Body Mass Index compares your weight to your height to give a single number that screens for underweight, healthy weight, overweight and obesity. It takes seconds to compute, needs no equipment, and is the measure most clinicians reach for first — but it describes populations far better than it describes individuals.
What BMI actually measures
BMI divides your mass by the square of your height. The squaring is what makes it a shape measure rather than a size measure: it roughly cancels out the fact that taller people are heavier, leaving a number that reflects build.
What it does not do is distinguish where that mass comes from. Muscle, bone, organs, fluid and fat all count identically on a scale, so BMI measures excess weight rather than excess fat. That distinction drives every limitation listed further down.
Metric: BMI = kg / m² Imperial: BMI = 703 × lb / in²
BMI categories for adults
The World Health Organization classification below applies to adults aged 20 and over, and is the same for men and women.
| Classification | BMI (kg/m²) |
|---|---|
| Severe thinness | under 16 |
| Moderate thinness | 16 – 17 |
| Mild thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese class I | 30 – 35 |
| Obese class II | 35 – 40 |
| Obese class III | over 40 |
Several health authorities across Asia apply lower thresholds — commonly 23 for overweight and 27.5 for obesity — because cardiometabolic risk rises at a lower BMI in many Asian populations.
Children and teenagers are read differently
Between ages 2 and 20, BMI is not compared against fixed cut-offs. Body composition changes constantly through growth, so a child's BMI is plotted against reference data for their exact age and sex and reported as a percentile.
A percentile says where a child sits relative to peers: the 60th percentile means 60% of children of that age and sex have a lower BMI.
| Category | Percentile |
|---|---|
| Underweight | below 5th |
| Healthy weight | 5th – 85th |
| At risk of overweight | 85th – 95th |
| Overweight | above 95th |
This calculator applies the adult classification. For anyone under 20, ask a paediatric provider to plot BMI on the appropriate growth chart.
BMI Prime
BMI Prime restates your BMI as a fraction of the upper healthy limit of 25. It is dimensionless, which makes it easy to read at a glance and easy to compare between populations that use different cut-offs.
A BMI Prime of 1.0 sits exactly at the top of the healthy range. 0.9 means you are at 90% of that ceiling; 1.3 means you are 30% above it.
BMI Prime = BMI / 25
| Classification | BMI | BMI Prime |
|---|---|---|
| Underweight | under 18.5 | under 0.74 |
| Normal | 18.5 – 25 | 0.74 – 1.00 |
| Overweight | 25 – 30 | 1.00 – 1.20 |
| Obese | over 30 | over 1.20 |
Ponderal Index
The Ponderal Index cubes height instead of squaring it. That single change makes it more stable at the extremes of the height range, where BMI systematically overstates body fat in very tall people and understates it in very short people.
For anyone of average height the two measures broadly agree, so PI is most useful precisely where BMI is least reliable.
PI = kg / m³
Where BMI misleads
BMI tracks body fat well for roughly 90–95% of people. The exceptions are systematic and worth knowing:
- Muscular people — athletes, and anyone who lifts seriously — routinely land in the overweight band with low body fat.
- Older adults tend to carry more fat and less muscle at the same BMI, so a "normal" reading can hide a genuinely unhealthy composition.
- Women carry more essential fat than men at equivalent BMI, yet the thresholds are identical for both.
- BMI says nothing about fat distribution. Visceral fat around the organs is far more dangerous than fat on the hips and thighs, and BMI cannot tell them apart.
- Pregnancy, significant fluid retention and large differences in bone density all distort the reading.
Waist circumference and waist-to-height ratio capture the visceral fat that BMI misses, and both take one tape measure to check. Use them alongside BMI rather than instead of it.
Why the healthy range matters
Sustained overweight and obesity raise the risk of type 2 diabetes, hypertension, coronary heart disease, stroke, several cancers, gallbladder disease, osteoarthritis and sleep apnoea.
Being underweight carries its own risks: nutrient deficiencies and anaemia, reduced bone density, impaired immune function, disrupted menstrual cycles and fertility, slower surgical recovery, and growth problems in children and adolescents.
Both tails of the distribution show higher all-cause mortality than the middle. A BMI outside the healthy range is a prompt to look further, not a diagnosis in itself.
Worked example
Using the values pre-loaded in the calculator above:
| Input | Value |
|---|---|
| Units | Metric (cm / kg) |
| Age (yrs) | 30 |
| Sex | Male |
| Height (cm or in) | 175 |
| Weight (kg or lb) | 75 |
| Output | Value |
|---|---|
| BMI | 24.49 |
| Healthy weight range | 56.7 kg – 76.3 kg |
| BMI prime | 0.98 |
| Ponderal index | 13.99 kg/m³ |
| Weight to reach BMI 25 | already below |
Frequently asked questions
What is a healthy BMI?
For adults, 18.5 to 24.9 kg/m². Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classified as obese. Many Asian populations use lower thresholds of 23 and 27.5.
Is BMI accurate for muscular people?
No. BMI counts all mass equally, so a lean, heavily muscled athlete can easily read as overweight or obese despite very low body fat. Use a body fat measurement instead.
Does BMI differ for men and women?
The thresholds are identical, though women naturally carry a higher proportion of essential body fat at the same BMI. That is one reason BMI should be read alongside other measures.
How do I lower my BMI?
BMI falls when weight falls, since height is fixed in adults. A deficit of roughly 500 kcal per day produces about 0.45 kg (1 lb) of loss per week. Combining a moderate deficit with resistance training preserves muscle while fat is lost.
What is the difference between BMI and BMI Prime?
BMI Prime is simply your BMI divided by 25, the top of the healthy range. It expresses how far above or below that ceiling you sit as a ratio, which makes comparisons across different national cut-offs straightforward.
Can I use this calculator for my child?
Not directly. Children and teenagers between 2 and 20 need BMI plotted as an age-and-sex percentile on a growth chart, because healthy ranges shift throughout development.