BMI & body composition

Body mass index is your weight in kilograms divided by your height in metres squared. That is the whole thing. It takes two numbers you already know, performs one division, and returns a figure between roughly fifteen and forty-five. No blood test, no scan, no appointment, no cost.

That cheapness is the reason it is everywhere, and also the reason it disappoints so many people who look it up. A ratio of mass to height cannot tell you what that mass is made of. It cannot see that you added four kilograms of muscle over a winter of training, or that you lost three kilograms of it during an illness. It cannot see where your body stores fat, which turns out to matter more than how much of it you carry. It does not know your age, your ancestry, your bone density, how much water you are holding, or whether you weighed yourself before or after breakfast.

The index was worked out in the 1830s by Adolphe Quetelet, a Belgian mathematician and astronomer who was trying to describe the average build of a population. He was explicit that it was not meant to diagnose individuals. It acquired its modern name and its clinical career in 1972, when the physiologist Ancel Keys tested several height-weight formulas against body fat measurements and found Quetelet's the best of a mediocre set — for population work. Nearly two centuries later it is still doing a job it was never designed for, largely because nothing else is this easy.

So read it the way a doctor reads a single blood pressure cuff: as one data point that raises or lowers the priority of looking closer. Not as a verdict on your body.

The World Health Organization's adult categories are the ones almost every calculator uses, this one included. Below 18.5 is classified as underweight. From 18.5 to 24.9 is the healthy range. From 25 to 29.9 is overweight. Thirty and above is divided into three classes of obesity, at 30, 35 and 40.

Those boundaries are conventions, not cliffs. Nothing physiological changes between 24.9 and 25.0. They were set where population risk curves begin to bend upward, and risk curves bend gradually. Someone at 25.2 who walks daily, sleeps well and has a waist under half their height is in a very different position from someone at 24.8 who does none of those things, despite the labels the two would be given.

The Asian cut-points on the calculator above are a separate WHO recommendation from 2004, and they are worth understanding because they demonstrate the whole problem in one move. The expert consultation found that people of South Asian, Chinese, Japanese and South-East Asian ancestry tend to develop type 2 diabetes and cardiovascular disease at a lower index than European-ancestry populations, often carrying more visceral fat at the same weight. The suggested action points are 23 and 27.5 rather than 25 and 30. Switch the toggle and the bands on the dial physically shift while your number stays exactly where it is. The body has not changed. Only the line drawn across it has.

If you would rather read it off a table than a dial, here is the healthy band converted into actual weights. The last column is the waist measurement that keeps you under the half-your-height line — in many cases a more useful target than the weight itself.

Read across your height rather than hunting for a single ideal weight. The band at 1.70 m is roughly nineteen kilograms wide. That width is not sloppiness — it is the honest range in which people of the same height live healthily, and it is far more generous than most of us allow ourselves to believe.

If you take one thing from this page, take this: fat stored around your organs behaves differently from fat stored under your skin. Visceral fat is metabolically active. It releases free fatty acids and inflammatory signals straight into the portal vein, and it is far more strongly associated with insulin resistance, high blood pressure and cardiovascular disease than subcutaneous fat on the hips and thighs.

BMI is blind to that distinction. Waist-to-height ratio is not, and it costs one tape measure. Keep your waist under half your height and you are on the right side of the line that most guidance now uses. At 1.75 m that means a waist under 87.5 cm. The rule scales across heights without adjustment, works reasonably across ancestries, and needs no chart.

Measure it properly or do not bother. Stand up, put the tape around your middle at the level of your navel, breathe out normally, and read it without pulling the tape into your skin. Do it at the same time of day each time — first thing in the morning is easiest to keep consistent. Trousers-size is not a waist measurement; the two disagree by several centimetres for most people, always in the flattering direction.

The most instructive case is the person whose two numbers disagree. A BMI of 23 with a waist at 0.55 of your height has a name in the literature — normal-weight central obesity — and it is associated with worse outcomes than a higher BMI with a smaller waist. The scale looks reassuring while the risk sits exactly where you cannot see it.

Body fat percentage answers the question BMI only gestures at: how much of you is fat, and how much is everything else. For women the fitness range sits roughly between 21 and 25%, with athletes lower and the population average nearer 30. For men the equivalent fitness range is roughly 14 to 18%, athletes lower again, average closer to 22. Women carry more essential fat for reasons of hormonal and reproductive function; comparing across sexes on a single scale is meaningless.

The tape method on this calculator is the one the US Navy has used for decades. It puts your waist, neck and — for women — hip circumference through a logarithmic equation and lands within about three to four percentage points of a DEXA scan for most people. That is not precise enough to argue over a single point, but it is genuinely measuring composition rather than inferring it from mass. If you skip the tape measurements, the page falls back to the Deurenberg equation, which estimates fat from your BMI and age and therefore inherits every weakness BMI has. It is labelled as such in the readout.

What matters more than either figure is the direction it moves. If your weight holds steady while your waist shrinks, you are trading fat for muscle and the scale is hiding good news. That is the single most common reason people abandon a programme that is working.

The trained athlete. Muscle is denser than fat. A rugby forward, an Olympic lifter or a sprinter can read 28 to 30 with body fat in the low teens. The index registers their training as a risk factor. Waist-to-height ratio and a tape-based fat estimate both sort this out immediately.

The older adult. Sarcopenia — age-related muscle loss — begins around forty and accelerates after sixty. Someone can lose several kilograms of muscle and gain the same in fat with the index never moving. In later life, the low end of the standard band is associated with frailty, falls and worse outcomes after illness, which is why several bodies suggest a higher healthy range after 65.

The pregnant or breastfeeding person. BMI is simply not interpretable here. Weight gain guidance in pregnancy is based on the pre-pregnancy index and is set by a midwife or doctor against your specific circumstances.

The person whose height the formula cannot handle. Dividing by height squared assumes a broadly typical geometry. Limb loss, marked scoliosis, achondroplasia and several other conditions break that assumption, and the resulting figure is not comparable to anyone else's.

The very tall and the very short. Human bodies do not scale as neat squares. The square term systematically over-reads tall people and under-reads short ones, which is why the Trefethen variant in the readout above uses height to the power of 2.5 instead. If you are well outside average stature, compare the two figures — the gap between them is the size of the distortion.

Start by deciding whether it needs to move at all. If your waist is under half your height, your blood pressure is fine, your bloods came back clean and you are active most weeks, a BMI of 26 is not an emergency and treating it as one tends to backfire. Chasing a category boundary is a poor reason to change your life.

If it does need to move, the sustainable rate is roughly 0.5 to 1% of your body weight per week. At 90 kg that is 450 to 900 grams. Faster than that and an increasing share of the loss is muscle and water, which brings the index down while making your composition worse — the exact opposite of the goal. A deficit of 300 to 500 calories a day gets most people into that range without the misery that makes people quit in week three.

Protect your muscle while you do it. Two things preserve it: eating enough protein, somewhere around 1.6 grams per kilogram of body weight a day for most people losing weight, and giving the muscle a reason to stay by loading it two or three times a week. Without those, a meaningful fraction of what you lose is the tissue you most want to keep.

Track the trend, not the day. Body weight swings a kilogram or two with salt, glycogen, hydration and the menstrual cycle. Weigh yourself at the same time under the same conditions, and only take a weekly average seriously. A fortnight of flat numbers followed by a sudden drop is completely normal and not a sign that anything is broken.

Our weight loss desk covers calorie deficits, plateaus and the psychology of all this in more depth, and the nutrition desk handles the protein side.

A BMI under 18.5 gets far less attention than the other end of the scale and deserves more. Being underweight is associated with reduced bone density, weakened immune response, fertility problems, poor recovery from illness and, in older adults, a materially higher risk of falls and fractures.

Some people sit naturally at 18 and are perfectly well; a lifelong low-normal reading with stable weight and good energy is a different matter from a recent unexplained drop. Losing weight you did not intend to lose is worth a GP appointment regardless of where the index started, because it is one of the more informative symptoms in medicine.

And if reading a number on this page produced a spike of anxiety, or if you find yourself calculating it repeatedly, that reaction is information too. Disordered eating is common, treatable, and not something a calculator can help with. In the UK, Beat runs a helpline on 0808 801 0677. Elsewhere, a GP is the right first call.

What is a good BMI?

For most adults the healthy band runs from 18.5 to 24.9. It is a range rather than a target, and the evidence behind it is about populations, not individuals. If your ancestry is South Asian, Chinese, Japanese or South-East Asian, the World Health Organization suggests a lower action point of 23, because cardiometabolic risk tends to appear earlier at the same index.

Is BMI accurate for women?

The formula is identical for men and women, and that is precisely the problem people notice. Women carry more essential body fat than men at the same index, so two people reading 23 can have very different compositions. BMI is reasonable for tracking a population and poor at describing an individual body of either sex. A waist measurement adds more than a sex-specific formula would.

What is a healthy weight for my height?

Multiply your height in metres by itself, then multiply that by 18.5 and by 25 for the two ends of the band. At 1.70 m that gives roughly 53.5 kg to 72 kg. The calculator on this page does it for you and shows the answer in kilograms, pounds or stone.

Why does BMI say I am overweight when I am fit?

Because the index cannot tell muscle from fat. It only knows your mass and your height. Trained lifters, rowers, sprinters and rugby players routinely read between 27 and 30 with body fat in the athletic range. If your waist is under half your height and your body fat is low, the index is measuring your training rather than your risk.

Does BMI change with age?

The formula does not, but what a given number means does. Muscle mass falls steadily from around forty, so an older adult can lose meaningful muscle while the index stays flat. Several bodies suggest the healthy range sits nearer 23 to 28 after 65, and that sitting at the low end of the standard band in later life carries its own risk of frailty.

Is waist-to-height ratio better than BMI?

For predicting cardiometabolic risk, generally yes. It needs one tape measure and no arithmetic: keep your waist under half your height. It captures where fat is stored, which matters more than how much you weigh, and it works across heights and ancestries better than a single BMI cut-point does.

How quickly can I lower my BMI safely?

Around 0.5 to 1% of your body weight a week is the range most clinicians consider sustainable. For someone at 90 kg that is roughly 0.45 to 0.9 kg a week. Faster than that and a larger share of what you lose is muscle and water, which lowers the number on the scale while making the underlying picture worse.

Can I use this BMI calculator for children?

No. Children and teenagers under twenty are assessed against age-and-sex percentile charts, because a growing body changes proportion faster than it changes weight. The adult bands used here will give a misleading answer. Ask a GP or paediatrician for the correct chart.