BMI Calculator

Body Mass Index takes two numbers — your height and your weight — and turns them into a single figure that estimates whether your weight sits in a healthy range. It is quick, it costs nothing, and clinics, researchers and public-health agencies lean on it as a first screen. Just hold on to what that word means. A screen flags what is worth a closer look; it does not diagnose. BMI can tell you something is worth checking, but it cannot tell you the whole story by itself.

Checking your BMI because you want to move it? Pair the result with our calorie calculator — it turns your numbers into a daily target for losing, maintaining or gaining weight.

How BMI is calculated

BMI is the Quetelet index: your weight divided by the square of your height. The formula is the same everywhere — only the units change, and this calculator accepts both metric (kg, cm) and imperial (lb, ft/in) inputs.

• Metric: BMI = weight (kg) ÷ height (m)²
• Imperial: BMI = 703 × weight (lb) ÷ height (in)²

A worked metric example: someone who weighs 70 kg and stands 175 cm (1.75 m) tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which sits in the healthy range. And in imperial units, a person who is 5 ft 4 in (64 in) and 145 lb comes out at 703 × 145 ÷ 64² = 24.9 — right at the top of that same healthy band.

The BMI chart: what your category means

The World Health Organization sorts adult BMI into four main bands:

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Normal (healthy) weight
25.0 – 29.9	Overweight
30.0 and above	Obese

These thresholds apply to most adults aged 20 and over, and they are the same for men and women. Clinically, the WHO splits the ends of the scale even finer — the obese band into Class I (30–34.9), Class II (35–39.9) and Class III (40 and above), and the underweight band into mild, moderate and severe thinness — but the four categories above are what matter for everyday screening, and they are what this calculator reports. Children and teenagers are read differently, which we cover below.

BMI for men and women

You will see BMI charts labelled “for women” or “for men”, but here is the thing: the formula and the adult cut-offs do not change with sex. A BMI of 27 is “overweight” whether you are a man or a woman. What changes is what that number tends to represent.

At the same BMI, women naturally carry more body fat than men — that is normal biology, not a warning sign. Men, on average, carry more muscle and denser bone, so at the same height and weight a man typically has less body fat than a woman. Two people with an identical BMI can therefore have quite different body compositions, which is one reason the number is best read alongside a second measure.

Waist circumference is the usual companion, and its thresholds do differ by sex. The NIH flags raised health risk once waist size passes 102 cm (40 in) for men and 88 cm (35 in) for women. So if your BMI sits right on a boundary, your waist measurement often settles which way to read it.

Your healthy weight range

Flip the formula around and it answers a more useful question: what should you weigh for your height? The healthy band runs from a BMI of 18.5 to 25, so your healthy weight range is simply those two numbers multiplied by your height in metres, squared.

For a height of 170 cm (1.70 m) that is 18.5 × 2.89 to 25 × 2.89 — about 53 to 72 kg. The calculator above works this out for you and marks it on the scale, but here are the ranges for some common heights:

Height	Healthy weight range
155 cm (5′1″)	44–60 kg (98–132 lb)
160 cm (5′3″)	47–64 kg (104–141 lb)
165 cm (5′5″)	50–68 kg (111–150 lb)
170 cm (5′7″)	53–72 kg (118–159 lb)
175 cm (5′9″)	57–77 kg (125–169 lb)
180 cm (5′11″)	60–81 kg (132–179 lb)
185 cm (6′1″)	63–86 kg (140–189 lb)

These are rounded to the nearest kilo, and a kilo either side changes nothing meaningful.

Does BMI work for every ethnicity?

BMI was built largely from data on white European populations, and the standard cut-offs do not fit every group equally. The clearest example is people of South and East Asian descent, who tend to develop weight-related conditions — type 2 diabetes and heart disease especially — at lower BMIs than the standard chart would predict.

So the WHO recommends lower trigger points for these groups: overweight from a BMI of 23, and obese from about 27.5, rather than 25 and 30. This calculator uses the standard international thresholds, so if you are of Asian descent it is worth reading your result against those lower numbers — a BMI of 24 that looks fine on the standard scale may already deserve a second look.

The CDC makes the general point plainly: the link between BMI and actual body fat varies with age, sex and ethnicity. The formula does not change — but the background you read it against does.

BMI Prime and Ponderal Index

Alongside your BMI, this calculator shows two related numbers that add context:

• BMI Prime is your BMI divided by 25 (the upper limit of the healthy range). A BMI Prime of 1.0 sits exactly at the overweight threshold, so 0.9 means you are at 90% of that limit, and 1.2 means 20% above it. It is a quick, unit-free way to see how far you are from the healthy ceiling.
• Ponderal Index (kg/m³) divides weight by height cubed instead of squared. Because BMI slightly over-rates tall people and under-rates short people, the Ponderal Index stays more consistent across extremes of height. Healthy adults typically fall around 11–15 kg/m³.

Health risks of a high BMI

A BMI in the overweight or obese range is linked to a higher risk of several conditions, including type 2 diabetes, high blood pressure, heart disease and stroke, certain cancers, and osteoarthritis. Risk generally rises as BMI increases, but it is also shaped by waist size, activity level, diet and family history — which is why BMI is a starting point, not a verdict.

Health risks of a low BMI

Being underweight carries its own risks: nutritional deficiencies, weakened immunity, reduced bone density (increasing fracture risk), and, in some people, fertility problems. A low BMI can also be a sign of an underlying condition. If your BMI is below 18.5, it is worth discussing with a clinician rather than simply trying to gain weight.

BMI for children and teens

For anyone under 20, BMI is read differently. Children and teenagers are still growing, and they grow at different rates, so their BMI is compared against age- and sex-specific percentile charts from the CDC rather than the fixed adult thresholds. The same BMI number can be perfectly healthy at one age and a concern at another. If you are checking a child’s result, use our child & teen BMI calculator, which plots it on the correct CDC percentile chart, and go over it with a paediatrician.

The limits of BMI

BMI is popular because it asks for only two numbers — but that is also where it falls short. It cannot tell muscle from fat. A muscular athlete can score as “overweight” on paper while carrying very little fat; an older adult who has lost muscle can score as “healthy” while carrying too much. And it says nothing about where fat sits, even though fat around the abdomen carries more risk than fat on the hips and thighs.

For a fuller picture, pair your BMI with something that reads body composition or fat distribution — a body fat percentage estimate or your waist-to-hip ratio — and weigh it against your own health history. No single number decides your health; BMI is one useful input among several.

If your BMI is outside the healthy range

A BMI outside 18.5–25 is not a verdict, but it can be a nudge worth acting on. If you want to change it, the sustainable path is a modest, steady shift in energy balance rather than a crash. Working out how many calories you actually burn in a day is a sensible first step: our TDEE calculator estimates that, and the calorie calculator turns it into a daily target for losing, maintaining or gaining weight. And if your BMI is low, treat unexplained weight loss or a persistently underweight reading as a reason to talk to a clinician — not just to eat more.