BMI Calculator: Work Out Your Body Mass Index Online

BMI chart: what each range means#

Once you have your number, find it in the chart. Categories follow the WHO classification for adults aged 18+; BMI is not read this way for children, adolescents or during pregnancy.

BMI, kg/m²	Category	What it means
below 16	Severe underweight	Health risk, see a doctor
16.0–18.4	Underweight	Below the healthy range
18.5–24.9	Normal	Healthy range
25.0–29.9	Overweight	Raised risk
30.0–34.9	Obesity class I	High risk
35.0–39.9	Obesity class II	Very high risk
40 and above	Obesity class III	Extremely high risk

The number alone is only a signpost: the 25–29.9 band holds both muscular people and people with a large belly and weak muscles. What your own result means is unpacked below.

How the calculation works#

The calculator produces four different numbers, and they answer four different questions.

Body mass index is weight divided by height in metres squared. It answers "how much of me is there relative to my height". Quetelet devised it in 1832 as a statistical tool for describing populations, and it was never meant as an individual measure of health.

Waist circumference answers where the fat actually sits. The absolute thresholds are 94 and 102 cm in men, 80 and 88 cm in women.

Waist-to-height ratio is the same waist adjusted for height: 90 cm means something different on a 195 cm frame and on a 160 cm one. The threshold is identical for everyone and easy to remember — keep your waist under half your height.

Body fat from circumferences is estimated with the US Navy method from neck, waist and, for women, hips. It is the only one of the four that tries to say what those kilograms are made of.

Where BMI fails#

It errs in both directions, and both matter clinically.

Overstating risk in muscular people. Muscle is denser than fat. A rugby player, or anyone three years into serious lifting, reaches a BMI of 27–29 at 12% body fat. The formula files them as overweight while they are metabolically healthier than most people at 22.

Understating it in older and sedentary people. After 60, muscle leaves and fat takes its place. The weight holds, the BMI holds, the composition changes entirely. There is a name for this — sarcopenic obesity — and it carries a worse prognosis than obesity alone.

Then there is a third group: normal weight, large belly. They are described as metabolically obese normal weight — BMI 23, waist 96 cm, insulin resistance and triglycerides in the range you would expect at obesity. BMI never sees them. A tape measure sees them immediately.

Читайте также: BMI and Body Composition: What the Index Shows and What It Misses

How to measure properly#

Accuracy of the circumferences decides everything: a 2 cm error at the waist shifts the estimated body fat by roughly a percentage point.

What	Where	How
Waist	At the navel, standing	Morning, before eating, tape horizontal, do not pull the stomach in
Hips	At the widest point of the buttocks	Feet together, tape parallel to the floor
Neck	Just below the larynx	Tape angled slightly forward, shoulders relaxed
Height	No shoes, back against a wall	In the morning: you lose 1–2 cm by evening

What to do with the result#

If everything sits in the healthy range: nothing. A stable weight and a waist that does not creep up year on year is itself a decent marker.

If the numbers are outside it, the direction is more useful than the value. A waist that dropped 4 cm over three months at the same body weight beats losing 4 kg of unspecified origin. That is why the calculator also shows fat-free mass index — it tells you how much muscle sits beneath the fat and stops you mistaking wasting for fat loss.

A scale that refuses to move while your waist shrinks is the normal course of events in the first months of resistance training, not a reason to stop.

Читайте также: Visceral Fat: Why It Is Dangerous and What Actually Removes It

What the calculator does not know#

It does not know your age in the sense of body composition — a BMI of 24 means different amounts of muscle at 25 and at 70. It knows nothing of ethnicity beyond the coarse Asian adjustment. It does not apply in pregnancy, in children and adolescents (who need age-specific percentile curves), in strength athletes, or in anyone with oedema or ascites, where retained fluid counts as body mass.

Above all: none of the four numbers diagnoses anything. Obesity is a clinical diagnosis, made alongside blood pressure, glucose, lipids and history — never from a single figure.

Читайте также: Health Check-ups by Age: Which Tests Are Actually Worth Doing