BMI Explained: How It Decides Treatment Eligibility

Body mass index (BMI) is the main measure used to decide who is eligible for weight loss injections in the UK. For Mounjaro (tirzepatide) and Wegovy (semaglutide), the licences cover adults with a BMI of 30 or more, or 27 to 30 with at least one weight-related health condition. NHS criteria are stricter, usually a BMI of 35 or more with a condition. For people from some ethnic backgrounds, NICE recommends thresholds about 2.5 points lower. BMI is a useful screening tool, but it has real limitations, which is why prescribers look at your wider health too. This guide explains how BMI works and how it shapes treatment decisions.

What BMI is

BMI is a simple calculation that relates your weight to your height. It was developed in the 19th century by the Belgian statistician Adolphe Quetelet as a way to describe populations, and was later adopted as a practical way to classify weight in clinical practice.

How to calculate it

The formula is:

BMI = weight (kg) ÷ height (m)²

For example, for a weight of 90kg and a height of 1.75m:

• 1.75 × 1.75 = 3.06
• 90 ÷ 3.06 = 29.4

So the BMI is about 29.4, which falls in the overweight range for most adults. Our BMI calculator does this for you, in metric or imperial units.

Standard BMI categories for adults

These categories apply to adults. For children and young people, BMI is interpreted using age- and sex-specific centile charts.

Lower thresholds for some ethnic groups

People from some ethnic backgrounds tend to develop weight-related conditions, particularly type 2 diabetes, at a lower BMI. NICE guideline NG246 therefore recommends lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background:

Our guide to BMI thresholds and ethnicity explores the evidence behind this.

How BMI decides eligibility for weight loss injections

Licensed criteria

The UK licences for weight loss injections, set out in each medicine's Summary of Product Characteristics (SmPC), use BMI as the main entry criterion:

At the time of writing, the newer GLP-1 tablets authorised by the MHRA, the Wegovy tablet (oral semaglutide) and Foundayo (orforglipron), were reported at approval to use similar BMI criteria. Check the current SmPC for each.

Weight-related conditions include prediabetes or type 2 diabetes, high blood pressure, abnormal blood fats, obstructive sleep apnoea and cardiovascular disease. See who can get weight loss injections for more.

NHS criteria

NHS criteria are based on NICE technology appraisals and are stricter than the licences:

• Wegovy (NICE TA875): within specialist weight management services, for adults with at least one weight-related condition and a BMI of at least 35, or 30 to 34.9 if they meet criteria for referral to specialist services. Lower thresholds apply to some ethnic groups.
• Mounjaro (NICE TA1026): adults with a BMI of at least 35 and at least one weight-related condition, with lower thresholds for some ethnic groups.

NHS England's phased roll-out of Mounjaro started in June 2025 with people who have a BMI of 40 or more (adjusted for ethnicity) and at least four of five named conditions. Later cohorts include people with a BMI of 35 to 39.9. See Mounjaro on the NHS and Wegovy on the NHS.

Why the thresholds differ

The licence reflects who was studied in the clinical trials and for whom the MHRA judged the benefits to outweigh the risks. Trials such as SURMOUNT-1 (tirzepatide) and STEP 1 (semaglutide) enrolled adults with a BMI of 30 or more, or 27 or more with at least one weight-related condition.

NHS criteria reflect cost-effectiveness: NICE considers which groups gain enough health benefit to justify the cost to the NHS, and NHS England then decides how to phase access given capacity and budget. People with higher BMIs and more conditions are generally expected to benefit most in health terms, which is why they are prioritised.

The limitations of BMI

BMI is widely used because it is simple, cheap and consistent. At a population level, higher BMI is associated with higher risk of type 2 diabetes, heart disease, some cancers and other conditions. But it has important limitations for individuals.

It does not measure fat directly

BMI cannot distinguish between fat, muscle and bone. A very muscular person can have a high BMI with relatively little body fat. Conversely, someone with low muscle mass may have a "healthy" BMI but a high proportion of body fat.

It does not show where fat is stored

Fat stored around the abdomen and internal organs (visceral fat) is more strongly linked to health risks than fat stored elsewhere. Two people with the same BMI may have very different risk depending on fat distribution.

It varies in meaning between groups

The same BMI can carry different risks depending on ethnicity, age and sex. That is why lower thresholds are recommended for some ethnic groups, and why BMI is interpreted with care in older adults, whose muscle mass often declines with age.

It is a snapshot

BMI says nothing about fitness, diet, blood pressure, blood sugar, mental health or how weight affects someone's daily life.

BMI in particular groups

Because BMI is a blunt tool, prescribers interpret it more carefully in some situations.

Older adults

With age, muscle mass tends to fall and fat tends to increase, even if weight stays the same. An older adult can therefore carry more body fat than their BMI suggests. On the other hand, intentional weight loss in older age carries particular risks, including loss of muscle and bone strength, frailty and falls. Prescribers will weigh these carefully, and protein intake and strength training become especially important. See GLP-1s over 65 and preserving muscle on GLP-1s.

Very muscular people

People who do a lot of strength training or physically demanding work may have a high BMI because of muscle rather than fat. In these cases, waist-to-height ratio and clinical judgement are particularly useful. Weight loss medicines are intended to reduce excess fat, not to cut weight for sport or appearance.

Pregnancy and after birth

BMI is not a useful measure during pregnancy, and weight loss medicines must not be used while pregnant or breastfeeding. After birth, weight changes over months, and any discussion of weight loss treatment should take account of breastfeeding and contraception plans. See GLP-1s, pregnancy and fertility.

Adolescents

For under-18s, BMI is interpreted using centile charts that account for age and sex, rather than adult thresholds. Wegovy and Saxenda have licences for some adolescents aged 12 and over, but treatment is usually managed by specialist services. Most private providers treat adults only.

People with limb differences or who cannot stand on scales

Standard BMI calculations may not be accurate for people with amputations or some disabilities, and standing scales may not be practical. Clinicians can use adjusted calculations or alternative measures. If this applies to you, discuss it with your GP or prescriber, who should make reasonable adjustments to the verification process.

Measuring yourself accurately at home

If you are checking your BMI before speaking to a professional, a few steps make the result more reliable:

• Weigh yourself in the morning, after using the toilet and before eating or drinking, wearing light clothing.
• Use the same scales on a hard, flat floor, not carpet. Bathroom scales vary in accuracy, so compare with a pharmacy or GP surgery scale if you can.
• Measure height without shoes, standing with your back against a wall and looking straight ahead. Many adults are slightly shorter than they remember, particularly with age.
• Measure your waist at the midpoint between your lowest rib and the top of your hip bone, after breathing out normally.
• Record the date and figures, so you can track change and share accurate information with a prescriber.

Remember that a prescriber will still need to verify your measurements, and that a single reading is less useful than a trend over time.

Other measures you may hear about

Several other measures are sometimes used alongside or instead of BMI:

• Waist circumference: a simple measure of central fat, used in some guidance and in the Lancet Commission's proposals.
• Body composition scales: home and gym devices estimate body fat percentage using a weak electrical current (bioimpedance). They are convenient but can be affected by hydration and are less accurate than clinical methods.
• DEXA scans: a type of low-dose X-ray that measures fat, muscle and bone precisely. It is mainly used in research and specialist settings.
• Staging systems: tools such as the Edmonton Obesity Staging System rate how much weight is affecting physical, mental and functional health, rather than size alone.

None of these currently replaces BMI in UK licensing or NICE criteria for weight loss injections, but they can add useful context about your health.

Waist-to-height ratio: the measure used alongside BMI

NICE guideline NG246 recommends measuring waist-to-height ratio in adults with a BMI under 35, as a practical way to assess central fat.

How to measure it

1. Find the point halfway between the bottom of your ribs and the top of your hip bones.
2. Measure your waist there, breathing out normally, with the tape snug but not tight.
3. Divide your waist measurement by your height, using the same units.

For example, a waist of 90cm and a height of 170cm gives a ratio of 90 ÷ 170 = 0.53.

Interpreting the result

A simple rule of thumb from NICE is to keep your waist to less than half your height. Waist-to-height ratio does not currently replace BMI in the licensed criteria for weight loss injections, but it can help you and your prescriber understand your overall risk.

How BMI is verified for treatment

Because BMI determines eligibility, it must be accurate. Self-reported weight is often underestimated and height overestimated, and inaccurate information can lead to unsafe prescribing.

On the NHS

Your GP or specialist team will usually measure your weight and height directly, or use recent recorded measurements.

Privately

General Pharmaceutical Council (GPhC) guidance, strengthened in 2025, expects online pharmacies to verify key information for weight loss medicines independently, rather than relying only on a questionnaire. Providers may:

• conduct a video consultation
• ask for a photo or video of you on scales, sometimes with ID visible
• request confirmation from your GP or another healthcare professional
• check your NHS record with your consent

See our guide to ID and weight verification and online weight loss consultations.

BMI during and after treatment

Starting BMI versus current BMI

Eligibility to start treatment is based on your BMI before treatment. As you lose weight, your BMI will fall, perhaps below the original threshold. Prescribers generally decide whether to continue based on your response, health benefits and goals, rather than requiring your BMI to remain above the starting threshold. Keep a record of your starting weight and any verification, particularly if you might switch provider. See switching providers mid-treatment.

Response thresholds

NICE uses weight change, not BMI, to judge response. Both TA875 and TA1026 advise reviewing treatment if less than 5% of initial weight has been lost after six months (for tirzepatide, on the highest tolerated dose). See GLP-1 non-responders.

Avoiding becoming underweight

Prescribers should monitor to make sure weight loss does not go too far. If your BMI approaches the healthy range, your prescriber may discuss reducing the dose, maintenance strategies or stopping. Trial results for medicines such as tirzepatide (average weight loss of about 21% at the highest dose in SURMOUNT-1) show how substantial weight loss can be.

Beyond the number

As treatment progresses, measures such as waist size, blood pressure, blood sugar, fitness, strength and how you feel can be as meaningful as BMI. See measuring progress beyond the scales.

Frequently misunderstood points about BMI and eligibility

"If my BMI qualifies, I will get a prescription." Not necessarily. BMI is the first step. A prescriber must also check that treatment is safe and appropriate for you.

"BMI 27 is enough on its own." Only with at least one weight-related condition, unless a lower threshold applies because of your ethnic background.

"NHS and private criteria are the same." NHS criteria are much stricter. See private vs NHS weight loss treatment.

"A healthy BMI means I'm healthy." BMI does not capture everything; central fat, fitness and blood markers matter too.

Next steps

If you think your BMI may make you eligible:

1. Use our BMI calculator and measure your waist-to-height ratio.
2. Try our eligibility checker for a general indication (it is not a prescribing decision).
3. Speak to your GP about NHS options and your overall health.
4. If considering private treatment, compare regulated services on our providers page and see what affects cost on our prices page.

For an overview of the medicines themselves, see weight loss injections and every UK weight loss injection compared.

When to speak to a professional

Speak to your GP, a pharmacist or a prescriber if your BMI or waist-to-height ratio suggests increased health risk, or if you are considering weight loss treatment. They can check your blood pressure, blood sugar and cholesterol and discuss options. If you are worried about your weight, eating or body image, including concerns about being underweight, speak to your GP; the eating disorder charity Beat also offers support.

If you are taking a weight loss medicine, seek urgent medical help (NHS 111, or 999 in an emergency) if you experience:

• severe, persistent abdominal pain, possibly spreading to your back, with or without vomiting (possible pancreatitis)
• pain in the upper right abdomen, fever, or yellowing of the skin or eyes (possible gallbladder problems)
• persistent vomiting or diarrhoea with signs of dehydration
• swelling of the face, lips or throat, or difficulty breathing
• feeling faint, very weak or unable to eat or drink