BMI calculator

How BMI is calculated

BMI divides your weight in kilograms by the square of your height in metres. A 70 kg adult at 1.70 m has a BMI of 70 divided by 1.70 squared, 70 divided by 2.89, which comes to 24.2.

The formula in words

Take your weight in kilograms. Divide it by your height in metres. Divide the result again by the same height in metres. Imperial height and weight convert to metric first; the formula itself only ever runs in kilograms and metres.

BMI has no upper or lower bound built into the maths. It simply describes the ratio of weight to height squared, which is why the categories in the table below exist, to turn that raw ratio into a plain label.

Self-reported numbers versus a measured reading

This calculator, like almost every BMI calculator online, takes whatever height and weight you type in, and it has no way to check either against a scale or a stadiometer. A systematic review by Gorber et al. found that self-reported height and weight consistently differ from a directly measured reading, with height tending to be overstated and weight understated, a combination that pushes a self-reported BMI lower than a measured one. The direction and size of that gap varies by how the weight is carried and by sex, so there is no single correction to apply. The practical takeaway is to weigh and measure yourself as carefully as you can before entering the numbers, rather than typing in a remembered or rounded figure, since the error introduced there is often larger than the difference between two adjacent WHO categories.

Why BMI, and what it does not measure

BMI became the standard screening number because it needs only a scale and a tape measure and correlates reasonably well with body fat across large populations. It was never built to assess an individual's body composition, only to flag population level risk cheaply and consistently.

A short history

The underlying formula is older than modern medicine's use of it. Adolphe Quetelet, a Belgian statistician, described the weight to height squared ratio in the 1830s while studying the average build of the population, and for over a century it was known as the Quetelet Index. Physiologist Ancel Keys renamed it body mass index in a 1972 paper comparing several weight indices against directly measured body fat, and found it the best of the simple options available at the time, which is why the name and the formula stuck.

Keys was explicit that BMI was a population comparison tool, not a diagnostic one for any single person. That caveat is still true over 50 years later; the formula has not changed, only how widely it gets applied to individuals rather than groups.

Waist to height ratio

Adds one more measurement, your waist, and correlates better with abdominal fat, the type most linked to cardiovascular risk. If you want a second opinion on the same reading, the waist to height ratio calculator is the natural next step.

Body fat percentage

From skinfolds, a scan or the Navy method, body fat percentage answers the question BMI cannot: how much of your weight is fat versus muscle. It takes more effort to measure but is the better number for anyone training seriously.

Lean body mass

The lean body mass calculator goes a step further and reports how much of your weight is not fat, muscle, bone, organs and water combined. It is useful alongside BMI for anyone whose BMI category does not match how they actually look or perform, since it separates the signal BMI is missing.

None of these alternatives replace BMI as a quick, free population screen. They answer a more specific question that BMI was never designed to answer in the first place, and each takes more time or equipment to measure well.

WHO weight categories at a glance

The World Health Organization sets six BMI bands for adults. They apply to the general adult population and are not adjusted for age, sex, ethnicity or muscle mass.

BMI prime: the same number as a ratio

BMI prime simply divides your BMI by 25, the upper edge of the normal range, so a BMI of 30 becomes a BMI prime of 1.2, or 20 percent above the normal weight ceiling. It carries exactly the same limitations as BMI itself since it is built from the same two measurements, but some clinicians find it a more intuitive way to communicate how far above or below the normal band someone sits.

It is worth knowing the term exists mainly so you are not confused if you come across it elsewhere. This calculator reports standard BMI and the WHO category directly, which is the version almost everyone actually uses.

The honest limit: BMI and muscular people

BMI cannot tell muscle from fat. It only weighs total mass against height, so a 96 kg, 178 cm rugby player at 12 percent body fat and a 96 kg, 178 cm sedentary adult at 30 percent body fat land on the exact same BMI of 30.3, technically obese by the WHO table, despite being in completely different physical condition.

This is not a rare edge case. Strength athletes, rugby and American football players and anyone who has trained hard for years routinely score overweight or obese on BMI while carrying well below average body fat. If you fall into this group, BMI is telling you almost nothing useful and a body fat measurement matters far more.

The reverse also happens. Someone with a normal BMI but very little muscle, sometimes called normal weight obesity, can carry a higher body fat percentage than their BMI suggests. A normal BMI is a reassuring average, not proof of good body composition on its own.

If your BMI looks off relative to how you actually look and perform, do not force the conclusion. Check body fat directly with the Navy body fat calculator or the lean body mass calculator instead.

Contact sport athletes show this most clearly. Rugby and American football players frequently land in the overweight or obese BMI bands because of the muscle and bone mass their sport selects for, not because of excess fat. The formula is not wrong in what it calculates, it is being asked a question about body fat that it was never built to answer.

How to use your BMI

Use it as a first screen, not a verdict

Treat a normal BMI as one good sign, not a clean bill of health, and treat a high BMI as a reason to look closer rather than a diagnosis on its own. Waist measurement, activity level and how you actually feel and perform matter more day to day.

Recheck alongside a body fat estimate

Pair your BMI with a body fat percentage wherever you can get one. The two numbers agreeing gives you real confidence; the two disagreeing, especially if you train regularly, usually means BMI is the one to discount.

Track the trend, not one reading

A single BMI reading is a snapshot. If you are trying to change your weight, a BMI trend measured monthly, alongside the scale weight itself, shows the direction far better than any one number in isolation.

BMI cutoffs differ by population

The 18.5 to 24.9 normal range was built mainly from European and North American data. Research since then, including a 2004 WHO expert consultation, found that several Asian populations carry higher health risk at lower BMI values than the standard cutoffs suggest, and some national health bodies now use lower action points, around 23 for overweight and 27.5 for obese, for people of South Asian and East Asian descent.

If you fall into one of these groups, treat the standard WHO categories shown above as a starting point and ask a clinician familiar with population specific cutoffs, rather than relying on this calculator's bands alone.

The same caveat runs the other way too. Certain population groups, including some Pacific Islander populations, tend to carry more lean mass at a given BMI than the original European and North American reference samples, which would make the standard cutoffs unnecessarily strict for them. The honest summary is that one fixed set of BMI bands was never going to fit every population equally well, which is exactly why it is a screening tool and not a diagnosis.

Common mistakes

Treating BMI as a body fat measurement. It is a weight to height ratio, nothing more. It does not measure fat directly, only correlates with it on average across large groups of people.

Ignoring the trend for a single number. Weight and hydration both shift day to day. A BMI reading taken a month apart tells you far more than obsessing over one measurement.

Applying adult categories to children or teenagers. BMI for under 18s uses age and sex specific percentile charts, not the fixed adult bands shown in the table above.

Assuming a normal BMI means good health. It rules out the extremes but says nothing about blood pressure, blood sugar, fitness or how much of that weight is muscle.

Comparing BMI across very different builds. Two people with the same BMI can have completely different waist size, muscle mass and health risk despite matching on paper.

Using BMI to compare progress week to week. Body weight, and therefore BMI, is a poor short term progress marker for anyone in a training programme, since water, glycogen and food weight swamp any real fat or muscle change over a few days.

How coaches use a BMI calculator

For a coach, BMI is a five second intake number, useful for spotting the small share of clients at a genuinely high or low weight who need a more cautious plan, not a metric to programme around week to week.

A better routine: note BMI at intake, then track the numbers that actually respond to training, waist measurement, progress photos, strength and, where possible, a body fat estimate. Weight alone, BMI included, moves slower and noisier than any of those.

This matters most in the first conversation with a new client. A client who has been told their whole life that a high BMI means they are unhealthy, despite training consistently and carrying real muscle, needs that context up front, or the number becomes a source of discouragement rather than a useful baseline.

Scraler logs weight, photos and check-in answers per client in one timeline, so you see the trend behind the BMI number instead of guessing from a single scale reading. See nutrition coaching and client check-ins.

BMI vs waist circumference as a risk screen

BMI and waist circumference answer closely related but not identical questions. BMI compares total weight to height and says nothing about where that weight sits on the body. Waist circumference measures one thing directly, the size of your midsection, which is where the fat most strongly linked to cardiovascular and metabolic risk tends to accumulate. Two people can share an identical BMI and carry very different amounts of visceral fat around the organs, and waist circumference is usually the number that catches the difference.

The clinical case for adding waist circumference rests on abdominal fat, sometimes called visceral or central fat, behaving differently from fat stored elsewhere on the body. It sits closer to the liver and other organs, releases more inflammatory signalling and free fatty acids into the bloodstream, and correlates more tightly with insulin resistance and heart disease risk than fat carried on the hips or thighs at the same total body weight. A 2008 WHO expert consultation set two commonly used thresholds, a waist above 102 cm for men and 88 cm for women, and a systematic review and meta-analysis by Ashwell et al. backs the simpler waist to height ratio cutoff of 0.5. Both aim to flag that specific risk pattern rather than total mass.

Where each measurement wins

BMI needs only a scale and a height measurement, takes seconds and is reproducible across almost any setting, which is why it remains the default first screen in a doctor's office or a gym intake form. Waist circumference needs a tape measure placed consistently, usually at the navel or the midpoint between the lowest rib and the top of the hip bone, and is more sensitive to measurement technique from one person to the next. Neither measurement alone tells the full story, which is why several major health bodies now recommend using both together rather than picking one.

Where they disagree, and why that matters

The cases where BMI and waist circumference disagree are exactly the cases where the extra measurement earns its place. A person with a normal BMI but a waist above the threshold, sometimes described as normal weight, central obesity, can carry meaningfully elevated metabolic risk that BMI alone would completely miss. The reverse also happens: someone with a high BMI built mostly from muscle and a proportionally narrow waist, the same muscular-athlete pattern that trips up BMI on its own, usually shows a normal waist to height ratio even while sitting in the overweight or obese BMI band. Waist circumference is not a replacement for BMI so much as a check on the specific failure mode BMI has around body composition and fat distribution.

For anyone whose BMI looks borderline or does not match how they feel and perform, adding a waist measurement takes under a minute and often resolves the ambiguity a single BMI number leaves open. Use the waist to height ratio calculator alongside this one for that second data point.