Your weight against your height, and the World Health Organization band that number falls in, with a straight account of what body mass index can and cannot tell you.
Body mass index
Enter your measurements and this fills in.
Under below 18.5
Healthy 18.5 to 25
Over 25 to 30
Obese 30 to 40
WHO category
not yet
Healthy weight for your height
not yet lb
Typically the WHO adult cut-offs are 18.5, 25 and 30
Taking the measurement
1
Measure your height
Stand against a wall without shoes, heels down, and mark the top of your head.
2
Weigh yourself
First thing in the morning, before eating, on a hard floor rather than carpet.
3
Enter both
The calculator divides your weight in kilograms by the square of your height in meters and reports the World Health Organization category it falls in.
Body mass index is your weight in kilograms divided by your height in meters squared. The World Health Organization classifies adults below 18.5 as underweight, 18.5 to 25 as healthy weight, 25 to 30 as overweight and 30 or above as obese. It measures mass against height and nothing else.
How this is worked out
Height
Without shoes, measured against a wall with your heels down.
Weight
First thing in the morning is the most repeatable time to weigh, because food and water move the figure by a kilogram or more across a day.
The calculation
Weight in kilograms divided by height in meters squared. That is the whole formula. It has been unchanged since the nineteenth century and it does not know anything about you beyond those two numbers.
What each measurement is
Reference bands are this tool's own, chosen for the landmarks it uses.
Where research reports a figure it is given separately, with the source,
because the two are different claims. And each one says what actually
moves it, which for most facial measurements is the more useful half.
Body mass index
Formula
weight in kilograms ÷ height in metres squared
Reference
the WHO adult cut-offs are 18.5, 25 and 30
Research
below 18.5 underweight, 18.5 to 25 healthy weight, 25 to 30 overweight, 30 and above obese - World Health Organization
Cannot distinguish muscle from fat, which is its best-known failure.
What moves it The WHO healthy band is 18.5 to 25 and it is a screening standard rather than an appearance one. If you train, read the body fat estimate instead: BMI cannot tell muscle from fat, and a lean trained man routinely reads as overweight on it. These are adult figures, and anyone still growing should take the question to a doctor.
WHO category
Formula
the band your index falls in
Healthy weight for your height kg
Formula
18.5 and 25 multiplied by your height in metres squared
Reference
the same two WHO cut-offs the category above comes from
The range is wide because BMI is a screening band and not a target weight. Where you sit inside it is a question about body composition, which this cannot see.
What moves it If you train, read this against the body fat estimate rather than on its own: muscle is denser than fat, and a lean trained man routinely reads as overweight here while measuring 12% body fat. The range is still the right place to start if you have no other measurement.
Body mass index is your weight in kilograms divided by your height in meters
squared. This calculator does that arithmetic and reports the World Health
Organization category the result falls in.
It’s the simplest measurement on this site and the most misread, so it’s worth
spending two minutes on what it actually does.
What the number means
The WHO classifies adults into four bands: below 18.5, 18.5 to 25, 25 to 30, and
30 or above. Those cut-offs are theirs. We report them as theirs and we don’t
add a target of our own.
The formula has been unchanged since the nineteenth century, when a Belgian
statistician built it to describe populations. That origin explains most of its
behavior. It was never designed to assess an individual, and nearly every
complaint about it is a version of that.
Where BMI fails, and why that is useful to know
It cannot tell muscle from fat. Muscle is denser, so a lean and
well-trained man can read as overweight on a measure that knows only his mass
and his height. The calculation isn’t malfunctioning. It’s answering a question
about mass when he asked one about composition.
It ignores where weight sits. Two people at the same BMI with very different
distributions of fat get the same figure, and distribution is the part that
carries most of the health signal.
It knows nothing about age, frame or history. Two inputs, no context.
Knowing this is worth something concrete: it tells you which measurement to
reach for instead. If your question is about composition, a circumference method
separates fat from mass, and the circumference method is
the published one. If your question is about appearance, torso shape has better
evidence behind it than either.
What it’s still good for
Two things, and they’re real.
Tracking one person over time. BMI’s weaknesses are mostly constant for a
given person. Someone whose BMI moves from 29 to 25 across a year has changed
something real, whatever the absolute figure fails to capture.
A starting point for a conversation. It’s the measure a doctor will already
have, and it costs two numbers to produce. That’s why it survives despite
everything above.
BMI and appearance are different questions
This site is about looksmaxxing, so it’s worth saying where BMI sits in that.
It sits outside it. The research on male appearance found torso shape predicted
ratings ahead of body mass, and there’s no body of work establishing a BMI that
people find attractive. Importing a health screening number and presenting it as
an appearance finding is a category error, and it’s the first thing anyone
checking the page would catch.
We publish the calculation and the WHO’s own classification. What to do about
the result, past that, is a question for a doctor rather than for a website.
If you’re under 18
The adult cut-offs on this page don’t apply to you. Under-18s are assessed
against age-and-sex percentile charts instead, because height and body
composition are both still changing, and the adult bands will read you wrong.
That isn’t a formality. If you’re worried about your weight and you’re still
growing, a doctor is genuinely the right place to take it.
Where to take this next
The body fat calculator answers the composition question BMI
can’t, using a published tape method.
Torso shape is the one of the three with real
appearance research behind it.
Weight in kilograms divided by height in meters squared. If you are 78 kg and 1.80 m, that is 78 divided by 3.24, which gives a BMI of 24.1.
What are the BMI categories?
The World Health Organization classifies adults below 18.5 as underweight, 18.5 to below 25 as healthy weight, 25 to below 30 as overweight, and 30 or above as obese. Those cut-offs are theirs, not ours.
Is BMI accurate for muscular people?
No, and this is its best-known failure. Muscle is denser than fat, so a lean, well-trained man can read as overweight on a scale that only knows his mass and his height. The number is not wrong; it is answering a different question from the one he is asking.
What is a better measurement than BMI?
For body composition, a circumference method separates fat from mass, which BMI cannot do. For appearance, torso shape has far better evidence behind it. BMI is useful mainly as a screening tool across populations.
Why is BMI still used if it has these problems?
Because it needs two numbers anyone can get, and across large populations it tracks health outcomes well enough to be worth collecting. It was built as a population statistic, and the trouble starts when it is read as a verdict on one person.
Does BMI apply to teenagers?
Not in this form. Under-18s are assessed on age-and-sex percentile charts rather than the adult cut-offs, because height and composition are still changing. If you are under 18, this calculator's categories do not apply to you and a doctor is the right place to ask.
Does BMI differ for men and women?
The WHO adult cut-offs are the same for both, even though body composition at a given BMI typically differs. That is one of the reasons it works better as a population screen than as an individual assessment.
Is BMI related to how attractive you look?
Not directly, and it is worth being exact. Research on male appearance found torso shape predicted ratings ahead of body mass. BMI is a health screening measure, and presenting it as an appearance finding is a category error.
What BMI is considered healthy?
The World Health Organization puts the healthy adult band at 18.5 to 25, overweight at 25 to 30 and obese at 30 or above. Those cut-offs are a population screening tool and they cannot tell muscle from fat, which is why a lean trained man often reads as overweight on them.
Why does my BMI change day to day?
Body weight moves by a kilogram or more across a day with food, water and salt. Weigh at the same time under the same conditions, and read the trend across weeks rather than the number on any one morning.
Does BMI account for age or frame size?
Neither. It takes two inputs and nothing else. Every criticism of BMI is really a version of that single fact.