Calculator Methodology and Sources

Last reviewed: 2026-09-28

Editorial policy

Every calculator uses a published formula; its source, valid range and limits are listed on this page.
We do not describe estimates as “accurate” or “certified”, and we do not claim medical review that has not happened.
When a formula changes, we update the review date above and the modified date of the affected pages.
Results are educational, for adults, and do not replace medical advice.

How it is calculated: BMI = weight (kg) ÷ height (m)². Imperial: 703 × weight (lb) ÷ height (in)².
Where it applies: Adults aged 18 and over. Categories: under 18.5 underweight; 18.5–24.9 healthy; 25–29.9 overweight; 30–34.9, 35–39.9 and 40+ obesity classes 1–3.
Limits: A screening measure, not a diagnosis. It does not measure body fat and can overrate muscular people. Children and teenagers need BMI-for-age percentiles.
Sources: WHO — BMI classification
CDC — Adult BMI Categories

How it is calculated: Devine, Robinson, Miller and Hamwi: a base weight plus a fixed amount per inch over 5 feet, with different constants for men and women. We show each result, their average, and separately the healthy range from BMI 18.5–24.9.
Where it applies: Heights of 5 ft (about 152 cm) and above. Below that, only the BMI-based range is shown.
Limits: The formulas come from drug dosing and historical insurance tables. They give a reference point, not a personal target.
Sources: Pai MP, Paloucek FP. Ann Pharmacother. 2000;34(9):1066–9
Peterson CM et al. Am J Clin Nutr. 2016;103(5):1197–203

How it is calculated: Log-based equations using height and neck and waist circumference (plus hip for women). Categories follow the American Council on Exercise (ACE).
Where it applies: Adults. Waist must be larger than neck; results outside 2–60% are rejected as likely measurement errors.
Limits: An estimate; can differ from DEXA by a few percentage points and depends on measuring technique.
Sources: Hodgdon JA, Beckett MB. Naval Health Research Center, 1984

How it is calculated: Mifflin–St Jeor for BMR, multiplied by an activity factor (1.2–1.9) for TDEE. Weight loss uses a 500 kcal/day deficit.
Where it applies: Adults. Weight-loss targets never go below 1,200 kcal (women) or 1,500 kcal (men).
Limits: Individual energy needs vary; treat the number as a starting estimate and adjust with real progress.
Sources: Mifflin MD et al. Am J Clin Nutr. 1990;51(2):241–7

How it is calculated: Calories = MET × body weight (kg) × hours.
Where it applies: About 30 common activities with MET values from the Compendium of Physical Activities.
Limits: MET values are population averages; individual burn can differ by 20% or more.
Sources: Ainsworth BE et al. Compendium of Physical Activities

How it is calculated: A 30–35 ml/kg rule of thumb for total water (not an official reference value); about 80% of total water usually comes from drinks; plus an approximate allowance for sweat losses from exercise and climate. Shown as a range.
Where it applies: Healthy adults. Shown next to an independent reference: EFSA adequate intake for total water, 2.0 L/day (women) and 2.5 L/day (men), including food.
Limits: Needs differ in pregnancy, breastfeeding, illness, extreme heat, prolonged activity and with some conditions or medicines. Never override a doctor's fluid instructions.
Sources: EFSA — Dietary Reference Values for water (2010)
National Academies — DRI for Water, Potassium, Sodium, Chloride and Sulfate (2004)