Body mass index, or BMI, is the most widely used measure of body size in the world. Doctors use it at check-ups, insurers use it on forms, and public health agencies use it to track obesity across whole countries. It is also one of the most misunderstood numbers in health. This guide explains how BMI is calculated, what the categories mean, where the measure is genuinely useful, and — just as importantly — where it falls short.
What is BMI?
BMI is a simple ratio of weight to height. It was devised in the 1830s by the Belgian mathematician Adolphe Quetelet, who was studying the statistics of the “average man”, and it was given its modern name in 1972 by the physiologist Ancel Keys. It was never designed to diagnose individuals. Its strength is that it needs only two measurements, both easy to take accurately, and that it correlates reasonably well with body fat across large groups of adults.
The BMI formula
In metric units, BMI is weight in kilograms divided by the square of height in metres:
In imperial units, the same calculation needs a conversion factor of 703, which accounts for the difference between pounds and inches and kilograms and metres:
Squaring the height is the important detail. Taller people are larger in every direction, not just taller, so weight rises faster than height. Dividing by height squared is an approximation that makes BMI broadly comparable between short and tall adults.
Step-by-step example
Take an adult who weighs 70 kg and is 1.75 m tall.
- Square the height: 1.75 × 1.75 = 3.0625.
- Divide the weight by it: 70 ÷ 3.0625 = 22.86.
- Round to one decimal place: 22.9.
The BMI is 22.9, which falls in the healthy-weight range of 18.5 to 24.9.
In imperial units, someone weighing 154 lb at 5 ft 9 in (69 inches) has a BMI of 703 × 154 ÷ 69² = 703 × 154 ÷ 4,761 = 22.7. The two people are almost the same size, and the two formulas agree. The BMI calculator handles both unit systems and shows the working.
BMI categories for adults
The World Health Organization groups adult BMI into these ranges:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy 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 |
These cut-offs apply to adults of both sexes. They are statistical boundaries, not cliffs: a BMI of 25.1 does not carry dramatically more risk than 24.9. What matters more is where you sit in the range and which direction you are moving over time.
Lower thresholds for Asian populations
At the same BMI, people of South Asian, Chinese and several other Asian backgrounds tend to carry more body fat and have a higher risk of type 2 diabetes and heart disease than people of European descent. A WHO expert consultation therefore suggested additional public-health action points, with increased risk starting at a BMI of 23 and high risk at 27.5. Health services in the UK, India and elsewhere now use lower thresholds for these groups. If they apply to you, a BMI of 24, for instance, is worth taking more seriously than the standard table suggests.
What healthy weight means for your height
Rearranging the formula turns the healthy BMI range into a weight range for any height. Multiply the lower and upper limits by your height squared:
That range is wide — almost 20 kg — which is a reminder that BMI is not a target to hit precisely. The ideal weight calculator compares this range with several other published formulas.
BMI Prime: a quicker way to read the number
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A result of 1.00 is exactly at the limit; 0.92 means you are 8% below it; 1.20 means 20% above it. It is the same information presented as a proportion, which some people find easier to interpret. The BMI Prime calculator works it out alongside the ordinary figure.
The limits of BMI
BMI is a screening tool, and its weaknesses are well known.
- It cannot tell muscle from fat. A muscular athlete can have a BMI over 27 with very little body fat. BMI will call them overweight.
- It ignores where fat is carried. Fat around the waist and internal organs carries more health risk than fat on the hips and thighs, and two people with the same BMI can differ greatly here.
- It changes meaning with age. Older adults tend to lose muscle, so the same BMI can hide more body fat at 70 than at 30.
- It varies by ethnicity, as the lower Asian thresholds show.
- It is not for children. Children’s BMI must be compared with growth charts for their age and sex, as a percentile — see the BMI percentile calculator for children.
None of this makes BMI useless. For most adults who are not unusually muscular, it gives a reasonable first indication, and changes in BMI over time are meaningful. It simply should not be the only number you look at.
Measures that complement BMI
Because waist fat matters so much, many clinicians now pair BMI with a waist measurement.
- Waist-to-height ratio — your waist divided by your height, in the same units. Keeping it below 0.5 (“keep your waist to less than half your height”) is a simple, well-supported guide. Try the waist-to-height ratio calculator.
- Waist circumference on its own — risk rises noticeably above about 94 cm (37 in) for men and 80 cm (31.5 in) for women in most guidelines.
- Body fat percentage — estimated from tape measurements with the US Navy method in the body fat calculator, or measured by scanning methods such as DEXA.
How BMI relates to health risk
Large studies following hundreds of thousands of people for many years consistently find a J-shaped relationship between BMI and health. Risk is lowest somewhere in the healthy range, rises gradually through the overweight range and more steeply in the obese ranges, and also rises at very low BMIs, where it often reflects illness or frailty. Conditions most closely linked to a high BMI include type 2 diabetes, high blood pressure, heart disease, fatty liver disease, sleep apnoea, osteoarthritis of the knees and hips, and several cancers.
It helps to remember that these are average risks across populations. Within any BMI band there are people in excellent health and people who are not. Fitness, diet, smoking, sleep and genetics all shift an individual’s risk, sometimes more than BMI does. That is why a doctor treats BMI as a prompt for further questions rather than a verdict.
What to do with your result
If your BMI is in the healthy range, the most useful thing is to keep an eye on the trend: slow weight gain of a kilogram or so a year is common in adulthood and easy to miss until it has added up. If it is in the overweight range and your waist is also high, even a modest loss of 5% of body weight is linked to real improvements in blood pressure, blood sugar and cholesterol. If it is below 18.5, or in the obese ranges, it is worth talking to a doctor, who can look at the wider picture. For planning a change in weight, the calorie calculator estimates how much energy you need each day.
Common mistakes
- Using centimetres instead of metres. 70 ÷ 175² gives 0.0023, not 22.9. Convert height to metres first.
- Forgetting to square the height. Dividing by height once gives a number around 40 — obviously wrong once you know the ranges.
- Applying adult categories to children. Children need age-and-sex percentiles.
- Reading a single number as a diagnosis. Use BMI alongside waist size, activity, blood pressure and a conversation with your doctor.
- Chasing an exact figure. The healthy range is broad; staying within it and moving in the right direction matters more than a particular decimal.
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI from 18.5 to 24.9 is classed as healthy. For people of South Asian, Chinese and some other Asian backgrounds, health services often use lower thresholds, with increased risk starting at 23.
Is BMI accurate for athletes?
Not very. Muscle is dense, so athletes and people who do a lot of strength training can have a high BMI with little body fat. For them, waist-to-height ratio or a body fat estimate gives a better picture.
How often should I check my BMI?
Occasional checks are enough — every few months, or when your weight changes noticeably. The trend over time tells you more than any single reading.
Final thoughts
BMI earns its place because it is quick, free and based on two measurements anyone can take. Understood properly — as a screening number for adults, blind to muscle and to where fat sits — it is a useful first check. Work yours out with the BMI calculator, add a waist measurement for a fuller picture, and talk to a health professional about what the numbers mean for you.