BMR (Basal Metabolic Rate) is what your body burns at complete rest — breathing, circulation, temperature regulation. The Mifflin-St Jeor equation (1990) estimates it from weight, height, age and sex: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 for men, the same with −161 instead of +5 for women. Across a 1990 validation study of 498 people, it predicted measured BMR within 10% about 82% of the time — the best of the four major formulas compared, and the reason it's the default in most clinical settings today. Multiply BMR by an activity multiplier (1.2 to 1.9) to get TDEE — the calories you actually burn in a day, the number that matters for any weight goal.
A 30-year-old man at 70 kg and 175 cm has a BMR of 1,649 kcal. Sedentary, his TDEE is 1,979 kcal; lightly active, 2,267 kcal — a 288 kcal difference from activity level alone, more than most single food choices.
Women carry proportionally more body fat and less metabolically active lean mass than men at the same weight and height, which lowers resting energy needs — the constant is simply what the original regression found fit the measured data, not a rounded assumption.
Because most people count workout intensity, not weekly total — three hard gym sessions plus a sedentary desk job the other four days is "lightly active," not "very active." Overestimating activity is the most common reason a TDEE estimate ends up too high.
Mifflin-St Jeor is typically accurate to within about 10% for most people — meaningful, but not exact. Treat the result as a starting point: eat at the target for 2-3 weeks, track actual weight change, and adjust by roughly 100 kcal/day if real results don't match the prediction.
Only if your chosen activity level didn't already include that exercise. If you picked "sedentary" and added a workout separately, eating back some of it is reasonable — but if you picked "moderately active" because you already train regularly, that training is already priced into the multiplier.
The naive 3,500 kcal/lb rule assumes a fixed deficit forever, but your BMR depends on your weight — as you lose weight, you need fewer calories just to stay alive, which quietly shrinks the deficit even if you eat exactly the same amount every day. The realistic line recalculates BMR at every step for exactly this reason, matching what published critiques of the fixed-deficit rule describe: real results consistently come in below the naive prediction, more so the longer the timeframe.
Protein is set first from your chosen grams per kilogram of body weight (typically 1.6–2.2 g/kg for active adults). Remaining calories after protein (4 kcal/g) are split 50/50 between carbohydrates and fat (4 and 9 kcal/g). That approximates a flexible ~30/40/30-style split when protein sits near the middle of the range, without locking you into fixed percentages.
This page has changed in 46 archived releases, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.