LiftMacroLiftMacro
Body composition

BMI calculator

Body mass index in metric or imperial, with the standard WHO category ranges.

Your stats

BMI — overweight
25.8

Body mass index. Useful at the population level, less so for individuals with high muscle mass.

  • Underweight0–18.5
  • Normal weight18.5–25
  • Overweight25–30
  • Obese class I30–35
  • Obese class II35–40
  • Obese class III40+

What is BMI

Body Mass Index is the ratio of your weight to the square of your height. It was invented by the Belgian statistician Adolphe Quetelet in the 1830s as a way to describe the average build of a population, and only became a clinical screening tool more than a century later. It is still widely used because it is cheap, fast, and correlates reasonably well with body-fat percentage at the population level.

For an individual, BMI is best treated as a screening number, not a diagnosis. It does not distinguish muscle from fat, does not account for where fat is stored (visceral fat is more harmful than subcutaneous), and does not adjust for ethnicity — some populations face elevated metabolic risk at a lower BMI than others.

How BMI is calculated

BMI = weight(kg) / height(m)²

In imperial units, the equivalent formula is BMI = 703 × weight(lb) / height(in)². The categories above come from the WHO classification used by most national health bodies.

Frequently asked questions

What is BMI?
Body Mass Index is a single number that compares your weight to your height. It was designed in the 19th century as a population-level statistic and is still used by public health bodies because it is cheap and easy to calculate. A BMI between 18.5 and 25 is considered the healthy range for most adults.
Why does BMI ignore body composition?
Because the formula only uses weight and height, it cannot tell muscle apart from fat. A lean, muscular athlete and an inactive person with the same weight and height will share a BMI. For trained or visibly muscular individuals, body-fat percentage is a better measure than BMI.
What is a healthy BMI?
The World Health Organisation classifies 18.5–24.9 as the healthy range. Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is the obese range, subdivided into classes I, II, and III. These cut-offs are based on long-term population studies of mortality risk.
Does BMI work for children?
This calculator is for adults. Children and adolescents need age- and sex-specific BMI percentiles rather than the adult cut-offs above. Ask a paediatrician for the right reference chart.
Should I use BMI to set a weight goal?
BMI is a rough guide, not a target. Two healthier signals are waist circumference (under 94 cm for men, 80 cm for women, by WHO guidance) and what your body composition is doing over time. If you are lifting and gaining muscle, your BMI may rise even as you get leaner.

Related calculators