Is "half your height" really where the line sits?

NICE places the healthy band at 0.40 to 0.49, which is the arithmetic behind the phrase "keep your waist to less than half your height". From 0.50 to 0.59 is described as increased central adiposity with increased health risk, and 0.60 or more as high central adiposity with further increased risk. Below 0.40 sits outside the healthy band in the other direction, which is worth a mention at an appointment rather than an assumption either way.

Do the thresholds change for men and women?

No, and this is the measure’s most useful property. NICE states the bands apply to both sexes, which puts it in contrast with almost every alternative: waist circumference has separate cut-offs by sex, waist-to-hip has separate cut-offs by sex, and BMI-for-age in children needs a separate reference for each. One set of three numbers covers everyone, which is why it survives being remembered as a sentence rather than looked up in a table.

Does it need adjusting for ethnicity?

NICE states the classification can be used for all ethnicities, and a 2010 systematic review reached the same conclusion — that waist-to-height may be advantageous precisely because it avoids the need for age, sex and ethnic-specific boundary values. That is a real practical advantage over waist circumference, where South and East Asian thresholds are meaningfully lower: 90cm for men against 94 or 102. Whether one number can truly serve every population is a fair question, but it is the position of the guideline.

Can I use this for a child?

From age five, and on the same three numbers as adults — NICE gives an identical classification for children and young people aged five and over, of both sexes and all ethnicities. That is a striking simplification against BMI, which for a child is meaningless until converted into an age-and-sex percentile, because a child’s BMI is expected to fall until about five and a half and then rise. Below five, growth is followed with different measurements and this ratio has no established interpretation.

Is it fair to tall and short people?

Not perfectly, and this is the measure’s known weakness. A fixed ratio treats everyone alike only if waist rises in proportion to height. Measurement puts that close to true for children aged seven to thirteen, but among adolescents the waist rises roughly with the square root of height, so identical builds read higher on shorter teenagers — around 0.54 at 145cm against 0.47 at 195cm. Adults show a smaller version of the same drift. The effect is real but modest, and it argues for reading the band rather than the decimal.

Is this better than BMI?

For central adiposity specifically, there is good evidence it discriminates better, and a substantial body of work has pointed that way for two decades. The practical argument is stronger still: BMI cannot tell where weight sits, so a person within the healthy BMI range can carry central adiposity that BMI does not register at all. Studies of UK survey data found a meaningful share of adults classified as healthy weight by BMI who sit above 0.50. That said, the two measure different things and clinical guidance uses them together rather than in place of one another.

Why does NICE limit the adult bands to a BMI under 35?

Because above that point central adiposity has stopped being the open question, and the classification adds little. Someone with a BMI of 40 will almost always be above 0.60 as well, so the extra measurement does not change what happens next. The guidance is not saying the number is wrong above 35 — only that it is no longer the figure that decides anything, and clinical attention has moved on to other things by then.

Do I need a tape measure at all?

Not really, and that is rather the point. A piece of string cut to half your height does the entire test: if it goes round your waist with slack you are under 0.50, and if it does not you are over. It gives you a band rather than a decimal, which is arguably the right precision for a screening measure anyway. The version with a tape is useful mainly for watching the figure over time, where a decimal has something to say that a yes-or-no does not.

How much does the reading move day to day?

A centimetre or two of waist, which on a typical adult height is about 0.005 to 0.01 on the ratio. That is small in absolute terms and not nothing near a boundary — enough to move 0.495 to 0.505 and change the band without anything changing about you. A meal, bloating, the time of day, and how much you contract your abdomen all contribute. Measure in the morning before eating, take two readings and average, and treat a single figure as approximate.

What should I do with the result?

Mention it at a routine appointment and let it sit alongside the things a tape cannot see — blood pressure, glucose, lipids, family history, how you sleep and how you move. Those are what the measurement stands in for, imperfectly. A reading above a boundary is a reason to look further rather than a conclusion, and one below is not evidence that there is nothing to find. If the reading concerns you about a child, that conversation belongs with a paediatrician and not with a change to what the child eats.

Waist-to-Height Ratio Calculator

Keep your waist to less than half your height. That is the entire rule, and its unusual quality is how far it travels. NICE puts central adiposity into three bands — 0.40 to 0.49 healthy, 0.50 to 0.59 increased, 0.60 or more high — and states plainly that these apply to both sexes and all ethniciti...

A screening figure, not a diagnosis. It says nothing about blood pressure, glucose, lipids, fitness or family history, which are what it stands in for and does imperfectly. A reading above a band boundary is a reason to mention it at a routine appointment, not a verdict — and one below is not evidence that all is well. Nothing here is a reason to change what anyone eats, and that applies with particular force to a child.

TWO MEASUREMENTS

WAIST (CM)

HEIGHT (CM)

AGE (YEARS)

Both in the same units — the ratio has none, so it does not matter which. Age changes none of the thresholds; it only tells you how well the fixed ratio behaves for your age group. Measure the waist midway between your lowest rib and the top of your hip bone, at the end of a normal breath out.

WAIST TO HEIGHT RATIO

0.50

Increased central adiposity — increased health risk, on the NICE bands. Half your height is 86.0cm, and your waist is 0.0cm above that.

RATIO

0.50

HALF YOUR HEIGHT

86.0cm

YOUR WAIST

86.0cm

One set of numbers for everyone, which is unusual. NICE states these bands apply to both sexes and all ethnicities, including adults with high muscle mass — and separately to children and young people from age five, on the same figures. Every one of those is a place where BMI or waist circumference needs a different threshold: waist cut-offs are lower for South and East Asian populations, and a child's BMI has to be read as an age-and-sex percentile before it means anything at all.

A fixed ratio is not quite fair across heights, and this is the known weakness. The rule only treats everyone alike if waist rises in proportion to height. Among adults it rises more slowly than that, so shorter people meet the 0.50 line at a slighter build than taller people do — the rule flags the short sooner and the tall later. It is a real limitation of an otherwise unusually portable measure.

NICE scopes these adult bands to a BMI under 35. Above that, central adiposity is not really the open question and the classification stops adding information. If that applies, the measurement is still worth taking to an appointment; it just is not the figure that decides anything.

Ratio

0.500

Band

Increased central adiposity

Waist

86.0 cm

Height

172.0 cm

Half your height

86.0 cm

Above that by

0.0 cm

What it cannot see. A tape measure does not distinguish visceral fat from muscle or from a large frame, and it knows nothing about the blood tests that actually describe cardiometabolic risk. A meal, bloating or the time of day move the waist by a centimetre or two, which is 0.005 to 0.01 on the ratio — small, but not nothing near a boundary. And the bands are boundaries drawn across a continuum: 0.49 and 0.51 describe almost identical bodies.

THE BANDS

The whole rule is one sentence: keep your waist under half your height. That is why it travels so well — no chart, no lookup by sex or age, and a piece of string cut to half your height performs the entire measurement without a number being written down.

NICE NG246 bands · live calculation

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HOW TO USE

1
Measure your waist midway between your lowest rib and the top of your hip bone, at the end of a normal breath out, with the tape level and snug but not compressing.
2
Enter waist and height in the same units. The ratio has no units of its own, so centimetres or inches give exactly the same answer — there is nothing to convert.
3
Add an age. It changes none of the thresholds — they are the same from five upwards — but it tells you how well the fixed ratio behaves for that age group, which is not the same at every age.
4
Read the band rather than the third decimal. NICE draws lines at 0.40, 0.50 and 0.60 across what is really a continuum, and 0.49 against 0.51 describes almost identical bodies.

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Last updated: August 1, 2026 · Bands from NICE NG246 · Applies to both sexes, all ethnicities, and from age five · A screening estimate, not medical advice.