Use weight as a signal—not a verdict.

For adults, the WHO puts the healthy range at 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, 30 to 34.9 is obesity class I, 35 to 39.9 class II and 40 or more class III. Those bands have not changed since 1997 and are the same for men and women.

Multiply your height in metres squared by 18.5 for the bottom of the range and by 24.9 for the top. At 5 ft 9 in that works out to roughly 125 to 168 lb. It is a window of about 40 lb, not a single number, which is why the ideal-weight formulas below should be read as a centre rather than a target.

None of them is "right". Devine (1974) exists to dose drugs and is still the pharmacology standard. Robinson (1983) came out of epidemiological data. Hamwi (1964) was built for diabetes meal planning. Lorentz (1929) is the simple European one. They disagree by several pounds, so this hub shows all four and their average, and compares that against the WHO healthy range.

No. The WHO uses the same 18.5 to 24.9 range for both. Sex does change three other things in this hub: the ideal-weight formula coefficients, the waist and waist-to-hip thresholds, and the child percentile curve. Body fat percentage bands also differ by about 8 to 10 points, which is why the composition case flags that explicitly.

Only with an adjustment. The ESPEN 2018 consensus places the healthy range at 22 to 27 in adults 65 and over, and the WHO suggests 23 to 28. Large mortality meta-analyses find the lowest risk around 27 to 28 in this group. Below 22 is a nutritional-risk flag, because a little reserve protects against sarcopenia and frailty.

Because muscle is denser than fat and BMI cannot tell them apart. A bodybuilder at 8% body fat can score 29. If you carry a lot of muscle, use body fat percentage, waist-to-height ratio or waist circumference instead, all of which are in this hub.

The WHO cut-offs are 37 inches (94 cm) for increased risk and 40 inches (102 cm) for substantially elevated risk in men, and 31.5 inches (80 cm) and 35 inches (88 cm) in women. The International Diabetes Federation uses a lower 35.5-inch line for men of Latin American and South Asian origin. The simplest rule of all is to keep your waist under half your height.

Where the fat is. Fat around the middle is metabolically active and drains into the liver; fat on the hips and thighs is far less harmful. The WHO 2008 bands put low risk below 0.90 for men and 0.80 for women, and high risk above 0.95 and 0.85 respectively. Two people with identical BMIs can sit on opposite sides of those lines.

As a percentile, never as an absolute number. The same BMI of 19 is obesity in a 5-year-old and healthy in a 15-year-old. This hub places the value on the WHO BMI-for-age curve for the child’s exact age and sex, interpolating between tabulated years: below the 5th percentile is underweight, 5th to 84th healthy, 85th to 96th overweight, and 97th or above obesity.

Subcutaneous fat sits under the skin; visceral fat wraps the organs inside the abdominal wall. Visceral fat drains directly into the portal vein and reaches the liver first, which is why it drives insulin resistance and dyslipidaemia out of all proportion to its mass. A visceral share above about 20% of total fat is where the risk markers start climbing.

Clinical dosing. Chemotherapy is prescribed in mg per m², cardiac output is normalised as cardiac index in L/min/m², and kidney function is reported per 1.73 m². Du Bois (1916) is the original, Mosteller (1987) is the practical default because it is a single square root, and Haycock (1978) is preferred in paediatrics.

About 1 to 2 lb a week, from a moderate deficit of 300 to 500 calories a day combined with resistance training two or three times a week. Faster than that and a large share of what you lose is muscle, which lowers your metabolic rate and makes the weight easier to regain.