The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
Understanding BMI Classification and Obesity
BMI classification divides weight into categories based on height, with obesity being the highest health risk tier. In the UK, the NHS uses a standard BMI formula and categorisation system that helps identify whether your weight poses health risks. Understanding where your BMI falls is the first step toward making informed health decisions. You can check your personal BMI instantly using our free BMI calculator above, which applies NHS guidelines for UK adults.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
NHS BMI Categories and Obesity Grades
The BMI formula is: weight (kg) divided by height (m)². The NHS BMI range classifications are:
- Underweight: Below 18.5 — associated with nutritional deficiencies and bone health risks
- Healthy weight: 18.5–24.9 — optimal health outcomes for most adults
- Overweight: 25.0–29.9 — increased risk of type 2 diabetes and cardiovascular disease
- Obese Class I: 30.0–34.9 — significantly increased health risks
- Obese Class II: 35.0–39.9 — high health risks including joint problems and sleep apnoea
- Severely Obese (Class III): 40.0 and above — substantial risk of serious health complications
For adults of South Asian, Chinese, Black African, or Black Caribbean backgrounds, the NHS recommends lower BMI thresholds: healthy weight is 18.5–22.9, and obesity begins at 27.5 — reflecting higher cardiovascular risk in these populations at lower weights.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
BMI Limitations and Complementary Health Measures
While BMI classification is a useful screening tool, it has important limitations. BMI does not distinguish between muscle and fat — athletes may have a high BMI but low body fat percentage. It also ignores where fat is stored; waist circumference is equally important for assessing health risk. The NHS recommends measuring your waist too: men over 94cm (37 inches) face increased risk, and over 102cm (40 inches) face high risk. Women over 80cm (31.5 inches) have increased risk, and over 88cm (34.5 inches) have high risk.
Your GP can assess your individual health picture alongside your BMI and provide personalised guidance on weight management, lifestyle changes, or treatment options.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
What BMI is classified as obesity in the UK?
In the UK, obesity begins at a BMI of 30.0 according to NHS guidelines. This is further divided into Class I (30.0–34.9), Class II (35.0–39.9), and Class III or severe obesity (40.0+). For people of South Asian or Black African heritage, obesity begins at a BMI of 27.5 due to higher health risks at lower weights.
Why are BMI thresholds lower for some ethnic groups?
Research shows that South Asian, Chinese, Black African, and Black Caribbean adults develop obesity-related health conditions like type 2 diabetes and heart disease at lower BMI levels than white European populations. The NHS therefore recommends lower thresholds for these groups to identify risk earlier and enable earlier intervention.
Can BMI classification be inaccurate?
Yes. BMI does not account for muscle mass, bone density, age, or fat distribution. Athletes or very muscular people may have a high BMI but low body fat. Additionally, BMI is less accurate in older adults and during pregnancy. Waist circumference and overall health context are important alongside BMI.
How do I find my healthy BMI range?
Use our free BMI calculator above to find your personal BMI instantly. For a healthy weight, your BMI should fall between 18.5 and 24.9 (or 18.5–22.9 if you are of South Asian, Chinese, Black African, or Black Caribbean heritage). Your GP can advise if this range is appropriate for your individual health.
What should I do if my BMI falls in the obese category?
Start by booking an appointment with your GP, who can assess your overall health and discuss weight management options. These may include lifestyle changes, behaviour change programmes, or in some cases prescription medications. Weight loss of even 5–10% can bring significant health benefits. Support is available through NHS weight management services.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.