The BMI Calculator estimates body mass index from height and weight, a single number that places a person's weight into a broad category used for population health screening. Enter measurements in US or metric units, and the calculator returns the BMI, the category it falls in, and the weight range considered healthy for that height.
BMI is a useful first look and a poor final word. It is quick, free and consistent, and it cannot tell muscle from fat or account for an individual body. This page returns the number and explains, plainly, what it does and does not mean, because a figure about your body deserves that context, not a verdict.
Calculate your BMI from height and weight
Body mass index compares weight to height with a single ratio. It was designed in the 19th century as a way to describe populations, not individuals, and it remains a screening tool: a fast way to flag who might benefit from a closer look, not a measurement of health or body fat.
Enter height and weight in whichever units are natural, feet and inches with pounds, or centimetres with kilograms. The calculator handles the conversion and returns the BMI immediately, along with the category and the healthy weight range for that height.
The number on its own is not the point. What matters is reading it in context, which is what the rest of this page is for.
Read the BMI categories
BMI sorts adults into broad ranges set by the World Health Organization. These describe the general adult population and are not diagnoses of any individual.
| Category | BMI range |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obese, class I | 30.0 to 34.9 |
| Obese, class II | 35.0 to 39.9 |
| Obese, class III | 40.0 and above |
A category is a range on a chart, not a statement about a person's health, fitness or worth. Two people with the same BMI can have very different amounts of muscle, fat and fitness. The category is a starting point for a conversation with a doctor, and nothing more final than that.
These ranges apply to adults. For children and teenagers the interpretation is completely different, and that is covered in its own section below.
Apply the BMI formula
BMI uses one formula in two unit systems. Metric: `` BMI = weight (kg) / height (m)² ``
US units: `` BMI = 703 × weight (lb) / height (in)² ``
The 703 factor in the US version converts pounds-per-inch-squared into the metric result, so both give the same number for the same person. Height is squared in both, which is the source of BMI's main quirk: because height enters as a square, the index tends to read slightly high for very tall people and slightly low for very short people, even at the same proportions.
The formula is identical for everyone. It does not change with sex, age or any other factor, a point that matters, because a separate "BMI for women" is a common search and there is no such separate calculation. What differs is interpretation, not the arithmetic.
Calculate BMI for a 70 kg adult at 175 cm
Take an adult weighing 70 kg at 175 cm tall. 1. Convert height to metres. 175 cm = 1.75 m 2. Square the height. 1.75² = 3.0625 3. Divide weight by that. 70 / 3.0625 = 22.9 A BMI of 22.9 sits within the range labelled healthy weight for adults, which spans 18.5 to 24.9.
The same person measured in US units, about 154 lb at 68.9 inches, gives 703 × 154 / 68.9² = 22.8, the small difference being rounding in the unit conversion.
What this does not tell you: whether those 70 kg are mostly muscle or mostly fat, how fit the person is, or whether they are healthy. It tells you their weight-to-height ratio falls in a range that, across large populations, is associated with lower weight-related health risk. That is genuinely useful, and it is all it is.
Find your healthy weight range
Rather than a single "ideal" number, BMI points to a range. Inverting the formula for the healthy band, 18.5 to 24.9, gives the span of weights that fall in that category for a given height. For someone 175 cm tall: `` lower = 18.5 × 1.75² = 56.6 kg upper = 24.9 × 1.75² = 76.2 kg ``
So roughly 57 to 76 kg reads as healthy weight at that height, a 19 kg span, which is wide on purpose. Healthy bodies come in a range of weights, and a person can be in good health across most of that band. The calculator shows this range for the height entered.
A range is a more honest answer than a target. There is no single number a person of a given height should weigh, and this page does not pretend otherwise.
Understand what BMI does not measure
This is the most important section on the page, and the one most calculators leave out. BMI cannot tell muscle from fat. It measures total weight against height, and muscle is denser than fat. A trained athlete can read as overweight or obese on BMI while carrying very little fat.
Rugby players, sprinters and weightlifters routinely do. Their BMI is not wrong; it is just answering a different question than the one they care about.
BMI says nothing about where fat sits. Fat carried around the abdomen carries more health risk than fat on the hips and thighs, and BMI cannot see the difference. A waist measurement or a waist-to-height ratio adds information BMI misses entirely.
BMI can miss low muscle in older adults. Someone who has lost muscle with age may carry a high proportion of body fat while their BMI still reads normal, because the lost muscle kept the total weight down. The number looks reassuring and the body composition may not be.
BMI is a screening tool, not a diagnosis. It flags who might benefit from a closer look. It does not diagnose anything, and no health decision should rest on it alone. A doctor combines it with other measurements, history and blood work to form an actual picture.
None of this makes BMI useless. It makes it a first step. Read it as one.
Understand how BMI applies across different groups
The formula is the same for everyone; how the result is best interpreted is not. Sex. There is no separate BMI formula for men and women, the calculation is identical. At the same BMI, women on average carry a higher proportion of body fat than men, so the same number can reflect different body composition.
This is why a single "healthy BMI" is a population guide rather than a personal verdict, and it is the honest answer to the common "BMI for women" question: same maths, context-dependent reading.
Ethnicity. Health risk at a given BMI varies between populations. Several health authorities apply lower thresholds for people of South Asian and East Asian descent, an overweight threshold around 23 and an obesity threshold around 27.5, because cardiometabolic risk tends to rise at a lower BMI in these groups. Someone of South Asian heritage reading 24 on the standard scale may sit in a higher-risk band on the scale their doctor uses.
Athletes and muscular builds. As above, high muscle mass inflates BMI without inflating health risk. For this group, body-fat percentage or a direct composition measure is far more informative.
Older adults. Age-related muscle loss can hide a high fat proportion behind a normal BMI, so the number alone is less reliable with age.
The BMI is the same three-digit result for everyone. Reading it well means knowing which of these applies.
Interpret BMI for children and teenagers
For anyone under 18, the adult categories above do not apply, and this calculator does not return them for a child's measurements. Children and teenagers are still growing, and healthy body composition changes rapidly with age and differs between boys and girls.
Their BMI is interpreted against percentile charts published by the Centers for Disease Control and Prevention, which compare a child's BMI to others of the same age and sex. The CDC considers a BMI between the 5th and 85th percentile for age to be a healthy range.
A raw BMI number means little for a child without that percentile context. For anyone under 18, use a paediatric BMI-for-age tool or, better, ask a paediatrician, a growing child's weight is best read by someone who can see the whole growth curve.
Frequently asked questions
Is BMI accurate?
BMI accurately calculates a ratio of weight to height, and that ratio correlates with body-fat levels across large populations. For an individual it is less precise, because it cannot distinguish muscle from fat or account for body-fat distribution. It works well as a population screening tool and should be combined with other measures for a personal assessment.
Does BMI differ for men and women?
No. The BMI formula is identical for men and women. At the same BMI, women on average carry a higher proportion of body fat, so the same number can reflect different body composition, but there is no separate calculation. A "BMI for women" search is answered by the standard formula with attention to how the result is interpreted.
What is a healthy BMI?
For adults, the World Health Organization defines a healthy BMI as 18.5 to 24.9. This is a broad range rather than a single ideal, because healthy bodies vary. The range is a population guideline, and where a person sits within or near it matters less than their overall health, fitness and habits.
Why is my BMI high if I am muscular?
BMI measures total weight against height and cannot tell muscle from fat. Muscle is denser than fat, so a muscular person weighs more for their height and reads higher on BMI despite carrying little fat. Athletes frequently register as overweight on BMI. A body-fat percentage measurement is more informative for muscular builds.
What BMI is considered underweight?
For adults, a BMI below 18.5 falls in the range labelled underweight. Being below this threshold can reflect many things, a naturally lean build, high activity, or an underlying issue, and the number alone does not identify which. Anyone concerned about a low BMI is best served by discussing it with a doctor rather than acting on the figure alone.
Does BMI work for older adults?
Less reliably. Age-related muscle loss can leave an older adult with a high proportion of body fat while their BMI still reads in the normal range, because the lost muscle lowered their total weight. For older adults, measures like waist circumference and body composition add information that BMI alone misses.
How do I lower my BMI healthily?
BMI falls as weight decreases relative to height, but the healthy approach is to focus on habits rather than the number: regular physical activity, a varied diet built around whole foods, adequate sleep and gradual change. A doctor or dietitian can give guidance suited to individual circumstances, which matters more than chasing a specific BMI figure.
Is BMI different for Asian populations?
The formula is the same, but the risk thresholds often differ. Several health authorities apply an overweight threshold around 23 and an obesity threshold around 27.5 for people of South and East Asian descent, because weight-related health risk tends to rise at a lower BMI in these groups. A doctor familiar with these guidelines can interpret a result accordingly.
How is BMI calculated for children?
For children and teenagers, BMI is calculated with the same formula but interpreted using CDC BMI-for-age percentile charts rather than the fixed adult categories. These compare a child's BMI to others of the same age and sex, with the 5th to 85th percentile considered healthy. A paediatrician is the best source for interpreting a child's BMI.
Summary
The BMI Calculator estimates body mass index from height and weight and places the result in a World Health Organization category, alongside the healthy weight range for that height. The formula, weight divided by height squared, or the 703-based US version, is identical for everyone regardless of sex or age, so there is no separate BMI for women.
BMI is a screening ratio, not a diagnosis: it cannot distinguish muscle from fat, cannot see where fat sits, and reads differently across athletes, older adults and some ethnic groups. It applies to adults only; children and teenagers are assessed by CDC percentile. Read as a first step and paired with other measures and a doctor's input, BMI is genuinely useful. Read as a verdict on a person, it is not.