BMI Calculator

Q: How much weight do I need to lose to change BMI category?

Enter your measurements into the calculator and it displays your healthy weight range along with the difference between your current weight and the nearest point of that range. As a rough guide, one BMI point is roughly 3 kg (about half a stone) for someone of average height.

Your measurements

Age

years

Sex

Height

ft in

Weight

st lb

Your result

Enter your measurements and press
Calculate my BMI

Your BMI

Underweight Healthy Overweight Obese

Healthy weight range

To reach healthy BMI

Underweight	Below 18.5
Healthy weight	18.5 – 24.9
Overweight	25 – 29.9
Obese	30 and above

This calculator is for informational purposes only and does not constitute medical advice. BMI is a screening tool, not a diagnostic measure — it does not account for muscle mass, bone density, or fat distribution. Always consult a qualified healthcare professional for personalised health guidance.

How BMI is calculated

Body Mass Index compares your weight against your height to produce a single number. The formula is deliberately simple — it needs only two measurements, which is exactly why it became the standard screening tool in general practice.

Metric

BMI = weight (kg) ÷ height (m)²

Imperial

BMI = [weight (lb) ÷ height (in)²] × 703

Take someone who is 5 ft 9 in tall and weighs 11 st 6 lb. That converts to 1.75 m and 72.6 kg. Squaring the height gives 3.06, and 72.6 divided by 3.06 comes to a BMI of 23.7 — comfortably inside the healthy range. The calculator above handles the stones-to-kilograms conversion for you, so you can enter your measurements however you normally think about them.

BMI was never designed for individuals. It was devised in the 1830s by Adolphe Quetelet, a Belgian statistician who was studying the characteristics of populations, not diagnosing patients. It was only adopted as a health screening measure in the 1970s. That history explains a lot about both its usefulness and its limits.

What each BMI category means

The thresholds below are the standard ranges used by the NHS and the World Health Organization for adults aged 18 and over.

Category	BMI range	What it suggests
Underweight	Below 18.5	May indicate insufficient nutrition, an underlying condition, or simply a naturally slight build. Worth discussing with a GP if it is unintentional or recent.
Healthy weight	18.5 – 24.9	Associated with the lowest risk of weight-related conditions across large population studies.
Overweight	25 – 29.9	Gradually increasing risk of type 2 diabetes, high blood pressure and heart disease — though the increase across this band is modest for most people.
Obese	30 and above	Meaningfully raised risk of several long-term conditions. Clinically subdivided into class I (30–34.9), class II (35–39.9) and class III (40 and above).

It is worth treating these boundaries as gentle gradients rather than cliff edges. A BMI of 25.1 does not represent a materially different level of health risk from 24.9 — the categories exist because population health needs round numbers, not because your body changes state when you cross one.

BMI thresholds are lower for some ethnic groups

This is the single most commonly missed detail about BMI, and it matters a great deal in the UK.

People of South Asian, Chinese, Black African, African-Caribbean and Middle Eastern family background tend to develop weight-related health risks — particularly type 2 diabetes — at a lower BMI than white European populations. This appears to relate to how and where the body stores fat rather than total fat alone.

NICE recommends acting at lower BMI levels for these groups: 23 or above indicates increased risk, and 27.5 or above high risk — against 25 and 30 for the general population. These are thresholds for offering advice and intervention earlier, rather than a redefinition of what counts as overweight or obese.

In practice this means a BMI of 24 might be read as unremarkable for one person and as a prompt for a conversation about diabetes risk for another. If this applies to you, it is worth raising with your GP rather than reading your category from the standard table.

Where BMI falls short

BMI knows your height and your weight. It knows nothing else about you — and several of the things it cannot see matter.

It cannot distinguish muscle from fat

Muscle is denser than fat, so a well-trained rugby player or weightlifter can register as obese while carrying very little body fat. This is the most frequently cited criticism of BMI, though it affects a fairly small share of the population.

It cannot see where fat is stored

Fat carried around the abdomen carries a higher metabolic risk than fat carried on the hips and thighs, but BMI treats both identically. Two people with the same BMI can have quite different risk profiles.

It is not designed for everyone

Children and teenagers under 18 — BMI is interpreted using age and sex-specific centile charts, not the adult thresholds above.
During pregnancy — BMI is not a meaningful measure; your midwife will use your pre-pregnancy weight as the reference point.
Older adults — muscle mass declines with age, so BMI can understate body fat in people over about 65.

A useful second measurement

Because BMI misses fat distribution, waist size is a valuable companion measure. Measure around your middle at the level of your belly button, without pulling the tape tight. As a simple rule of thumb, your waist should be less than half your height — so someone 5 ft 9 in (69 in) is aiming for a waist under about 34.5 in. This waist-to-height ratio works across different body types and ethnic backgrounds, which is part of why NICE now recommends it alongside BMI.

What to do with your result

A BMI figure is a starting point for a question, not an answer in itself. What it is genuinely good at is flagging when something might be worth a closer look.

If your result sits in the healthy range and you feel well, there is little to act on. If it sits outside that range, the useful next step is context: how is your waist measurement, has your weight changed noticeably in the past year, and how active are you? The NHS recommends at least 150 minutes of moderate activity a week for adults, alongside strengthening exercise on two days — and improvements in fitness produce real health benefits even when the number on the scale barely moves.

For a persistent result well outside the healthy range, an unexplained change in weight, or a result you are unsure how to read, a GP can interpret it alongside blood pressure, blood glucose and your family history. That combined picture is far more informative than BMI alone.

Frequently asked questions

Is BMI accurate?

BMI is accurate at what it measures — the ratio of weight to height — and it is a reliable screening tool across large groups. It is less reliable as a statement about any one individual, because it cannot distinguish muscle from fat or account for where fat is stored. Treat it as a prompt rather than a verdict.

What is a healthy BMI for my age?

For all adults aged 18 and over, the healthy range is 18.5 to 24.9 regardless of age. The thresholds do not shift as you get older. Bear in mind that muscle mass tends to decline with age, so an unchanged BMI in later life can mask a gradual increase in body fat.

Does BMI differ between men and women?

The BMI thresholds themselves are identical for both. Women typically carry a higher proportion of body fat than men at the same BMI, which is one reason the measure is best read alongside a waist measurement rather than on its own.

Can I have a healthy BMI and still be unhealthy?

Yes. BMI says nothing about blood pressure, cholesterol, blood glucose, fitness, diet quality or smoking — all of which affect long-term health independently of weight. A healthy BMI is one reassuring signal among many, not a clean bill of health.

How much weight do I need to lose to change category?

The calculator above shows this directly: enter your measurements and it displays your healthy weight range along with the difference between your current weight and the nearest point of that range. As a rough guide, one BMI point is roughly 3 kg (about half a stone) for someone of average height.

Sources and further reading

World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva, 2000. — the international BMI classification used in the table above.
Eknoyan G. Adolphe Quetelet (1796–1874) — the average man and indices of obesity. Nephrology Dialysis Transplantation, 2008;23(1):47–51. — the origin of the index, and why it was built to describe populations.
Keys A, Fidanza F, Karvonen MJ, et al. Indices of relative weight and obesity. Journal of Chronic Diseases, 1972;25(6):329–343. — the paper that named it “body mass index” and brought it into health screening.
National Institute for Health and Care Excellence. BMI: preventing ill health and premature death in black, Asian and other minority ethnic groups (PH46), 2013. — the source of the lower action thresholds discussed above.
Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obesity Reviews, 2012;13(3):275–286. — the evidence behind the “waist under half your height” rule.
NHS UK — Obesity and the NHS BMI healthy weight calculator. — UK guidance on interpreting your result.

Thresholds and guidance on this page follow NHS and NICE recommendations for adults in the UK, and the World Health Organization's international BMI classification. This page is reviewed periodically for accuracy. For a fuller explanation of the measure, see our article on what BMI means.