How to calculate BMI and read the WHO adult bands

The number places the weight-to-height ratio on a shared population scale. It does not say whether extra mass is fat, muscle, or water. For a given height, each BMI point equals height squared in kilograms: about 3.06 kg at 1.75 m, 2.56 kg at 1.60 m. That conversion turns a band target into concrete kilos. This page uses the metric formula and returns one decimal place, the WHO label, and the 18.5–24.9 weight range for the height you enter.

A BMI of 24.8 and a BMI of 25.2 land in different bands, yet the weight gap is small. Treat the cutoff as a zone marker, not a verdict. A four-week trend is more useful than a single morning value.

• Below 18.5: underweight
• 18.5 to 24.9: normal weight
• 25.0 to 29.9: overweight
• 30.0 to 34.9: obesity class I
• 35.0 to 39.9: obesity class II
• 40.0 and above: obesity class III

The formula also ignores fat distribution. Visceral fat raises metabolic risk more than subcutaneous fat on the limbs. Waist circumference, a body-fat estimate, and clinical labs fill the gaps. This calculator flags underweight and obesity class II or III, ranges where changing training or intake without a professional raises risk. Pregnancy, edema, dehydration, and post-workout fluid shift the scale. Weigh under similar conditions and compare weekly averages, not one reading.

Weighing at random times adds noise. A salty dinner, yesterday’s training volume, or a menstrual cycle can move 1 to 2 kg, enough to change the first decimal and, near a cutoff, the band. Average three to seven mornings. Treating 18.5–24.9 as an aesthetic target is another miss. That band describes population risk, not the weight where you lift best. Strength athletes often sit above 25 with stable labs. Chasing BMI 20 with an aggressive deficit can cost lean mass.

If BMI is under 18.5, the priority is a supervised surplus of 250 to 400 kcal, the same protein range, and progressive loading. Moving from 17.5 to 18.5 at 1.65 m takes about 2.7 kg. If you already sit between 18.5 and 24.9 and you train, treat BMI as background and track circumferences, loads, and photos. FITTA logs weight, training, meals, and water in one stream so the index becomes a time series. This page is a screening estimate. Clinical calls, especially in underweight or class II and III obesity, belong to a physician or dietitian.

A working rule: if BMI and waist both rise, the gain is mostly fat. If BMI rises, waist holds, and loads go up, the gain is leaner. If BMI falls and lifts stall, the deficit is too large. Expect BMI to cross bands during a cut or a hypertrophy block. That does not void the plan. Adjust calories and volume from a four-week trend, not from one calculation.