Waist size and waist-to-height ratio

Waist-to-height ratio is your waist measurement divided by your height. UK guidance from NICE says to keep your waist to less than half your height, a ratio under 0.5, and links 0.5 to 0.59 with increased health risks.^[1] It is a quick screen for fat around the middle, and the NHS says not to use it to diagnose any condition.^[2]

How to measure your waist and work out the ratio

Waist-to-height ratio

Your waist measurement divided by your height, both in the same units. NICE uses it as a practical estimate of central adiposity, meaning fat carried around the middle.^[1]

NICE gives a short method for measuring yourself at home.^[1] All you need is a tape measure and your height.

1. 1Find the spot. Find the bottom of your ribs and the top of your hips. Wrap the tape midway between them, which NICE says will be just above the belly button.^[1] Diabetes UK notes that the belly button is not the right guide for everyone, so find the midpoint.^[6]
2. 2Keep the tape flat and snug. The World Health Organization (WHO) survey method keeps the tape level with the floor, snug but not pressing into the skin, over bare skin or light clothing.^[4]
3. 3Breathe out, then read it. Breathe out naturally before you read the tape.^[1]
4. 4Measure twice. The WHO expert method repeats the measurement and averages the two if they are within 1 cm of each other.^[5]
5. 5Divide. Use the same units for both numbers, then divide your waist by your height.^[1]

US agencies use a different spot. The NIH protocol measures at the top of the hip bone.^[5] The US heart and lung institute tells people to measure just above the hipbones, just after breathing out.^[9] One expert group accepts either spot, as long as you use the same one every time.^[15]

What the 0.5 boundary means

NICE sorts adults into three bands.^[1] The health risks it names are type 2 diabetes, high blood pressure and heart disease.^[1]

The bands apply to adults with a BMI under 35, of both sexes and all ethnicities, including adults with high muscle mass.^[1] For those muscular adults, NICE says BMI may be a less accurate measure of central fat.^[1] The studies NICE reviewed put the cut-off generally around 0.5 for all ethnicities and sexes.^[3]

NICE named the single cut-off for everyone as a key benefit, and noted that 0.5 can be shared as a simple message: "keep your waist to less than half your height".^[3] It called the quality of the evidence mixed.^[3]

Why the middle matters: visceral fat is the fat inside the belly, and it carries the strongest health risk of any fat store.^[15] Measured by CT or MRI scans, it is a risk marker for heart and metabolic disease on its own.^[7]

The American Heart Association calls belly fat measured by waist size a heart disease risk marker that is independent of BMI.^[8] A tape measure does not see visceral fat directly, which is why NICE calls the ratio a practical estimate.^[1]

A worked example

The same person shows how the older waist lines differ. Her 35-inch waist sits right at the US line, which flags risk above 35 inches for women.^[9] On the Diabetes UK target of under 80 cm for women, her 88.9 cm waist is already over.^[6] You can check BMI on our BMI calculator, and read what BMI measures and who it is wrong for.

Waist size alone: the inch and centimeter lines

Fixed waist sizes are another common check. The lines differ by sex, by country and by ethnic group.^{[9, 6, 10]}

The 94 cm and 102 cm lines for men, and 80 cm and 88 cm for women, are often called WHO cut-offs, but a WHO expert report says that in an earlier WHO report they were an example only, and not WHO recommendations.^[5]

Is it better than BMI?

For flagging risk factors such as diabetes and high blood pressure, usually yes, by a small margin. A 2010 review of 78 studies found average accuracy scores of 0.704 for the ratio, 0.693 for waist size and 0.671 for BMI, where higher is better.^[11] The authors said the ratio and waist size were similar predictors, both stronger than BMI.^[11]

A 2012 analysis of 31 papers covering more than 300,000 adults found the ratio improved on BMI by 4 to 5%.^[12] It also found the ratio better than waist size alone.^[12] Both reviews share an author, Margaret Ashwell, and both back the "half your height" message.^{[11, 12]}

Other researchers are less sure the height step adds much. In 15 studies of new diabetes cases, the ratio did modestly better than BMI, but adding height to waist size gave no extra benefit.^[13]

A WHO expert consultation in 2008 found too little data to favor the ratio over other measures.^[5] And in 58 cohorts of 221,934 people in developed countries, BMI and waist measures did not importantly improve heart disease and stroke prediction once blood pressure, diabetes and cholesterol were known.^[14] That study did not test the ratio itself.

Where the ratio falls short

• The tape spot changes the number. Experts have not agreed on one place to measure.^[15] The top-of-hip and midpoint methods generally differ little for men.^[15] For women, one study found a 1.8 cm gap, and another, in Japanese adults, about 9 cm, as summarized by one expert group.^[15] Pick one spot and keep it.^[15]
• Waist is harder to repeat than BMI. In the 58-cohort analysis, repeat waist readings agreed with each other less closely than repeat BMI readings.^[14]
• It only moves with your waist. In grown adults height is fixed, so the ratio changes only when your waist does. One expert group suggests waist size itself for tracking lifestyle changes.^[15]
• It adds little at a BMI over 35. NICE says people in that range are always likely to have a high ratio, so it may not be useful for them.^[3]
• Children are different. NICE uses the same bands from age 5, but says the evidence in children is not as good as in adults.^{[1, 3]}
• Some bodies need another method. With some disabilities or conditions such as scoliosis, sitting height may replace standing height, and a specialist assessment may be needed.^[3]

A waist of more than half your height is one sign among several. Body fat estimates are another, covered in body fat percentage: scans, scales, calipers and the Navy method. For bringing the number down, start with how to lose weight and keep it off.

When to talk to a clinician