BMI, BMR, TDEE — what they actually mean

Most online calculators give you only one number. This tool combines three because they answer different questions:

• BMI tells you whether your weight is in a healthy range for your height.
• BMR is the calories your body burns at complete rest — the absolute floor.
• TDEE is BMR multiplied by an activity factor — what you actually burn in a normal day.

TDEE is the number that matters for weight goals. Eat below it to lose, above it to gain. BMR is the floor you should never eat below for extended periods.

BMI categories (CDC standard)

• Underweight: BMI < 18.5
• Healthy weight: 18.5 – 24.9
• Overweight: 25 – 29.9
• Obesity Class I: 30 – 34.9
• Obesity Class II+: 35+

BMR formula — Mifflin-St Jeor (1990)

We use Mifflin-St Jeor instead of the older Harris-Benedict (1919) because it's more accurate for modern populations. It's what most registered dietitians and the Academy of Nutrition and Dietetics use.

• Men = 10 × kg + 6.25 × cm − 5 × age + 5
• Women = 10 × kg + 6.25 × cm − 5 × age − 161

Imperial inputs (lbs/ft·in) are converted to metric internally — same formula, no precision loss.

Activity multipliers — pick honestly

• Sedentary (desk job, no exercise): × 1.2
• Light (1-3 days/wk): × 1.375
• Moderate (3-5 days): × 1.55
• Active (6-7 days): × 1.725
• Very active (2× per day, manual labor): × 1.9

Most people overestimate by one tier. Count only sessions where you broke a sweat for 30+ minutes. Your daily walk to work doesn't count toward "exercise" — it's already factored into the sedentary baseline.

Weight loss / gain math

One pound of fat ≈ 3,500 kcal. So a 500 kcal/day deficit gives ≈ 1 lb/week loss. Going faster (1,000+ kcal/day deficit) is harder to sustain and risks muscle loss. For weight gain, +300 to +500 kcal works for most people without excessive fat gain.

Macros — 50/25/25 default

We default to 50% carbs · 25% protein · 25% fat, which works for most balanced eaters. Adjust based on goal:

• Cutting (fat loss): bump protein to 30-35% to preserve muscle.
• Bulking (muscle gain): 1.6-2.0 g protein per kg body weight.
• Athletes / endurance: 55-60% carbs.
• Keto / very-low-carb: ~70% fat, ~20% protein, ~10% carbs (medical supervision recommended).

BMI's blind spots

BMI doesn't distinguish muscle from fat. A muscular athlete may register "overweight" without being unhealthy. Conversely, a person within "healthy" BMI can have high visceral fat (skinny-fat). For a fuller picture, combine BMI with waist circumference (≥40 in men / ≥35 in women indicates higher cardiometabolic risk per CDC) and a body composition scan if available.

Three BMI/BMR/TDEE traps that mislead Americans

1. WHO standard 25/30 vs Asian-Pacific 23/27.5 — same body, different risk category

Standard WHO BMI bands are: normal 18.5–24.9, overweight 25–29.9, obese ≥ 30. ButSouth and East Asian populations carry higher cardio-metabolic risk at lower BMIs(Lancet 2004; WHO Expert Consultation), so Asian-Pacific thresholds are 23 (overweight) and 27.5 (obese). The American Diabetes Association recommends screening Asian-American adults for diabetes at BMI ≥ 23, not 25. If you have East/South Asian heritage, the lower threshold is the more medically relevant one — a BMI of 24 puts you in "normal" by US default but "overweight" by Asian standard, and your diabetes risk is closer to a White American at BMI 26.

2. BMR formula — Mifflin-St Jeor (1990) beats Harris-Benedict (1919) for accuracy

The Harris-Benedict equation is from 1919 (White adults of that era). TheMifflin-St Jeor equation (1990) was validated on a modern, diverse population and is endorsed by the Academy of Nutrition and Dietetics as first-line. Harris-Benedict tends to overestimate BMR by ~5% in modern populations. This calculator uses Mifflin-St Jeor by default; if body fat % is known, Katch-McArdle (which uses lean mass) is most accurate of all. BMR represents calories burned at complete rest — never set diet targets below your BMR (risks hypoglycemia and muscle loss).

3. TDEE activity multiplier — Americans overestimate their activity by 15–25%

TDEE = BMR × activity multiplier. This calculator uses sedentary 1.2 / light 1.375 / moderate 1.55 / very active 1.725 / extra active 1.9. Accelerometer-based research (Westerterp 2017, IJBNPA) shows self-rated activity is overestimated by 15–25% on average. "Hit the gym 3x/week + walk to work" feels "very active" but objectively often measures as "light" once you account for the 80%+ of waking hours spent sitting or standing still. For weight loss targets, drop one activity tier from your self-rating, then adjust based on 4-week weight/composition trends. Tracker step counts and Apple Health/Google Fit data are more reliable than self-reported tier.

* Educational only. Consult a healthcare professional for personal advice, especially if pregnant, chronically ill, or under 18.