How we calculate: Health & Fitness

BMI = weight (kg) ÷ height (m)² Imperial: BMI = 703 × weight (lb) ÷ height (in)² Known as the Quetelet Index after Belgian statistician Adolphe Quetelet, who devised it between 1830 and 1850; physiologist Ancel Keys renamed it "Body Mass Index" in a 1972 paper, and it's the formula WHO uses for adult population screening today.

BMI is a screening measure, not a health verdict. It can't distinguish muscle from fat, doesn't account for where on the body weight is carried, and its standard ranges were developed primarily from adult population studies — so results can be misleading for athletes, older adults, and some ethnic groups with different risk thresholds. If your result concerns you, a healthcare provider can put it in context alongside other measures.

• WHO — Obesity and overweight (fact sheet) — accessed 2026-08-19
• WHO Global Health Observatory — Body mass index (adults) — accessed 2026-08-19
• Wikipedia — Body mass index (Quetelet/Keys history) — accessed 2026-08-19

Men: BMR = 10×weight(kg) + 6.25×height(cm) − 5×age + 5 Women: BMR = 10×weight(kg) + 6.25×height(cm) − 5×age − 161 TDEE = BMR × activity multiplier Mifflin-St Jeor equation (Mifflin et al., 1990), derived from indirect-calorimetry measurements of 498 adults and one of the better-validated resting-energy-expenditure formulas for the general population.

• Mifflin MD, St Jeor ST, et al. — A new predictive equation for resting energy expenditure in healthy individuals (Am J Clin Nutr, 1990) — accessed 2026-08-19

Aggressive cut = TDEE − 500 · Mild cut = TDEE − 250 · Maintain = TDEE Mild bulk = TDEE + 250 · Aggressive bulk = TDEE + 500 The ±500 kcal/day step traces back to Max Wishnofsky's 1958 estimate that roughly 3,500 kcal corresponds to one pound of body fat (500 × 7 days ≈ 3,500). Modern research shows this rule overstates real-world results, since the body adapts its energy needs as weight changes — treat it as a starting point, not a precise predictor.

• Today's Dietitian — Farewell to the 3,500-Calorie Rule (origin and modern critique of the Wishnofsky estimate) — accessed 2026-08-19

Protein grams = (calories × protein %) ÷ 4 Carb grams = (calories × carb %) ÷ 4 Fat grams = (calories × fat %) ÷ 9 The 4 / 4 / 9 calories-per-gram figures are the FDA's general Atwater conversion factors for protein, carbohydrate, and fat. The Balanced/Low-carb/High-protein percentage splits are editorial presets, not a single official prescription — each falls within the Acceptable Macronutrient Distribution Range (AMDR) published by the (U.S.) National Academies: 10–35% protein, 45–65% carbohydrate, 20–35% fat.

• Cornell LII — 21 CFR § 101.9, Nutrition labeling of food (general Atwater calorie factors) — accessed 2026-08-19
• National Academies — Rethinking the Acceptable Macronutrient Distribution Range for the 21st Century — accessed 2026-08-19

Men: 50kg + 2.3kg per inch over 5 feet Women: 45.5kg + 2.3kg per inch over 5 feet Healthy range = estimate ± 10% Devised by B.J. Devine (1974) originally for drug-dosing calculations, not derived from a population study — it was an empirical estimate that became the most widely used ideal-body-weight formula in clinical practice. Always computed in kg internally, then converted for display when Imperial units are selected.

• Pai MP, Paloucek FP — The Origin of the 'Ideal' Body Weight Equations (Annals of Pharmacotherapy, 2000) — accessed 2026-08-19

Men: 495 ÷ (1.0324 − 0.19077×log10(waist−neck) + 0.15456×log10(height)) − 450 Women: 495 ÷ (1.29579 − 0.35004×log10(waist+hip−neck) + 0.22100×log10(height)) − 450 Hodgdon & Beckett circumference method (Naval Health Research Center, 1984), developed and validated against U.S. Navy and Army personnel. Independent research finds it correlates reasonably well with lab methods for group averages, but tends to underestimate body fat in men and overestimate it in lean women — useful for tracking trends, not a precise individual measurement.

• Tomczak et al. — Comparing Alternate Percent Body Fat Estimation Techniques for U.S. Navy/Marine Corps Body Composition Assessment (accuracy discussion of the Hodgdon-Beckett method) — accessed 2026-08-19

Epley: 1RM = weight × (1 + reps ÷ 30) Brzycki: 1RM = weight × 36 ÷ (37 − reps) Both are long-standing strength-training formulas estimating a one-rep max from a lower-rep, sub-maximal set. Both are most accurate in roughly the 1-10 rep range this tool is built for — accuracy drops for both formulas at higher rep counts, since muscular endurance increasingly outweighs pure strength the more reps you do.

These are estimates, not guarantees — actual 1RM depends on technique, fatigue, and day-to-day variation. Use the estimate to guide programming, not as a number to test against without a proper warm-up and a spotter.

• Epley B — Poundage Chart, Boyd Epley Workout (1985) — as cited in modern 1RM-formula reviews — accessed 2026-08-25

Estimated max heart rate = 220 − age Heart rate reserve (HRR) = max heart rate − resting heart rate Zone target = (HRR × zone %) + resting heart rate The Karvonen formula (Martti Karvonen, 1957) is widely considered more personalized than a flat percentage of max heart rate, since it factors in your resting heart rate as a proxy for cardiovascular fitness — two people with the same age and max heart rate but different resting heart rates get different zone targets.

"220 minus age" is a widely used estimate, not a precise measurement — actual max heart rate varies meaningfully between individuals of the same age. A supervised max-effort test gives a more accurate number if you need precision for serious training.

• American Heart Association — Target Heart Rates Chart — accessed 2026-08-25

Daily protein target = body weight (kg) × grams per kg for your activity level Sedentary: 0.8 g/kg — the U.S. RDA, calibrated to prevent deficiency in a generally inactive population, not to optimize performance or muscle retention. Recreationally active / Building muscle: 1.4-2.0 g/kg — the range the International Society of Sports Nutrition's position stand on protein and exercise cites as sufficient for building and maintaining muscle mass in most exercising individuals.

These are population-level guidelines, not individually prescribed targets — actual optimal intake varies by training volume, goals, age, and total calorie intake. The ISSN position stand notes intakes above 2.0 g/kg show diminishing returns for most people, though some research suggests higher intakes (>3.0 g/kg) may still benefit body composition in resistance-trained individuals during a calorie deficit specifically.

• Jäger et al. — International Society of Sports Nutrition Position Stand: protein and exercise (Journal of the ISSN, 2017) — accessed 2026-08-25

Recommended time = target time ± (90 minutes × number of cycles) − 14 minutes (typical time to fall asleep) Based on the ~90-minute (80–120-minute) ultradian sleep cycle described in sleep researcher Nathaniel Kleitman's basic rest-activity cycle (BRAC) work — an average, not a fixed length for any one person or night.

• Wikipedia — Basic rest–activity cycle (Kleitman's ~90-minute ultradian sleep cycle) — accessed 2026-08-19

Base intake = weight (kg) × 0.033 L Activity addition = (exercise minutes ÷ 30) × 0.35 L Total = Base + Activity addition The 0.033 L/kg baseline sits within the 30–35 mL/kg/day general fluid-requirement guideline used in clinical nutrition; ESPEN's geriatric guideline recommends at least 1.6 L/day for women and 2.0 L/day for men from beverages alone as a floor.

• ESPEN practical guideline — Clinical nutrition and hydration in geriatrics — accessed 2026-08-19

Due date = LMP + 280 days Current week = (Today − LMP) ÷ 7, rounded down Naegele's rule, formalized by German obstetrician Franz Naegele in his 1812 textbook and still the global standard estimation method, though a 2021 review found actual gestational length varies by ethnicity, height, cycle length, parity, and maternal weight.

• Lawson GW — Naegele's rule and the length of pregnancy, a review (Aust N Z J Obstet Gynaecol, 2021) — accessed 2026-08-19

Ovulation ≈ LMP + (cycle length − 14) days Fertile window ≈ 5 days before ovulation through 1 day after Based on ACOG's fertility-awareness guidance: the luteal phase (ovulation to next period) is typically 14 days, sperm can survive up to 5 days in the reproductive tract, and a released egg survives about 24 hours.

• ACOG — Fertility Awareness-Based Methods of Family Planning (FAQ) — accessed 2026-08-19