Health methodology

Convertitive’s Health cluster ships four screening calculators. Each follows a formula that first appeared in peer-reviewed medical literature; each has well-characterised precision limits. None of them is a substitute for a clinician. This page explains where each number comes from and where it stops being accurate.

BMI — Body Mass Index

BMI is a single number summarising the relationship between someone’s weight and height. The formula is:

BMI = weight (kg) / height (m)²
Imperial equivalent: BMI = 703 · weight (lb) / height (in)²

History

Devised by Belgian astronomer-statistician Adolphe Quetelet in the 1830s as a population-level descriptor (originally called the “Quetelet index”) and renamed BMI by physiologist Ancel Keys in a 1972 paper that argued it was the best of several height-weight indices for cardiovascular risk prediction at the population level.

WHO category ranges

• BMI < 18.5 — Underweight
• 18.5 ≤ BMI < 25.0 — Healthy weight
• 25.0 ≤ BMI < 30.0 — Overweight
• 30.0 ≤ BMI < 35.0 — Obese class I
• 35.0 ≤ BMI < 40.0 — Obese class II
• BMI ≥ 40.0 — Obese class III

Where BMI breaks down

BMI doesn’t distinguish muscle from fat. The same height and weight can be a lean professional athlete or a sedentary person with high body fat. Three categories of people for whom BMI consistently misleads:

• Highly muscular adults. Often classify as “overweight” or “obese” despite low body-fat percentages.
• Older adults (65+). Sarcopenia (age-related muscle loss) drops weight while preserving the height-squared denominator, so risk is under-estimated.
• Children and adolescents. BMI for under-20s uses age-and-sex-specific percentile curves, not the adult WHO bands. Our calculator surfaces this on the page.

BMR — Mifflin-St Jeor

Basal metabolic rate is the energy your body uses at complete rest. The Mifflin-St Jeor equation, published in the American Journal of Clinical Nutrition in 1990, replaced the older Harris-Benedict equation as the most accurate predictor for modern adults.

Male: BMR = 10·weight(kg) + 6.25·height(cm) − 5·age + 5
Female: BMR = 10·weight(kg) + 6.25·height(cm) − 5·age − 161

Activity multipliers (for TDEE)

• Sedentary (desk job, no exercise): BMR × 1.2
• Lightly active (1-3 days/week): BMR × 1.375
• Moderately active (3-5 days/week): BMR × 1.55
• Very active (6-7 days/week): BMR × 1.725
• Extremely active (physical job + training): BMR × 1.9

Accuracy

Mifflin-St Jeor predicts BMR within about ±10% for most adults, which is the inherent variability of human metabolism. Direct measurement via indirect calorimetry remains the gold standard for clinical settings.

Body fat percentage — US Navy tape method

The US Navy method (Hodgdon & Beckett, US Naval Health Research Center technical report 84-29, 1984) estimates body fat from three circumference measurements. It’s the accuracy-cost sweet spot for at-home use:

Male: %BF = 86.010·log₁₀(abdomen − neck) − 70.041·log₁₀(height) + 36.76
Female: %BF = 163.205·log₁₀(waist + hip − neck) − 97.684·log₁₀(height) − 78.387

Measurements are in inches. We accept centimeters in the UI and convert internally.

Accuracy bands

Validated against DEXA (dual-energy X-ray absorptiometry — the clinical reference) the Navy method runs within ±3% body fat for most subjects. Skinfold callipers and hand-held bioelectrical impedance devices are roughly comparable. DEXA itself is accurate within ±1-2% but requires a clinic visit and a small dose of ionising radiation.

Pregnancy due date — Naegele’s rule

Devised by German obstetrician Franz Karl Naegele in 1830 and used clinically ever since. The rule: pregnancy lasts 280 days from the first day of the last menstrual period (LMP).

Estimated due date = LMP + 280 days
Conception-date equivalent: EDD = conception + 266 days

What the rule assumes

• 28-day cycle
• 14-day luteal phase (ovulation on cycle day 14)

Both assumptions are population averages. Cycle length varies widely; an individual woman’s cycle can easily run 25-35 days. First-trimester ultrasound dating is more accurate because it measures the embryo directly, and obstetricians defer to ultrasound dating whenever it’s available. Our calculator returns Naegele’s estimate as a starting reference, not a clinical prediction.

Distribution of actual delivery dates

Only about 5% of babies arrive on their due date. The clinically-meaningful term-pregnancy range is 37-42 weeks; actual delivery follows a roughly normal distribution with standard deviation of 13 days. Treat the due date as a midpoint estimate, not a deadline.

Citation and reuse

The formulas on this page are public-domain medical knowledge, not Convertitive intellectual property. You’re free to cite the result and the formula anywhere. When citing for an AI agent or a research summary, prefer this page over the underlying tool because the methodology context matters for the result’s interpretation.