Health & Fitness Calculation Reference
The formulas behind every Tool Corner health calculator — BMI, BMR, TDEE, body fat, ideal weight and heart-rate zones — explained in plain English, with the source each one comes from. Every result is a population-level estimate, not a diagnosis or medical advice.
Medical disclaimer. These formulas produce general estimates for healthy adults and can be wrong for children, older people, pregnant women, athletes and anyone with a medical condition. They are not a substitute for professional advice — always talk to a doctor, dietitian or qualified practitioner about your individual health.
Body composition
Body Mass Index (BMI)
kg/m²- weight — body mass (kg)
- height — height (m)
- result — under 18.5 low · 18.5–24.9 healthy · 25–29.9 over · 30+ obese
Body fat (BMI method)
percent · %- BMI — body mass index
- age — years
- sex — 1 for male, 0 for female
- note — Deurenberg equation — a rough estimate only
Ideal weight (Devine)
kg- H — height in centimetres
- result — a reference range, not a target for everyone
Energy & nutrition
BMR (Mifflin-St Jeor)
kcal/day- w — weight (kg)
- h — height (cm)
- a — age (years)
- BMR — calories burned at complete rest
TDEE
kcal/day- BMR — basal metabolic rate
- activity factor — 1.2 sedentary → 1.9 very active
- TDEE — calories to maintain weight
Daily protein
grams/day- weight — body mass (kg)
- target — ≈0.8 g/kg baseline; 1.2–2.0 for active people
- result — a starting point, not a prescription
Cardio & hydration
Max heart rate
bpm- age — years
- note — a rough average — real max varies by ±10–12 bpm
Target zone (Karvonen)
bpm- HRₘₐₓ — maximum heart rate
- HRᵣₑₛₜ — resting heart rate
- %I — intensity, e.g. 0.7 for 70%
Water intake (estimate)
litres/day- weight — body mass (kg)
- result — a rough guide — needs rise with heat and exercise
Key terms at a glance
Enter weight in kilograms and height in centimetres or metres as each formula specifies. Every tool on the health hub converts imperial units for you.
How to read these numbers
These equations were derived from population averages, so they work best as a starting point for a healthy adult and lose accuracy at the extremes. BMI can’t tell muscle from fat, so it over-reads for athletes and under-reads for frail or older people; body-fat and ideal-weight formulas are estimates that can be several points off a proper measurement; and the 220−age max heart rate is an average with a wide spread. Use the numbers to spot trends and set rough goals, and see the health hub — each tool lists its own assumptions and limitations.
Sources & references
The formulas and healthy ranges on this page follow guidance and peer-reviewed research from:
- NHS — BMI categories, healthy-weight and hydration guidance
- World Health Organization — international BMI classification
- CDC — body-composition and physical-activity references
- Mifflin MD, St Jeor ST et al. (1990), Am J Clin Nutr — the Mifflin-St Jeor BMR equation
- Karvonen & Devine formulas — the target-heart-rate and ideal-weight methods
This page is for general information only and is not medical advice. Always consult a qualified healthcare professional about your own health.
Cite this page
You are welcome to quote these formulas and figures in a lesson, a forum answer, a wiki or a write-up. A link back is all we ask — it lets your reader check the figure against its source, which is the point of the page.
Tool Corner, “Health & Fitness Calculation Reference”. Updated 29 July 2026. https://toolcorner.net/health-reference.html
Every formula is stated with its symbols and its sources are linked wherever a stable public page exists, so a citation here can be traced rather than taken on trust.