What does your body burn at rest?

BMR (Basal Metabolic Rate) is the calorie cost of staying alive — measured in laboratory conditions: 12 hours fasted, 8 hours of sleep first, lying still in a temperature-controlled room. It excludes the energy your body spends digesting food and regulating temperature.

RMR (Resting Metabolic Rate) is BMR + a small extra slice (the residual thermic effect of yesterday's food, mild thermoregulation). Almost every "BMR calculator" on the web — including this one — actually estimates RMR, which runs roughly 10% higher than true BMR.

TDEE adds the calories you burn through movement (NEAT + EAT) and digestion (TEF). Eating at TDEE holds weight; below it cuts; above it gains.

• Mifflin–St Jeor (1990) — accurate within ±10% for 71% of subjects vs indirect calorimetry. Current gold standard for the general adult population.
• Katch–McArdle — drives off lean body mass, not total weight. Beats Mifflin once you have a reliable body-fat reading, especially for lean trainees.
• Cunningham (1980) — ~5% higher than Katch-McArdle, validated on athletes. Use it if you're trained and lean (under ~15% body fat).
• Harris–Benedict (1984 revised) — kept for legacy. Tends to overestimate by 5–8% in modern sedentary adults.
• Henry / Oxford (2005) — outperforms Schofield/Mifflin in tropical and South Asian populations. Use it if you're of native South Asian descent.

When the spread between formulas exceeds 10%, your body composition or ethnicity sits outside the reference population. Trust the formula closest to your demographic.

The Pontzer et al. Science 2021 study — 6,600 subjects across 29 countries using doubly-labelled water — showed daily energy expenditure is essentially flat from age 20 to 60. The decline begins around age 60 and runs ~0.7% per year after that. The "metabolism slows after 30" trope is wrong.

What changes between 30 and 60 isn't metabolism — it's activity (NEAT and EAT shrink), muscle mass (sarcopenia begins), and food awareness. The good news: those are fixable.

Other myths this tool debunks: "1 lb of muscle burns 50 kcal" (reality: ~6–13 kcal/day), "starvation mode shuts down BMR" (reality: 5–15% partial drop over months of deficit, not a shutdown), "metabolism is fixed" (reality: 26% variance between people of the same age, sex, weight, and height).

• Lean body mass — every pound of skeletal muscle burns ~6–13 kcal/day at rest. A 10-lb muscle gain adds 60–130 kcal/day. Real, not life-changing.
• Thyroid status — hyperthyroid can raise BMR 30–60%; untreated hypothyroidism drops it 10–25%. If you have either, get bloodwork before trusting any calculator.
• Illness and injury — fever raises BMR ~13% per °C; recovery from surgery or burns can raise it 20–50%.
• Pregnancy & lactation — third-trimester pregnancy adds ~300 kcal/day; lactation adds 400–500 kcal/day.
• FTO gene variant — present in ~16% of the global population; can shift BMR by ±160 kcal/day. You can't change it.
• Adaptive thermogenesis — after 8+ weeks of cutting, BMR can drop 5–15% beyond what weight loss alone would predict. Plan diet breaks.

Mifflin–St Jeor and Harris–Benedict were derived from predominantly European reference populations. Indian and other South Asian adults run BMR roughly 5–10% lower than these equations predict, partly due to body composition (lower lean mass for height) and partly due to genuine metabolic differences (FAO/WHO 2004, ICMR-NIN 2020).

Toggle the South Asian context flag under Precision controls — it applies a documented ×0.95 multiplier. Or just pick the Henry/Oxford formula in the comparison strip; it was derived on tropical and Indian samples specifically.