Getting a BMI you can actually rely on
1. Enter your height in whichever units you think in
Centimetres, or feet and inches — switch the unit and the second box appears or disappears to match. Most people know their height in one system and their weight in another, and there is no reason to make you convert anything by hand before you start. Measure without shoes, standing straight, heels together, looking forward.
2. Enter your weight the same way
Kilograms or pounds. If you are weighing yourself at home, do it at the same time of day each time, ideally in the morning before eating. Body weight swings by a kilogram or two across a normal day from food and fluid alone, which is enough to move a BMI by half a point and mean nothing at all.
3. Add your age
Not decoration. Below 20 the adult categories genuinely do not apply, and rather than print a label that would be wrong, the tool says so and explains what is used instead. Above 20 your age is used to frame the result, because the same BMI carries different weight at 25 and at 75.
4. Tick the Asian cut-offs if they apply to you
If you are of South Asian, East Asian or South-East Asian descent, the public-health action points sit lower than the standard ones. Ticking the box switches the thresholds to 23.0 and 27.5 and tells you it has done so. This is a documented WHO position, not a rounding preference — the section below explains where it comes from.
5. Read the range, not just the word
Underneath the category you get the actual weight range that would put you in the healthy band at your height, in your own units. That range is the useful output. “Overweight” tells you almost nothing; “the healthy band for you runs from 59.9 kg to 80.9 kg” tells you where you stand and how far away the edges are.
6. Open the worked example if the arithmetic matters to you
The page shows the calculation with your own numbers substituted in, step by step, so nothing about the result has to be taken on trust. If you want to check it against a calculator on your phone, everything you need is on screen.
One line of arithmetic, and why it survived
Body mass index is weight in kilograms divided by height in metres squared. That is the whole formula. Someone who is 1.70 m and 70 kg has a BMI of 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2. If you work in pounds and inches, the same thing is expressed as weight in pounds × 703, divided by height in inches squared — the 703 is just the unit conversion folded into a single number.
The index is nearly two hundred years old. Adolphe Quetelet, a Belgian mathematician, described the weight-to-height-squared relationship in the 1830s while studying the statistical distribution of human measurements across a population. He was not designing a health screen and never claimed to be. The name “body mass index” was attached much later, by Ancel Keys and colleagues in 1972, who compared several weight-for-height indices and concluded that Quetelet’s was the best of the simple ones for population studies.
That history explains both why BMI is still everywhere and why it attracts so much criticism. It survived because it needs only two measurements that anyone can take, it is reproducible, and across large groups it tracks body fatness well enough to be useful. It attracts criticism because people keep using a population instrument on one individual and expecting a diagnosis.
Where 18.5, 25 and 30 actually come from
The boundaries are not arithmetic. They were chosen because that is roughly where, in large cohort studies, the risk of illness and death starts to climb. The World Health Organization set out the classification in its 1995 report on physical status and confirmed it in the 2000 technical report on obesity; the US National Heart, Lung, and Blood Institute adopted matching adult cut-offs in its 1998 clinical guidelines, which is why American and international charts agree.
The categories this page uses are the full WHO set, not the four-band summary most calculators stop at:
- Below 16.0 — severe thinness
- 16.0 to 16.9 — moderate thinness
- 17.0 to 18.4 — mild thinness
- 18.5 to 24.9 — healthy range
- 25.0 to 29.9 — overweight, or pre-obesity
- 30.0 to 34.9 — obesity class I
- 35.0 to 39.9 — obesity class II
- 40.0 and above — obesity class III
Collapsing everything from 30 upwards into one word called “obese” is the single most common shortcut in online BMI calculators, and it throws away the distinction that changes what a clinician would actually do. A BMI of 31 and a BMI of 46 are not the same finding. The class boundaries are where treatment guidelines in most health systems change their recommendations, so the class is the part worth reading.
Why 23 and 27.5 appear when you tick the Asian cut-offs
In January 2004 a WHO expert consultation published its conclusions in The Lancet on whether the standard BMI thresholds were appropriate for Asian populations. The finding was that a substantial proportion of Asian people carry a higher percentage of body fat, and more of it around the abdomen, at a BMI below the conventional cut-off of 25 — and that the risk of type 2 diabetes and cardiovascular disease begins to rise from a BMI of about 22 to 25, well before the standard overweight line.
The consultation deliberately declined to redraw the international classification, because the shift was not uniform across all Asian populations. Instead it recommended additional public-health action points at 23.0 and 27.5, to be used alongside the standard categories for identifying people who need attention sooner. That is precisely what the toggle on this page does: it does not rename your category, it moves the thresholds at which the page starts flagging risk, and it tells you plainly that it has done so.
For a site with a large Indian and South-East Asian readership this is not a footnote. A BMI of 24 sits comfortably inside the healthy band on the standard chart and above the action point on the Asian one. Which of those two readings is more useful to you depends on your ancestry, and it is not a question a calculator should silently decide on your behalf.
Several national bodies have gone further than WHO. NICE in the United Kingdom recommends lower thresholds for people of South Asian, Chinese, Black African and African-Caribbean family background when assessing risk of type 2 diabetes and cardiovascular disease. If the number matters to you clinically, that difference is worth raising with a doctor rather than resolving on a web page.
The healthy weight range for your height, which is the part you can use
Because the formula is reversible, any height has a weight range that corresponds to the 18.5–24.9 band. At 1.70 m that is 53.5 kg to 71.9 kg. At 5 ft 6 in it is 114.6 lb to 154.2 lb. The tool works this out in whichever units you entered and shows it under the result.
This is deliberately the most prominent number after the index itself, because a category on its own is a dead end. Knowing you are 3 kg above the top of your healthy band, or 8 kg below the bottom of it, is actionable in a way that the word “overweight” never is. It also makes the arbitrariness visible in a healthy way: a range roughly 18 kg wide at average adult height is not a target weight, and anyone presenting a single “ideal” figure to two decimal places is overselling what this measurement can do.
What a BMI cannot see
BMI knows two things about you: your height and your mass. Everything it appears to say about your health is inference from those two numbers, and the inference breaks in several well-documented ways.
It cannot tell muscle from fat. Muscle is denser than fat tissue. A rugby forward, a competitive swimmer or someone several years into serious strength training can carry a BMI in the overweight or obese range with a low body-fat percentage and excellent cardiovascular health. The number is not lying — they genuinely are heavy for their height — but the health inference attached to it does not hold.
It cannot see where the fat sits. Two people with identical BMIs carry very different risk if one stores fat around the abdomen and the other around the hips and thighs. Visceral fat around the organs is the pattern most strongly associated with metabolic disease, and BMI is completely blind to it. This is why waist circumference and waist-to-height ratio are used alongside BMI rather than in place of it, and why measuring your waist is a genuinely useful next step.
It drifts with age. Adults lose muscle mass and bone density from roughly the fourth decade onwards, so an unchanged BMI at 70 usually represents more fat and less lean tissue than the same figure at 30. There is also consistent evidence that in older adults, being at the very bottom of the healthy band is associated with worse outcomes than being slightly above it. The page frames your result with your age for this reason.
It was derived from European populations. The cohorts behind the original cut-offs were overwhelmingly white and European. That is the entire reason the 2004 consultation happened, and the reason the toggle on this page exists.
It does not apply during pregnancy. Weight gain in pregnancy is expected and is assessed against your pre-pregnancy BMI using an entirely separate framework. Running a current-weight BMI while pregnant produces a number with no meaning. Our pregnancy weight gain calculator uses the right one.
It says nothing about anyone under 20. Children and adolescents are assessed on age-and-sex-specific percentile charts, because body composition changes continuously through growth. Enter an age below 20 and this tool refuses to print an adult category, and explains why, rather than producing a confident-looking answer that a paediatrician would disregard.
Can you get body fat percentage out of a BMI?
Approximately, and with error bars wide enough that we chose not to print a figure. Deurenberg and colleagues published age- and sex-specific prediction formulas in the British Journal of Nutrition in March 1991, derived from 1,229 subjects, estimating body fat percentage from BMI, age and sex. The equations work reasonably at the level of a group. For one individual the prediction interval is wide — comfortably several percentage points — and a number like “23.4% body fat” printed to one decimal place implies a precision the method does not have.
The previous version of this page produced exactly that figure and was asked in its own FAQ how accurate it was. We removed it rather than continue answering that question. If body composition is what you are after, the body fat calculator uses circumference measurements, which is a measurably better input than height and weight alone, and is honest about its own margin.
What this page will not tell you to do
The previous build responded to a low reading with “You may need to gain weight” and to a high one with “You should consider weight loss” — directive instructions issued from one number, with no source and no disclaimer anywhere on the page. Those are gone.
A BMI is a screening measurement. It can tell you that your weight relative to your height sits outside the range associated with the lowest risk in population studies. It cannot tell you why, whether it matters for you, or what to do about it. Those questions need someone who can look at your waist, your blood pressure, your blood glucose and lipids, your medicines, your family history and your actual life. This page gives you the measurement and the context, and stops there. Nothing here is medical advice or a diagnosis.
If your result surprises you, or has moved a long way without a change in how you eat and move, that is a reason to speak to a doctor rather than a reason to act on a web page.
Nothing you type here leaves your device
The calculation runs entirely in your browser as JavaScript. Your height, weight and age are never transmitted to us or to anyone else, nothing is stored, and there is no account, no upload and no log. Closing the tab is all the deletion there is to do. This also means the tool works with the connection dropped once the page has loaded.
Questions people actually ask about BMI
What is a healthy BMI for my height?
The healthy band is 18.5 to 24.9 at every adult height — the index is designed so the target does not change. What changes is the weight range that produces it, and the tool prints that range for your height in your own units.
Is BMI accurate for muscular people?
The number is accurate; the health inference is not. If you carry substantially more muscle than average, expect BMI to overstate your risk, and use waist measurement and body composition instead. This is the single most common legitimate objection to the index.
Why does my BMI differ slightly from another calculator?
Rounding, almost always. Some tools round your height to the nearest centimetre before squaring it, some round the final index to a whole number, and imperial conversions introduce a little more. Differences of 0.1 to 0.3 are normal and never change anything meaningful. A difference larger than that usually means one of the two is using different units than you think.
Which BMI chart should I use if I am Indian or South-East Asian?
Tick the Asian cut-offs box. It applies the WHO 2004 action points of 23.0 and 27.5 rather than 25 and 30. Both sets of numbers are legitimate; the lower ones reflect that cardiometabolic risk rises at a lower BMI in many Asian populations.
Does BMI work over 65?
Less well. Height loss from spinal compression inflates BMI slightly, muscle loss means the same index represents more fat, and the lowest-risk range appears to shift upward in later life. Treat a result in older age as a rough marker and pay more attention to changes over time than to the label.
My BMI is 26. Should I be worried?
On its own, no — it is a prompt to look at the things BMI cannot see, not a finding. Waist circumference, blood pressure, resting heart rate, blood glucose and cholesterol all say more about cardiovascular risk than a BMI of 26 does. If several of those are also unfavourable, that combination is worth a conversation with a doctor.
What is the difference between obesity class I, II and III?
Class I is 30.0 to 34.9, class II is 35.0 to 39.9, and class III is 40.0 and above. They matter because clinical guidelines change at those lines — the threshold at which particular treatments are considered, and the intensity of follow-up recommended, are tied to class rather than to the single word.
Can BMI be too low?
Yes, and it is taken as seriously in the other direction. Below 18.5 is associated with reduced bone density, impaired immune function, fertility problems and slower recovery from illness. The WHO classification grades thinness in three bands for the same reason it grades obesity in three: severity changes the response.
How often is it worth checking?
Monthly is plenty for tracking a deliberate change; weekly tells you mostly about fluid and food. Day-to-day weight variation of one to two kilograms is normal and moves BMI by around half a point at average adult height, which is noise rather than signal.
Do you keep my measurements?
No. Nothing is saved, sent or logged. The earlier version of this page had a reading-history feature and an FAQ answer about its privacy; the rebuild does not store anything, so there is nothing to keep private.
Where these numbers come from
- WHO — obesity and overweight fact sheet, updated 8 December 2025. The source of the international classification used here.
- WHO Expert Consultation, “Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies”, The Lancet, 2004 (PMID 14726171). The origin of the 23.0 and 27.5 action points.
- CDC — Adult BMI Calculator, for the US framing of the same categories.
- NHLBI, National Institutes of Health — Calculate Your BMI, whose 1998 clinical guidelines set the matching adult cut-offs.
- Deurenberg P, Weststrate JA, Seidell JC, “Body mass index as a measure of body fatness: age- and sex-specific prediction formulas”, British Journal of Nutrition, March 1991 (PMID 2043597), on why a body-fat percentage estimated from BMI alone is not printed here.
- NIDDK Body Weight Planner, if you want to model how a change in intake and activity would move your weight over time.
Tools people open next to this one
- BMR & TDEE Calculator — what your body burns at rest and across a normal day.
- Blood Pressure Calculator — one of the measurements that tells you far more than a BMI does.
- Diabetes Risk Calculator — a validated score that uses BMI as one input among several, which is how it is meant to be used.
