BMI Calculator

What BMI Is

A ratio, not a measurement of fat. BMI divides weight by height squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was describing populations rather than assessing individuals, and it carried his name until the 1970s.

It became standard because it is cheap. Height and weight are quick to measure, need no equipment beyond scales and a tape, and require no training. For public health work across millions of people that practicality outweighs the loss of precision.

Population level, it works well. Across large groups, average BMI tracks rates of type 2 diabetes, cardiovascular disease and several cancers closely enough to be genuinely useful for planning and research.

Individually, it is a starting point. A high or low BMI is a reason to look more closely, not a conclusion in itself. Two people with identical numbers can be in completely different states of health.

The Formula and How to Use It

Metric: BMI equals weight in kilograms divided by height in metres squared. Someone 1.75 metres tall weighing 70 kilograms has a BMI of 70 divided by 3.0625, which is 22.9.

Imperial: BMI equals weight in pounds divided by height in inches squared, multiplied by 703. That constant simply converts the units. This calculator converts imperial input to metric first, so both routes give the identical answer.

Measure carefully. Height changes by up to two centimetres over a day as spinal discs compress, so measure in the morning without shoes. Weigh yourself at the same time of day, ideally before breakfast, since a large meal and drink can shift the scale by a kilogram or more.

Working the range backwards: Multiplying 18.5 and 24.9 by your height in metres squared gives the weights at each end of the healthy band. At 1.75 metres that is roughly 56.7 to 76.3 kilograms.

The WHO Categories

Below 18.5 is underweight, subdivided into mild, moderate and severe thinness below 17 and 16. Being underweight carries real risks of its own, including reduced bone density, weakened immune response and fertility problems, and it is often overlooked in conversations that focus only on excess weight.

From 18.5 to just under 25 is the healthy range. This band is where population studies find the lowest rates of weight related illness. It is a wide range, and where you sit within it matters far less than people assume.

From 25 to just under 30 is overweight, sometimes called pre-obesity. Many people in this band are metabolically healthy, particularly if they are active, which is exactly where BMI alone stops being informative.

Thirty and above is obesity, divided into class I to 35, class II to 40 and class III above that. Risk of associated conditions rises with each class, which is why the subdivisions exist rather than one broad label.

Where BMI Breaks Down

It cannot tell muscle from fat. Muscle is considerably denser than fat, so a lean, muscular person can register as overweight while carrying very little fat at all. The formula sees only total mass.

It ignores where fat sits. Fat around the abdomen, surrounding the organs, carries far more metabolic risk than the same amount on hips and thighs. Two people with the same BMI and very different fat distribution face different risks.

The squared term distorts at the extremes. Because height is squared rather than cubed, BMI tends to overstate the weight of very tall people and understate that of very short people. Quetelet chose the square because it fitted average adults, not because bodies scale that way.

It says nothing about behaviour. Fitness, diet quality, sleep, smoking and alcohol all shape health outcomes, and none of them appear anywhere in the calculation. An active person with a BMI of 27 may well be healthier than a sedentary one at 23.

Athletes and Muscle Mass

The classic illustration is a rugby forward or a sprinter registering as obese on BMI while carrying single figure body fat. Their weight is muscle and bone, which the formula counts identically to fat.

It affects far more than elite athletes. Anyone who lifts weights seriously, works a physically demanding job or is simply built heavily can land a category higher than their body composition warrants.

Use a different measure if this is you. Body fat percentage from a skinfold assessment or bioelectrical impedance, or simply a waist measurement, will tell you far more than BMI can.

Children and Older Adults

These categories do not apply under 20. Children and adolescents are assessed on BMI-for-age percentile charts specific to age and sex, because normal body composition changes substantially through growth and puberty. Applying adult thresholds to a child produces misleading and sometimes harmful conclusions.

Older adults may do better slightly higher. Several large studies find that people over 65 have the lowest mortality at BMI values in the mid twenties rather than the low twenties, possibly because some reserve helps during illness. Losing weight in later life is not automatically beneficial.

Pregnancy makes BMI meaningless. Weight gain is expected and is monitored against pre-pregnancy weight instead. Use the figure from before conception if you need one.

Ethnicity and Different Cut-offs

The standard thresholds came from European populations. Research since has found that risk of type 2 diabetes and cardiovascular disease rises at lower BMI values in people of South Asian, Chinese and other Asian descent.

Several countries use adjusted figures. The WHO has suggested lower action points for Asian populations, and UK guidance recommends thresholds of 23 for overweight and 27.5 for obesity in South Asian, Chinese, other Asian, Middle Eastern, Black African and African Caribbean groups.

This calculator shows the standard bands. If lower thresholds apply to you, interpret the number accordingly, and raise it with a clinician who can advise on your particular background.

Measures That Tell You More

Waist circumference is the easiest upgrade. Measured at the midpoint between the lowest rib and the top of the hip bone, it captures abdominal fat directly. Above 94 centimetres for men and 80 for women indicates raised risk, with 102 and 88 marking high risk.

Waist to height ratio is simpler still. Keeping your waist below half your height is a single rule that works across ages and populations, and in several studies predicts metabolic risk better than BMI does.

Body fat percentage measures the thing itself, though accessible methods vary in accuracy. Bioelectrical impedance scales are convenient but sensitive to hydration; skinfold calipers are reliable in trained hands; DEXA scanning is the reference standard.

Blood markers matter more than any of them. Blood pressure, fasting glucose, HbA1c and a lipid panel describe what is actually happening in your body, and no measurement of shape can substitute for them.

Using the Number Sensibly

Treat it as one reading among several. BMI, waist measurement, activity level and how you actually feel together give a picture. Any one of them alone does not.

Direction matters more than the value. A BMI drifting upward year on year is worth attention even while it remains inside the healthy band. A stable figure slightly above it is far less concerning.

Do not chase a specific number. The healthy range is wide precisely because there is no single correct value. Aiming for a particular decimal is a good way to develop an unhealthy relationship with the scale.

Take it to a professional if it concerns you. A doctor or registered dietitian can interpret the figure alongside your history, medication, family background and blood work, which is the context a calculator cannot have.

Frequently Asked Questions