BMI Calculator

Body mass index is your weight in kilograms divided by your height in meters squared, and for most adults a result between 18.5 and 24.9 is the healthy band. The calculator below does that arithmetic, places the result in the World Health Organization categories, and works out what weight range those categories correspond to at your height. Nothing you type leaves your browser.

The formula, and how to measure so it is right

BMI = weight (kg) ÷ height (m)². At 70 kg and 1.75 m that is 70 ÷ 3.0625 = 22.9. In imperial units the same index is weight (lb) ÷ height (in)² × 703, which is the conversion factor and nothing more — both routes give the same number.

Two measurement habits decide whether the result means anything. Weigh yourself in the morning, after the toilet and before breakfast, in light clothing; the same scale each time. Measure height without shoes, heels together, looking straight ahead — adults lose height through the day and through the decades, and an out-of-date height quietly inflates the index.

The index is deliberately crude. It was built by Adolphe Quetelet in the 1830s to describe populations, and Ancel Keys revived it in 1972 as the best of the simple weight-for-height indices for exactly that purpose. Applying a population statistic to one person is where every limitation on this page comes from.

How to read your result

The categories below are the WHO cut-offs used for adults of 20 and over. They are the same for men and women.

BMI	Category	What it signals
Below 16.0	Severe thinness	Nutritional risk, low bone density, weakened immune response
16.0–18.4	Underweight	Possible nutrient deficiencies, anemia, low energy availability
18.5–24.9	Healthy weight	Lowest average risk across large cohorts
25.0–29.9	Overweight	Raised risk of hypertension and dyslipidemia, strongly modified by waist
30.0–34.9	Obesity class I	Clearly raised cardiometabolic and type 2 diabetes risk
35.0–39.9	Obesity class II	High risk, including sleep apnea and osteoarthritis
40.0 and above	Obesity class III	Very high risk; shortest average life expectancy in cohort data

A result near a boundary is not a verdict. Measurement noise of a kilogram and a centimeter moves BMI by roughly 0.4 units, so 24.8 and 25.1 are the same reading taken twice. The Body Mass Index (BMI) tool reports the same categories alongside a mortality-risk ratio and a Smart BMI score, which is the more useful view once you are near a cut-off.

Add your waist, because BMI cannot see fat distribution

BMI knows your mass, not what the mass is. Two people at BMI 27 can differ by 20 kg of muscle, and two people at BMI 23 can differ completely in where their fat sits. Fat inside the abdomen — Visceral fat — is the metabolically active kind, and it is what turns an unremarkable BMI into real risk.

That is why the calculator asks for waist circumference and why it is worth the twenty seconds. Waist divided by height above 0.5 flags central adiposity at any BMI, and it does so earlier than the index does. The Waist-to-height ratio (WtHR) tool scores that ratio properly against age-adjusted thresholds, and Body fat assessment estimates the fat fraction itself when you have a few more measurements to hand.

Measure the waist midway between the lowest rib and the top of the hip bone, at the end of a normal breath out, with the tape snug but not compressing the skin.

What BMI is bad at

It cannot distinguish muscle from fat, so trained athletes are routinely classified as overweight while carrying single-digit body fat. It says nothing about where fat sits, so Body composition has to be measured separately. It drifts with age as muscle is lost and height is shed, which is why the healthy band for older adults sits higher than the textbook one. And it was derived largely in European populations, so the risk thresholds are lower for people of South, South-East and East Asian descent.

The honest summary: BMI is a fast screening step that tells you whether to look further, and a poor final answer about any individual. Treat a result outside the healthy band as a prompt to check waist, blood pressure and fasting glucose, not as a diagnosis.

BMI Prime, the version that needs no table

BMI Prime is your BMI divided by 25 — the upper limit of the healthy band. A BMI of 27.5 is a Prime of 1.10, and you can read that directly: you are 10% above the top of the healthy range. Below 0.74 is underweight, 0.74 to 1.00 is the healthy band, and anything above 1.00 is the percentage you are over. The calculator reports it beside the index.

Frequently asked questions

Is BMI accurate for one person?

As a screen, yes; as a verdict, no. It sorts populations well but cannot tell muscle from fat in an individual. Pair it with waist circumference, and with a body-composition measurement if the two disagree.

What is a healthy BMI?

18.5 to 24.9 for most adults aged 20 to 64. For adults over 65 the lowest-mortality range in cohort studies sits higher, roughly 23 to 30, because a little reserve protects against illness and muscle loss.

Does BMI differ for men and women?

The cut-offs do not. Body composition does: at the same BMI women carry more essential fat and men more muscle, which is one reason the index should not be read alone.

Does the calculator store my data?

No. Every calculation runs in your browser. Nothing is sent to a server, and nothing is saved when you close the page.

Sources

Sources (4)

1. WHO Expert Committee. Physical status: the use and interpretation of anthropometry. World Health Organ Tech Rep Ser. 1995;854:1–452. PubMed →
2. Keys A, et al. Indices of relative weight and obesity. J Chronic Dis. 1972;25:329–343. PubMed →
3. Nuttall FQ. Body mass index: obesity, BMI, and health — a critical review. Nutr Today. 2015;50:117–128. PubMed →
4. Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI. Obes Rev. 2012;13:275–286. PubMed →

Related reading

• What a Healthy BMI Range Actually Means
• BMI vs Body Fat Percentage: Which to Track
• Waist-to-Height Ratio Beats BMI for Risk
• What to Do After You Calculate Your BMI

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Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.