BMI Calculator

About the BMI Calculator

This page turns two measurements — your height and your weight — into a Body Mass Index, and then into the handful of numbers that are actually derived from it: your BMI category, your BMI Prime, the weight range that would put someone of your height between 18.5 and 25, and how far you currently sit from that range. Add an optional waist measurement and you also get a waist-to-height ratio, which tells you something BMI structurally cannot.

You can enter height in feet and inches or in centimetres, weight in kilograms, pounds or stone, and waist in centimetres or inches. The unit selectors sit next to each field and your choice is remembered between visits. Weight in stone takes a single decimal number rather than a stone-and-pounds pair, so 12 st 6 lb is entered as 12.43 st.

Two fields on the form — age and gender — are optional and do not enter the calculation at all. They exist only to label saved results in your history so you can tell one person's entries from another's. The arithmetic is identical whatever you put there, and Section 11 explains why.

Everything is computed in your browser. Nothing you type is sent to a server.

Using the BMI Calculator, step by step

1. Set the height unit, then enter your height. The dropdown beside the field reads FT or CM. On FT you get two boxes — feet in the first, inches in the second, beside a fixed IN label. Type 5 and 9. On CM the pair collapses to one box, which opens empty: switching the dropdown converts nothing, so you enter 175.26 yourself.
2. Enter your weight and pick its unit. One box, one dropdown offering KG, LB and ST. Set it to LB and type 165. The same caveat bites harder with one box: changing the dropdown re-reads the number in it rather than converting it.
3. Optionally, add a waist measurement. Same arrangement, CM or IN. Type 86 with CM selected; leave it blank and everything else still works.
4. Press Calculate Your BMI. Missing height or weight gets you a message naming it, not a result.

Then read down the results column. The top card carries the headline figure, 24.4, titled with your name if you gave one; Share puts My BMI is 24.4 (Normal). on the clipboard or into your phone's share sheet. The next card is BMI Prime: 0.97. The third gives the category message, then the healthy range for your height — for 5'9", 56.8–76.8 kg, or 125.3–169.3 lb with the selector on LB. The waist-to-height ratio, 0.49, sits on that line; a weight outside the range adds one more giving the gap. Below it, the coloured 15-to-38 scale marks where 24.4 falls with a triangle.

With history on, the result joins the table underneath, storing height and weight as typed (5'9", 165 lb). Reset clears the fields but keeps your unit choices, which live in settings.

Measuring yourself accurately

The arithmetic is exact. Your measurements are not, and that is where nearly all the error comes from.

Height. Shoes off, heels together and against a wall, back straight, eyes level and looking straight ahead — tipping your chin up costs you height. Read it at the end of a normal breath out. Care here counts for more than the same care over weight, because height is squared: a percentage error in height moves BMI twice as far as the same error in weight. At 5 ft 9 in and 165 lb, shoes adding 2.5 cm drop the result from 24.4 to 23.7; a whole kilogram out on the scale moves it 0.3. People also get height most wrong, usually quoting a figure last checked years ago.

Weight. Same scale, same time of day, minimal clothing, hard floor rather than carpet — carpet lets a scale's feet sink unevenly, so the reading shifts with where you put it. Day-to-day swings of one to two kilograms are normal, and they are water, glycogen and gut contents rather than fat. So compare Tuesday morning with Tuesday morning, not tonight with yesterday lunchtime.

Waist. Bare skin, never over a jumper or a belt, at the midpoint between your lowest rib and the top of your hip bone, tape snug but not compressing, read at the end of a normal exhale. Check in a mirror that the tape is level all the way round; one riding up at the back reads short. Use the same landmark every time: consistency between your own measurements matters more than which protocol you picked.

Three situations worth walking through

Meera, five months apart

Meera is 163 cm and weighed 78 kg in January, for a BMI of 29.4. She recalculates on the first Sunday of each month, same scale, before breakfast. By June she is 75.5 kg and reads 28.4. Six rows sit in the history table, and the trend across them says what no single reading can.

At her height one BMI point is 2.7 kg, and a single kilogram of water moves the figure 0.38. A month where the number rises two-tenths tells her nothing. Six readings leaning the same way tell her something.

Tom, who lifts

Tom is 180 cm and 92 kg. His BMI is 28.4 and the bar puts him in Overweight. His 84 cm waist is a ratio of 0.47 against his height, inside the under-half-your-height guideline. The two disagree because they measure different things: BMI knows only his total mass, the ratio only where that mass sits.

Neither settles it. A low waist ratio in a heavy person is consistent with plenty of lean tissue but is not proof of it, and the BMI is not wrong — it answers a question about mass that Tom was not asking. Settling it needs an assessment that can see body composition: a clinician, not a calculator.

Ruth, between units

Ruth thinks in stones; her GP works in kilograms. She enters 12.4 st at 5 ft 4 in and gets 29.8. Curious whether the metric side agrees, she switches to KG — which leaves 12.4 in the box, now read as 12.4 kg, until she retypes it as 78.7. That gives 29.8 again.

It was always going to agree: the conversions are exact definitions, not approximations. The only thing that moves is the rounding Ruth adds by typing 78.7 instead of 78.74 — a hundredth of a BMI point, which never reaches the screen.

The formula, and a worked example

BMI is weight in kilograms divided by height in metres, squared:

BMI = kg ÷ m²

Everything else on the page is downstream of that one division. If you enter imperial units, the calculator converts first, using the exact legal definitions: 1 inch = 2.54 cm, 1 foot = 30.48 cm, 1 pound = 0.45359237 kg, 1 stone = 14 pounds. These are definitions, not approximations, so nothing is lost in the conversion.

Take someone 5 ft 9 in tall weighing 165 lb.

• Height: (5 × 30.48) + (9 × 2.54) = 152.4 + 22.86 = 175.26 cm, which is 1.7526 m
• Height squared: 1.7526 × 1.7526 = 3.0716 m²
• Weight: 165 × 0.45359237 = 74.843 kg
• BMI: 74.843 ÷ 3.0716 = 24.366, displayed as 24.4

A useful thing to notice at this height: one whole BMI point is 3.07 kg, about 6.8 lb. That conversion factor is different for every height, which is why "how many pounds is one BMI point" has no general answer.

What the categories mean

The bands this calculator uses come from the World Health Organization's adult classification. The exact boundaries in the code, and which side each boundary falls on, are:

• Below 18.5 — Underweight
• 18.5 up to but not including 25 — Normal
• 25 up to but not including 30 — Overweight
• 30 up to but not including 35 — Obese
• 35 and above — Severely Obese

Every boundary belongs to the band above it. A BMI of exactly 25.000 is labelled Overweight, not Normal. One departure from WHO worth naming: WHO splits obesity into class I (30–34.9), class II (35–39.9) and class III (40+). This calculator stops at 35 and calls everything above it Severely Obese, so it does not distinguish class II from class III. The result bar is also drawn on a fixed 15–38 scale; a BMI above 38 pins the marker at the right-hand end rather than running off the chart.

These are the standard international adult thresholds, and they are not universal. Following a WHO expert consultation published in *The Lancet* in 2004, WHO identified additional public-health action points at 23 and 27.5 for Asian populations, because cardiometabolic risk — type 2 diabetes and cardiovascular disease in particular — rises at a lower BMI in people of South Asian, Chinese and other Asian descent, at least partly because more of the body fat at a given BMI is visceral fat around the organs. NICE, which writes clinical guidance for the NHS in the UK, now recommends lower thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds: overweight from 23.0, obesity from 27.5.

This calculator does not adjust for ethnicity. It applies the standard thresholds to everyone. For the worked example above, the difference is not academic: at 175.26 cm, BMI 23 is 70.6 kg (155.7 lb) and BMI 27.5 is 84.5 kg (186.2 lb). Our 165 lb example is comfortably "Normal" on the standard scale and in the overweight range on the Asian-specific one. If the lower cut-offs apply to you, read your number against them rather than against the coloured bar.

Reference: WHO fact sheet on obesity and overweight.

BMI Prime

BMI Prime is simply your BMI divided by 25 — your position relative to the top of the healthy range, expressed as a ratio. Below 1.00 means at or under that ceiling; above 1.00 means over it, and by how much proportionally.

Our example: 24.366 ÷ 25 = 0.97. Read that as "97% of the upper limit", or about 3% below it. Someone at BMI 30 has a Prime of 1.20 — 20% above the ceiling. The advantage over the raw number is that the same Prime means the same thing at every height, so it travels well between people.

Your healthy weight range

The range shown with your result is the formula run backwards. Rather than dividing weight by height squared, it multiplies height squared by each end of the Normal band:

• Lower end: 18.5 × 3.0716 = 56.8 kg
• Upper end: 25 × 3.0716 = 76.8 kg

For 5 ft 9 in that is 56.8–76.8 kg, or 125.3–169.3 lb. If your weight falls outside it, the page also shows the gap: someone at 185 lb (83.9 kg) is 15.7 lb above the top of the range, with a BMI of 27.3.

The range looks wide because it is: nearly 20 kg, or 44 lb. That width is 6.5 BMI points multiplied by your height squared, so it grows with height. At 155 cm the healthy range spans 44.4–60.1 kg, a 15.6 kg window. At 190 cm it spans 66.8–90.3 kg, a 23.5 kg window. The same six-and-a-half BMI points buy a taller person eight more kilograms of latitude.

One boundary quirk: weighing exactly the top of the printed range produces a BMI of exactly 25, which the classifier assigns to Overweight. The printed range and the coloured label disagree at that single point by design.

Waist-to-height ratio: the second signal

If you enter a waist measurement, the page divides it by your height. Both are converted to centimetres first, so the units you chose do not matter. The rule of thumb is simple enough to remember without a calculator: keep your waist to less than half your height — a ratio under 0.5.

NICE recommends this measure in the UK for adults with a BMI under 35, alongside BMI rather than instead of it. Our 5 ft 9 in example has a half-height of 87.6 cm (34.5 in); a 86 cm waist gives a ratio of 0.49, just inside.

To measure properly:

• Find the midpoint between the bottom of your lowest rib and the top of your hip bone, and run the tape horizontally around you at that level. This is the WHO protocol; some national guidance uses the level just above the navel instead, which is usually close but not identical — pick one and reuse it.
• Measure against skin or light clothing, not over a jumper or a belt.
• Keep the tape snug without compressing the skin.
• Breathe out normally and read the tape at the end of that breath. Do not hold your breath and do not pull your stomach in.

Why bother, when you already have a BMI? Because BMI knows your total mass and nothing about where it sits. Fat stored viscerally, around the liver and other abdominal organs, is associated with metabolic risk in a way that fat stored on the hips and thighs is not. Two people at BMI 26 can have very different waist measurements, and the waist figure is the one that distinguishes them.

Who BMI does not describe well

BMI measures mass against height. It has no way to ask what the mass is made of, so it misreads anyone whose body composition is unusual — in both directions.

Muscular athletes. Skeletal muscle is denser than fat tissue, so building it adds weight without adding fat. A rugby forward or a competitive powerlifter can land in the Obese band at single-digit body fat. The number is arithmetically correct and clinically meaningless.

Older adults. Two changes work in opposite directions and both distort the result. Height is lost with age as the spinal discs compress and vertebrae change shape, and because height is squared in the denominator, losing height raises BMI without any weight gain. At the same time, muscle mass declines, so a stable BMI over decades quietly represents more fat and less lean tissue than it used to.

Different body compositions generally. Bone density, limb-to-torso proportion and where fat is deposited all vary between individuals and between populations, and BMI is blind to every one of them. This is the same mechanism behind the ethnicity-specific thresholds above.

Pregnancy. Weight gained in pregnancy is fetus, placenta, amniotic fluid, expanded blood volume and breast tissue. Running that weight through a formula built for non-pregnant adults produces a number that describes nothing. Clinicians work from pre-pregnancy BMI instead.

After limb amputation. Removing a limb removes weight but not height, so BMI drops — often by several points — while body fat percentage has not changed at all. Clinical practice applies estimated corrections for this; a general-purpose calculator cannot.

Children and teenagers

For ages 2 to 19, BMI is calculated with the same formula and then interpreted completely differently. A child's number is plotted against growth reference charts specific to their exact age and sex — the CDC growth charts in the US, WHO growth references elsewhere — because normal body composition changes month by month through growth and puberty. The result is a percentile, and categories are defined by percentile position rather than by fixed values.

This calculator applies the adult bands to whatever it is given. It does not read the age field, and it has no percentile data. The category label it produces is not appropriate for anyone under 20. The BMI figure itself is still arithmetically correct; take it to a paediatrician or practice nurse who can plot it, and ignore the coloured bar.

The history and the limits of BMI

The formula is nearly two centuries old. Adolphe Quetelet, a Belgian astronomer and statistician, described the weight-over-height-squared ratio in the 1830s while developing what he called "social physics" — an attempt to characterise the average person in a population statistically. He was studying populations, not patients, and he never proposed it as a measure of an individual's health.

The name came much later. Ancel Keys and colleagues, in a 1972 paper in the *Journal of Chronic Diseases*, compared several simple height-weight indices and concluded that Quetelet's was the best of them for population studies. They called it the body mass index — while noting explicitly that it was suited to population work rather than individual diagnosis.

That lineage explains most of the criticism BMI attracts. It is cheap, reproducible and genuinely useful for comparing groups and tracking trends across millions of people. It was never designed to diagnose anything about one person standing on a scale, and it still does not.

What to do with the number

Treat it as one input among several, and one that is easy to over-read. A BMI just inside or just outside a band tells you very little; the bands are administrative lines drawn across a continuum, not cliff edges where risk changes.

If you want to do something with it, the productive move is to bring it to a GP, nurse or other qualified healthcare professional who can read it alongside the things BMI cannot see: your waist measurement, blood pressure, blood glucose or HbA1c, cholesterol, family history, medications, and what your activity and sleep actually look like. Those measurements together support a conversation that a single number cannot.

Tracking your own number over months is generally more informative than any single reading, which is why the optional history exists. Weigh yourself at a consistent time of day if you do — normal within-day fluctuation of one to two kilograms is enough to move BMI by half a point at average heights.

Before you read too much into it

BMI is a screening measure. It sorts populations into bands that correlate with risk; it diagnoses nothing about the person on the scale. Nothing here should change what you do about your health without a clinician reading it alongside everything BMI cannot see. The terms covering this calculator are in our Terms of Service.

Medical disclaimer

This calculator is an educational tool. It is not a medical device, it does not provide medical advice, diagnosis or treatment, and its output must not be used as a substitute for consultation with a qualified healthcare professional. BMI is a screening measure with the documented limitations described on this page; a result inside the Normal band does not rule out a health problem, and a result outside it does not establish one. Never delay or disregard professional medical advice because of something you read here. If you have concerns about your weight, your health, or a specific symptom, speak to a doctor or other qualified clinician. If you are pregnant, under 20, an athlete, or living with a condition that affects body composition, the category labels on this page are particularly unlikely to apply to you.