🥗 BMI Calculator (Body Mass Index)
Check your body mass index to evaluate if your weight is in a healthy range.
What BMI measures, what it misses, and how to read your number
Body mass index is weight divided by height squared. That is the entire calculation. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way of describing populations, and it was never designed to diagnose an individual. Understanding that origin explains both why it is still used everywhere and why it is so often misleading.
BMI = weight (kg) / height (m)2 BMI = 703 × weight (lb) / height (in)2
The WHO categories
| Category | BMI range | Notes |
|---|---|---|
| Underweight | below 18.5 | Associated with nutritional deficiency and reduced bone density |
| Healthy weight | 18.5 – 24.9 | Lowest average all-cause mortality in large cohort studies |
| Overweight | 25.0 – 29.9 | Raised average risk, but highly dependent on body composition |
| Obesity class I | 30.0 – 34.9 | Clearly raised risk of type 2 diabetes and cardiovascular disease |
| Obesity class II | 35.0 – 39.9 | Substantially raised risk |
| Obesity class III | 40.0 and above | Highest risk category |
For someone 1.75 m tall, the healthy range of 18.5 to 24.9 corresponds to roughly 57 kg to 76 kg, a spread of nearly twenty kilos. BMI is a wide net, not a target weight.
Where BMI breaks down
The formula knows your weight and your height and nothing else. It cannot distinguish muscle from fat, it cannot see where fat is stored, and it does not adjust for frame size, age or ethnicity. That produces predictable failures:
- Muscular people. A rower or a rugby forward can register as obese on BMI while carrying 10% body fat. Muscle is denser than fat, so the same volume weighs more.
- Older adults. Muscle mass falls with age while fat rises, so a stable BMI at 70 can conceal a very different composition from the same BMI at 30.
- Fat distribution. Visceral fat around the organs carries far more metabolic risk than subcutaneous fat on the hips, and BMI treats them identically.
- Ethnic variation. Health risks associated with a given BMI appear at lower thresholds in South Asian and East Asian populations. Several health authorities use 23 rather than 25 as the overweight threshold for these groups.
- Height extremes. Because the denominator is squared rather than cubed, BMI slightly overstates for tall people and understates for short ones.
Better measurements to use alongside it
Waist circumference is the single most useful addition and takes ten seconds with a tape measure. Measured at the midpoint between the lowest rib and the top of the hip bone, it is a direct proxy for visceral fat:
| Measure | Increased risk | Substantially increased risk |
|---|---|---|
| Men | 94 cm / 37 in | 102 cm / 40 in |
| Women | 80 cm / 31.5 in | 88 cm / 35 in |
Waist-to-height ratio is simpler still and works across heights: keep your waist under half your height. For someone 175 cm tall, that means under 87.5 cm. Several studies have found it predicts cardiometabolic risk better than BMI alone.
A worked example
A man of 1.82 m weighing 94 kg has a BMI of 94 / 1.82² = 28.4, which places him in the overweight band. His waist measures 89 cm, below the 94 cm threshold, and his waist-to-height ratio is 0.49, just under half. Taken together the picture is of a heavier-than-average but not high-risk build, which a BMI of 28.4 on its own would not tell you.
This page explains published formulas and population-level ranges. It is general information, not medical advice, and it cannot account for your individual health. Talk to a doctor or a registered dietitian before changing how you eat, train or treat a condition.
Frequently Asked Questions (FAQs)
Is BMI accurate for athletes?
No. Muscle weighs more than fat for the same volume, so well-trained athletes routinely land in the overweight or obese categories while carrying very low body fat. For anyone who trains with resistance regularly, body fat percentage and waist measurement are far more informative.
What is a healthy BMI for my age?
The adult range of 18.5 to 24.9 applies from 20 upwards, though some research suggests slightly higher BMI is protective after 65, where being underweight carries more risk than being mildly overweight. Children and teenagers use age and sex-specific percentile charts instead, not the adult bands.
Does BMI differ for men and women?
The formula and the thresholds are the same, but the meaning is not. Women naturally carry a higher body fat percentage at the same BMI. This is one of the reasons body fat percentage, which does have sex-specific ranges, is the more useful measure.
How much weight do I need to lose to change category?
Divide the target BMI by your height in metres squared. At 1.70 m, dropping from a BMI of 27 to 24.9 means going from 78 kg to 72 kg, so about 6 kg. The calculator shows the full healthy weight range for your height.
Why do some countries use different thresholds?
Because risk appears at different BMIs in different populations. Health bodies in several Asian countries use 23 for overweight and 27.5 for obesity, reflecting evidence that type 2 diabetes and cardiovascular disease emerge at lower BMI values in those groups.
Is BMI useful at all then?
Yes, at population scale and as a first screen. It is free, repeatable and correlates well with health outcomes across large groups. The error is treating a single individual's BMI as a diagnosis rather than as one number among several.