What is a healthy BMI?

The WHO defines 18.5 to 24.9 as the healthy range for adults, with under 18.5 underweight, 25 to 29.9 overweight and 30 or above obese. These are population thresholds drawn from mortality statistics, not individual diagnoses, and a number just outside the range is not in itself a medical finding.

Why is BMI criticised so heavily?

Because it uses only height and weight, so it cannot distinguish muscle from fat or tell where fat is stored. A muscular athlete is routinely classed as overweight while someone with low muscle mass and high abdominal fat can score as healthy. Waist circumference and body fat percentage are better individual indicators.

Should I use BMI at all?

As a rough screening tool it is reasonable, which is why doctors still calculate it. Treat it as one number among several rather than a verdict. If it puts you outside the healthy range, that is a prompt to look at waist measurement, blood pressure and blood markers, not a conclusion on its own.

How do I enter my height in feet and inches?

Convert to total inches: multiply the feet by 12 and add the inches. Five foot ten becomes 70, six foot two becomes 74. A single field avoids the mistake of leaving one of two boxes blank, which is the most common source of wildly wrong BMI results.

What is the healthy weight range shown here?

It is the span of weights that would place you between 18.5 and 24.9 BMI at your current height. It is generally a wider range than people expect — often 20 kilograms or more — which is a useful corrective to the idea that there is one correct weight.

BMI Calculator: Body Mass Index and Category

How to use this calculator

Choose metric (kilograms and centimetres) or imperial (pounds and inches).
Enter your weight.
Enter your height. In imperial this is total inches — five foot ten is 70, six foot two is 74.

The healthy weight range shown is the span of weights that would place you between 18.5 and 24.9 BMI at your height.

How the calculation works

BMI = weight (kg) / height (m)²

That is the whole formula. Imperial input is converted to metric first, which is why there is one implementation rather than two that could drift apart.

A worked example

78 kg at 175 cm.

height in metres = 1.75
1.75² = 3.0625
BMI = 78 / 3.0625 = 25.47


BMI	25.5
Category	Overweight
Healthy weight range	56.7 kg – 76.3 kg
Distance from range	1.7 kg

A BMI of 25.5 sits just over the 25.0 threshold — 1.7 kilograms above the top of the healthy band. Whether that is meaningful depends entirely on what those 78 kilograms are made of, which BMI cannot see.

The WHO categories

BMI	Category
Under 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obese, class I
35.0 – 39.9	Obese, class II
40.0 and over	Obese, class III

These are population thresholds derived from mortality statistics. They are screening boundaries, not diagnoses, and a number a fraction outside the range is not in itself a medical finding.

What BMI genuinely cannot tell you

This is worth being blunt about, because BMI is treated with far more authority than it earns.

It cannot distinguish muscle from fat. A rugby player and a sedentary person of the same height and weight get the same BMI. Muscle is denser than fat, so athletic builds are routinely classified as overweight while carrying very little body fat.

It cannot see where fat is stored. Abdominal fat carries substantially more metabolic risk than fat on the hips and thighs. Two people with identical BMI can have very different risk profiles, and waist circumference detects this while BMI does not.

It was never designed for individuals. The formula comes from Adolphe Quetelet in the 1830s, a statistician studying populations, not a physician assessing patients. It was popularised for individual screening more than a century later largely because it is cheap and easy.

It is not equally valid across populations. Health risks begin at lower BMI values in South Asian, Chinese and other Asian populations, and several countries use a lower overweight threshold of 23 as a result. It is also unsuitable for children, who need age-and-sex-specific percentiles, for pregnant women, and for adults over about 65, where slightly higher BMI is associated with better outcomes.

So is it worth calculating?

As a rough screen, yes — which is why doctors still do it. It correlates reasonably well with body fat at a population level, costs nothing, and needs no equipment.

Treat it as one number among several. If it puts you outside the healthy range, that is a prompt to look at waist measurement, blood pressure, resting heart rate and blood markers — not a conclusion on its own. If it puts you inside the range, that is not a clean bill of health either; someone with low muscle mass and high abdominal fat can score perfectly well.

Common mistakes to avoid

Entering height in feet. In imperial mode the field wants total inches. Entering 5.8 for five foot eight produces a BMI in the thousands, which at least is obviously wrong — entering 58 produces a plausible-looking number that is completely incorrect.

Weighing yourself inconsistently. Body weight fluctuates by one to two kilograms across a day with food, fluid and salt. Weigh first thing in the morning, after using the bathroom, before eating, and compare weekly averages rather than single readings.

Chasing a specific number inside the range. The healthy band spans roughly 20 kilograms at a typical adult height. There is no single correct weight, and the evidence for outcomes is a broad plateau, not a point.

When this calculator is not the right tool

For body composition, the body fat calculator uses tape measurements and distinguishes fat from lean mass, which is the limitation BMI cannot overcome. For a weight target, the ideal weight calculator compares four classical formulas against the healthy BMI range. And for what to eat, the BMR and calorie calculators start from energy expenditure rather than weight alone.

BMI Calculator