BMI and TDEE: Two Numbers, One Formula Each

Fitness planning runs on two estimates: how your weight compares to your height (BMI), and how much energy you burn in a day (TDEE). The BMI & TDEE Calculator computes both from the same inputs — and is equally clear about what each one can and cannot tell you.

The formulas, for the record

BMI = weight ÷ height², classified on the WHO adult scale from severe thinness through normal (18.5–24.9) to obesity class III. The tool draws the full band scale with a marker at your position and, more usefully, converts it back into your healthy weight window for your height — an actionable kilogram range beats a dimensionless index.

BMR uses Mifflin-St Jeor (1990), the standard in clinical nutrition:

• male: 10·kg + 6.25·cm − 5·age + 5
• female: 10·kg + 6.25·cm − 5·age − 161

TDEE = BMR × activity multiplier (1.2 sedentary → 1.9 athlete). Cut (−20%) and bulk (+10%) targets round it out.

What they miss

BMI cannot distinguish muscle from fat — athletes routinely classify as overweight with low body fat. It's a population screening metric, not a body composition scan. TDEE estimates are ±10% even under lab conditions; activity multipliers are self-reported averages. The calculator is restricted to ages 15–100 because children get age-specific percentile charts, not adult bands. Treat outputs as starting points to adjust against real-world weight trends over weeks, and as estimates — not medical advice, as the built-in disclaimer states.

Units and accuracy

Metric and imperial inputs both work (conversion happens before the formulas, math stays metric). Need the consistency habit around it? The same discipline that compounds money in the Compound Interest Calculator applies to habits — time them with the Pomodoro Focus Timer.