About

Misclassifying your weight status leads to poorly calibrated dietary plans and delayed medical intervention. A single metric like BMI (weight ÷ height²) cannot distinguish between lean mass and adiposity. This tool cross-references five independent indices: BMI, BMI Prime, PI (Ponderal Index), WHR (Waist-to-Hip Ratio), and ABSI (A Body Shape Index). Each captures a different dimension of body composition risk. The WHO classification bands apply to adults aged 20 - 65. Results diverge for athletes, pregnant individuals, and the elderly because the underlying regression models assume average lean-mass-to-fat ratios.

Four clinical ideal weight formulas (Devine, Robinson, Miller, Hamwi) are computed simultaneously. These were derived from actuarial mortality tables and pharmaceutical dosing studies. They agree within roughly ±5 kg for most adults but diverge at extreme heights. Treat the output as a screening indicator. It does not replace a DEXA scan or hydrostatic weighing for true body fat percentage.

Formulas

The primary screening metric is the Body Mass Index, defined by Adolphe Quetelet in the 1830s and adopted by the WHO:

BMI = wh²

where w = body mass in kg and h = height in m. The normal band spans 18.5 - 24.9 kg/m².

BMI Prime normalizes to the upper threshold of the normal range:

BMI Prime = BMI25

A value of 1.0 marks the boundary. Values above 1.0 indicate overweight status.

The Ponderal Index corrects for the height bias inherent in BMI by cubing the denominator:

PI = wh³

Normal range: 11 - 15 kg/m³.

Waist-to-Hip Ratio captures central adiposity risk independently of total mass:

WHR = waisthip

where both measurements are in identical units. Males above 0.90 and females above 0.80 enter the elevated-risk zone.

A Body Shape Index (ABSI) isolates waist circumference contribution after removing the expected correlation with BMI and height:

ABSI = WCBMI²³ × h¹²

where WC = waist circumference in m. Higher ABSI correlates with higher mortality risk, independent of BMI. The population mean is approximately 0.0808 m^11/6/kg^2/3.

Reference Data

Category (WHO)	BMI Range kg/m²	BMI Prime	Risk Level
Severe Thinness	< 16.0	< 0.64	Very High
Moderate Thinness	16.0 - 16.9	0.64 - 0.68	High
Mild Thinness	17.0 - 18.4	0.68 - 0.74	Moderate
Normal Weight	18.5 - 24.9	0.74 - 1.00	Low
Overweight (Pre-obese)	25.0 - 29.9	1.00 - 1.20	Increased
Obese Class I	30.0 - 34.9	1.20 - 1.40	High
Obese Class II	35.0 - 39.9	1.40 - 1.60	Very High
Obese Class III	≥ 40.0	≥ 1.60	Extremely High
Waist-to-Hip Ratio Thresholds
Low Risk (Male)	WHR < 0.90		Low
Moderate Risk (Male)	0.90 - 0.99		Moderate
High Risk (Male)	≥ 1.00		High
Low Risk (Female)	WHR < 0.80		Low
Moderate Risk (Female)	0.80 - 0.84		Moderate
High Risk (Female)	≥ 0.85		High
Body Frame Size (r = height ÷ wrist)
Small Frame (Male)	r > 10.4		-
Medium Frame (Male)	9.6 - 10.4		-
Large Frame (Male)	r < 9.6		-
Small Frame (Female)	r > 11.0		-
Medium Frame (Female)	10.1 - 11.0		-
Large Frame (Female)	r < 10.1		-
Ideal Weight Formulas (for height h in inches over 60)
Devine (Male)	50 + 2.3 × (h − 60) kg
Devine (Female)	45.5 + 2.3 × (h − 60) kg
Robinson (Male)	52 + 1.9 × (h − 60) kg
Robinson (Female)	49 + 1.7 × (h − 60) kg
Miller (Male)	56.2 + 1.41 × (h − 60) kg
Miller (Female)	53.1 + 1.36 × (h − 60) kg
Hamwi (Male)	48.0 + 2.7 × (h − 60) kg
Hamwi (Female)	45.4 + 2.2 × (h − 60) kg

Frequently Asked Questions

BMI divides total mass by height squared without distinguishing lean tissue from adipose tissue. A person with high skeletal muscle density (e.g., rugby players, powerlifters) can register a BMI above 25 kg/m² while carrying low body fat. Cross-check with WHR and ABSI - if both are in the low-risk zone, the elevated BMI is driven by lean mass, not fat.

The WHO protocol specifies measuring waist circumference at the midpoint between the lowest palpable rib and the iliac crest, with the tape horizontal and snug but not compressing the skin. Measuring at the navel - a common shortcut - typically yields values 2-5 cm higher, which inflates WHR and may push the result into a higher risk category. Consistency matters more than absolute position: always measure at the same anatomical landmark.

Large meta-analyses (Flegal et al., 2013; Global BMI Mortality Collaboration, 2016) show a J-shaped curve. All-cause mortality is lowest in the BMI range of 20-25 kg/m², rises modestly in the 25-30 range (overweight), and increases significantly above 30. Below 18.5, mortality also rises, partly due to confounding from illness-related weight loss. The inflection point varies by age: for adults over 65, a BMI of 23-28 is often associated with the lowest mortality.

Devine (1974) was derived for drug dosing calculations, not body composition. Robinson (1983) adjusted Devine's coefficients using actuarial data. Miller (1983) used a broader population sample with a flatter slope, yielding higher values for shorter individuals. Hamwi (1964) predates all three and uses round coefficients from insurance mortality tables. The average of all four provides a reasonable central estimate. For heights below 152 cm (5 ft) or above 190 cm (6 ft 3 in), the formulas diverge by up to ±8 kg.

Yes. Krakauer & Krakauer (2012) demonstrated that ABSI predicts premature mortality independently of BMI. Two individuals with identical BMI can have very different ABSI values if one carries visceral fat centrally and the other stores fat peripherally. A high ABSI (above 0.083) with a normal BMI is a stronger mortality predictor than a moderately elevated BMI with low ABSI. It captures the "metabolically obese, normal weight" phenotype that BMI alone misses.

WHO BMI classifications are calibrated for adults aged 20-65. For children and adolescents, BMI-for-age percentile charts (CDC or WHO growth references) are the correct tool because body composition changes during growth. For adults over 65, age-related muscle loss (sarcopenia) means a normal BMI can mask excess body fat. In this age group, WHR and waist circumference alone are more reliable predictors of cardiometabolic risk than BMI.