Patient guide
BMI limits, waist measurement and ethnicity-adjusted thresholds
Body mass index is easy to calculate but a blunt tool. UK obesity guidance uses it alongside waist measurement and health conditions, and recommends lower thresholds for people from some ethnic backgrounds. This guide explains where BMI helps, where it misleads and what else clinicians consider.
Last checked: 2026-10-03
Key facts
At a glance.
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What BMI is
Weight in kilograms divided by height in metres squared. It does not distinguish fat from muscle or show where fat is stored.
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Ethnicity matters
NICE guidance advises using lower BMI thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds.
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Waist adds information
Fat carried around the abdomen is linked to higher cardiometabolic risk than fat on the hips and thighs.
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Funding tool
In practice BMI is also used as an NHS access threshold, which is a different purpose from diagnosing health.
What BMI does well
BMI is cheap, quick and gives an approximate picture for populations. In large groups, higher BMI is linked to higher rates of type 2 diabetes, high blood pressure, heart disease, sleep apnoea and some cancers. For individuals it works as a starting point for a conversation and a threshold for pathways.
Where BMI misleads
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It does not distinguish muscle from fat, so very muscular people can have a high BMI without excess fat.
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It does not show fat distribution, which affects risk.
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It can underestimate risk in older people with low muscle mass.
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It is less accurate for people at the extremes of height.
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It does not capture health, fitness or wellbeing.
Ethnicity-adjusted thresholds
Research shows that people of South Asian, Chinese and several other family backgrounds tend to develop type 2 diabetes and heart disease at lower BMI than white European populations. Because of this, NICE advises clinicians to use lower BMI thresholds when assessing risk and deciding on interventions in these groups. If you belong to one of these groups, ask whether this adjustment has been applied.
| Group | What guidance advises |
|---|---|
| White European and other backgrounds not listed | Standard BMI thresholds |
| South Asian, Chinese, other Asian, Middle Eastern, Black African, African-Caribbean | Lower thresholds for assessing risk and offering interventions |
Waist measurement
Measuring waist circumference, or comparing waist to height, gives additional information, because abdominal fat is more closely linked to metabolic risk. Clinicians may use it particularly when BMI is borderline. The measurement is taken with a tape around the middle of the abdomen, between the lowest rib and the top of the hip bone, with the person standing and breathing normally.
Body composition tools
Body composition scans, such as DEXA, and bioelectrical impedance devices estimate fat and lean mass. DEXA is more accurate, whereas many consumer scales are not reliable. These tools are not routinely used for NHS eligibility but can add information about muscle and fat, for example in older adults or athletes.
Using the numbers sensibly
Treat BMI as a prompt, not a verdict. Combine it with blood pressure, blood sugar, cholesterol, sleep, mobility, mood and family history. A person with a BMI above a threshold may be metabolically healthy, and someone below it may have significant risk. Discuss the overall picture with a clinician rather than relying on a single measure.
Frequently asked
Questions people ask.
Quick, plain-English answers to the questions we hear most.
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Is BMI wrong for athletes?
It can be, since muscle weighs more than fat. Clinicians consider body composition and waist measurement alongside it.
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Why is BMI used at all?
It is simple, standardised and correlated with risk in populations, which makes it a practical screening and access measure.
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Does the lower threshold apply to me?
It applies if you identify with one of the groups named in guidance. Ask your clinician to confirm how your service applies it.
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How do I measure my waist?
Use a tape measure around the middle of your abdomen, between the lowest rib and the top of the hip bone, after breathing out normally.
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Is waist-to-height ratio better than BMI?
It may flag central fat risk better in some people, and NICE obesity guidance describes using it as an additional measure.
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Do children use BMI?
Children use age and sex-specific centile charts, not adult cut-offs.
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Can BMI change what treatment I am offered?
Yes. BMI and conditions set eligibility for many NHS pathways, which is why the threshold applied matters.
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Are home scales reliable?
Weight scales are usually fine. Body fat readings from consumer scales are much less reliable.
Where this comes from
Sources and checking.
This guide is for information, not medical advice.
Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.
Published 2026-10-03, next review 2027-10-03.
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NICE guideline NG246, Overweight and obesity management (nice.org.uk)
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NHS website, Obesity and weight-loss medicines pages (nhs.uk)
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NICE technology appraisal TA1026, Tirzepatide for managing overweight and obesity (nice.org.uk)
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NICE technology appraisal TA875, Semaglutide for managing overweight and obesity (nice.org.uk)