BMI Calculator & Weight Loss Injection Eligibility

What BMI Do You Need for Weight Loss Medication?

In the UK, weight loss medications such as Mounjaro (tirzepatide) and Wegovy (semaglutide) are usually prescribed to adults who meet certain Body Mass Index (BMI) thresholds.

In most cases, eligibility criteria include:

BMI of 30 or above
This falls within the clinical category of obesity. People in this range may be considered for prescription weight loss medication if lifestyle changes alone have not been effective.

BMI of 27 or above with a weight-related health condition
Weight loss injections may also be considered for people with a BMI of 27 or higher if they also have a health condition linked to excess weight, such as:

• Type 2 diabetes
• High blood pressure
• High cholesterol
• Obstructive sleep apnoea
• Cardiovascular disease

These criteria are based on clinical trial evidence and prescribing guidance used by healthcare professionals when assessing treatment suitability.

However, BMI alone does not determine whether weight loss medication is appropriate. A clinician must review your medical history, current medications, and overall health before treatment can be prescribed.

With obesity rates continuing to rise across the UK, more people are exploring clinically proven weight loss treatments to support long-term weight management. One of the first steps in understanding whether these treatments may be suitable is checking your Body Mass Index (BMI).

Use our BMI calculator above to calculate your BMI and see how it relates to typical eligibility thresholds for prescription weight loss injections such as Mounjaro and Wegovy.

Please note that BMI is only one factor used during a clinical assessment. Suitability for treatment is always confirmed through a clinician-led consultation that considers your overall health, medical history, and current medications.

What is BMI and how is it calculated?

BMI (Body Mass Index) is a widely used measurement that compares your weight to your height to indicate whether you are within a healthy weight range.

It is calculated using the following formula:

BMI = weight (kg) ÷ height (m²)

Healthcare professionals use BMI as a screening tool to assess weight-related health risk and to help guide treatment decisions.

BMI categories

• Underweight: below 18.5

• Healthy weight: 18.5–24.9

• Overweight: 25–29.9

• Obesity: 30 and above

A BMI of 27 sits within the overweight category. This is the point at which weight-related health risks begin to increase, which is why it is often used as an eligibility threshold for medically supervised weight loss treatments.

How to use our BMI calculator

Using the BMI calculator is quick and simple:

1. Enter your height in centimetres

2. Enter your weight in kilograms

3. Click “Calculate BMI”

The calculator will:

• Show your BMI number

• Indicate which BMI category you fall into

• Explain what your result may mean in relation to weight loss treatment eligibility

What does this BMI calculator tell me?

Our calculator provides an estimate of your BMI and explains how it fits within clinically recognised BMI categories.

Depending on your result, it will also indicate whether you may meet the BMI criteria commonly used when assessing suitability for prescription weight loss injections.

Important:
Meeting BMI criteria alone does not guarantee treatment. A full online consultation is required so a clinician can assess your overall health, medical history, and suitability.

Why is a BMI of 27 often used as the cut-off?

The BMI threshold of 27 is based on a combination of clinical evidence, trial data, and UK prescribing guidance.

1. Health risks increase at BMI 27+

Research shows that from a BMI of around 27, the risk of conditions such as the following begins to rise:

• Type 2 diabetes

• High blood pressure

• Sleep apnoea

• Joint and mobility problems

• Cardiovascular disease

Early intervention at this stage can help prevent progression to obesity and more serious long-term health complications.

2. Clinical trials use BMI ≥27

Weight loss injections such as Mounjaro (tirzepatide) and Wegovy (semaglutide) were studied in large clinical trials involving people with a BMI of 27 or above, particularly where additional weight-related conditions were present.

These trials demonstrated meaningful, sustained weight loss in appropriately selected patients.

3. UK prescribing guidance

In the UK, prescribing guidance typically allows weight loss medication for:

• Adults with a BMI of 30 or above, or

• Adults with a BMI of 27 or above who also have at least one weight-related health condition

This ensures treatments are prescribed safely, ethically, and in line with clinical best practice.

Eligibility criteria can differ depending on whether treatment is accessed privately or via the NHS. You can learn more about NHS BMI eligibility for weight loss treatments and how it compares to private options in our dedicated guide.

Will this calculator tell me if I can get weight loss medication?

The calculator can indicate whether your BMI meets the usual eligibility thresholds for prescription weight loss medication.

However, it cannot determine whether treatment is right for you on its own.

To request treatment, you must still complete a secure online consultation. One of our clinicians will review your answers and decide whether medication is safe and appropriate for you.

What if my BMI is under 27?

If your BMI is below 27, prescription weight loss injections are not usually recommended.

This is because:

• The risks may outweigh the benefits

• There is limited evidence of benefit in lower BMI ranges

• UK regulations restrict off-label prescribing

If you are concerned about your weight or overall health, a clinician can still offer guidance on lifestyle, nutrition, and other appropriate options.

Are BMI calculators accurate?

BMI calculators are accurate when:

• Your height and weight are measured correctly

• You enter precise figures

However, BMI does have limitations. It does not:

• Differentiate between muscle and fat

• Reflect fat distribution

• Account for differences across some ethnic groups

For this reason, BMI is used as a screening tool, not a diagnosis. Clinicians will always consider BMI alongside your wider health profile.

Overview of weight loss treatments (learn more)

If you meet eligibility criteria, a clinician may discuss prescription options such as:

• Mounjaro (tirzepatide) – a dual GIP and GLP-1 receptor agonist

• Wegovy (semaglutide) – a GLP-1 receptor agonist

• Orlistat – an oral medication that reduces fat absorption

You can learn more about each treatment on our dedicated guides and product pages. If you would like to estimate how weight loss may progress over time, our weight loss calculator can help you calculate a personalised calorie deficit and explore how long it may take to reach your target weight.

Safety, regulation, and choosing a trusted pharmacy

Due to rising demand, unregulated and counterfeit weight loss injections are increasingly common online. These products can pose serious health risks.

When using Happy Pharmacy, you are assured of:

• MHRA-approved medicines

• Prescriptions issued by UK-registered clinicians

• Secure and discreet delivery

• Ongoing clinical support

We are a fully regulated UK pharmacy and follow strict prescribing standards.

Next steps

1. Use the BMI calculator above

2. Use our calorie deficit calculator to estimate how long it may take to reach your target weight

3. Complete our secure online consultation

4. Receive a clinician review of your eligibility

5. If appropriate, begin treatment with professional support

If you decide to continue, you can learn more about how our online consultation and prescribing process works before requesting treatment.