to be stored as fat rather than turned into lean tissue.
How accurate is this for me personally?
The BMR estimate is within ~10% for most adults, and the activity multipliers can vary by another 10–15% person to person. Treat the result as a starting point. Track weight weekly for 3–4 weeks: if weight stays flat, you're at maintenance; if it moves more or less than expected, adjust calories by 100–200 kcal/day.
What about macros — protein, carbs, fat?
The most defensible default: 1.6–2.2 g protein per kg of bodyweight (preserves muscle in a deficit, supports growth in a surplus), 0.8–1.0 g fat per kg as a floor for hormone health, the rest as carbs. Athletes can go higher on carbs; lower-activity individuals can go lower. Calorie target matters most; macros matter at the margins.
Should I eat back exercise calories?
Usually no — your activity multiplier already includes typical weekly exercise. Eating back tracker-reported burns on top of that risks double-counting (and most fitness trackers overestimate burn by 20–50%). If your activity changes a lot week to week, just shift to a higher or lower activity factor instead.
What's the difference between BMR and TDEE?
BMR (Basal Metabolic Rate) is what your body would burn if you stayed in bed all day — only the energy required to keep you alive: heart, lungs, brain, kidneys, liver, body temperature. For most adults this is 1,200–1,800 kcal. TDEE (Total Daily Energy Expenditure) is BMR plus everything else: walking, working, exercise, fidgeting, and the thermic effect of food (digestion costs ~10% of intake). For weight management, TDEE is the number that matters — eat at it to maintain, below it to lose, above it to gain. BMR is the building block, but you don't live at BMR unless you're in a coma.
I'm in a deficit but my weight has stalled — why?
Three culprits, in order of likelihood. (1) You're eating more than you think — undercounting is normal and averages 20–30% in self-report studies, especially with restaurant food, oils, and bites of other people's plates. Weigh everything for 10 days to recalibrate. (2) Water-retention masking real loss — fat is gone but the scale isn't showing it because of training, sodium, or hormonal cycles. Compare 7-day rolling averages, not daily numbers. (3) Genuine metabolic adaptation — after 4–8 weeks of dieting, TDEE typically drops by 5–10% as your body reduces NEAT and shaves a bit off everything else. Solution: cut another 100–200 kcal/day, take a 7–10 day diet break at maintenance, or accept slower progress.
Are all calories the same? Does the source matter?
For body weight, calories are mostly fungible — a 200 kcal surplus is a 200 kcal surplus regardless of where it came from. But the source matters in three smaller ways. Protein has a higher thermic effect (20–30% of its calories spent on digestion vs 5–10% for carbs and 0–3% for fat), so 2,000 kcal of high-protein food has a slightly lower net energy than 2,000 kcal of high-fat food. Protein and fibre also keep you full longer per calorie, which makes hitting the target easier without willpower. And sleep, performance, and long-term health depend heavily on micronutrient quality, even when total calories are identical. Calories run the weight side of the equation; food quality runs almost everything else.