Methodology & sources

Every number on this site is computed in code from the constants below. Nothing is estimated by an AI model at runtime, and if a constant can't be traced to a source, it doesn't ship.

Resting metabolism (BMR)

Mifflin-St Jeor (default): men 10W + 6.25H − 5A + 5, women 10W + 6.25H − 5A − 161 (W kg, H cm, A years). Source: Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51(2):241-247. Chosen because comparative validation (e.g. Frankenfield 2005 review) found it the most consistently accurate common equation for healthy adults.

Katch-McArdle (when body fat % is provided): 370 + 21.6 × lean mass (kg). Better for very lean or very muscular bodies, since it works from lean mass rather than total weight.

Activity multipliers

The standard five-level scale: sedentary ×1.2, light ×1.375, moderate ×1.55, very ×1.725, extra ×1.9. These are the conventional multipliers used with predictive BMR equations; they are the least precise part of any TDEE estimate, which is why we recommend adjusting from real-world weight trend after 2–3 weeks.

Protein ranges (g per kg body weight per day)

General health 0.8–1.2 (RDA baseline, NIH Dietary Reference Intakes) · regular training 1.4–2.0 (Jäger R, et al. ISSN Position Stand: protein and exercise. J Int Soc Sports Nutr 2017;14:20) · fat loss with muscle retention 1.8–2.4 (Helms ER, et al. 2014 review of dieting lean athletes) · muscle gain 1.6–2.2.

Macro presets

Protein anchored in g/kg (per above), fat 25–30% of calories (conservative hormonal-health floor of 20%), carbohydrates from remaining calories at 4 kcal/g (protein 4, fat 9). Cut = −20% of TDEE with protein 2.0 g/kg; lean bulk = +10% with 1.8 g/kg.

Deficit guardrails

Deficit levels −10% / −20% / −25% of TDEE, with weekly-rate estimates from the ~3,500 kcal per pound of fat approximation. Targets below computed BMR are flagged and not recommended; very-low-calorie diets belong under professional supervision.

What this site is not

KcalMath is an educational calculator, not a medical device and not individualized advice. Predictive equations carry ±10% error for most people and more at the extremes; treat every output as a starting point to adjust against two to three weeks of real-world data.