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
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Weight and BMI Explained
BMI (Body Mass Index) is a measure that uses your weight and height to assess whether you're at a healthy weight for your size. It's one of the most common tools the NHS uses to screen for weight-related health risks in UK adults. Your BMI falls into different categories—from underweight through to severely obese—and understanding where you sit helps you and your GP identify whether lifestyle changes or medical support might be beneficial. This guide explains how BMI works, what the ranges mean, and what your personal BMI tells you about your health.
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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.
BMI Formula and NHS Categories
The BMI formula is straightforward: divide your weight in kilograms by your height in metres squared. For example, if you weigh 70kg and are 1.73m tall, your BMI is approximately 23.4, which falls in the healthy weight range.
According to NHS BMI guidelines, the standard categories for UK adults are:
- Below 18.5: Underweight
- 18.5–24.9: Healthy weight
- 25.0–29.9: Overweight
- 30.0–34.9: Obese (Class I)
- 35.0–39.9: Obese (Class II)
- 40.0+: Severely obese (Class III)
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds—healthy weight is 18.5–22.9, and overweight begins at 23.0. This is because these groups face higher cardiovascular and diabetes risk at lower BMI levels. Use our free BMI calculator above to check your personal BMI instantly.
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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 What Else Matters
While BMI is useful for screening, 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 doesn't account for where fat is stored; waist circumference is equally important for assessing health risk.
The NHS recommends measuring waist circumference alongside BMI: men should aim for under 94cm (37 inches), women under 80cm (31.5 inches). For a complete picture of your health, your GP may also consider your blood pressure, cholesterol, blood sugar, and family history. These factors together give a far better indication of your actual health risk than BMI alone. Our weight converter tool helps you check your weight in different units—kg, stone, and pounds—so you can compare easily.
- 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 is a healthy BMI range in the UK?
According to NHS guidelines, a healthy BMI for most UK adults is between 18.5 and 24.9. For people of South Asian, Chinese, Black African, or Black Caribbean backgrounds, the healthy range is 18.5 to 22.9 due to higher health risks at lower BMI levels in these groups. Your GP can advise on your specific healthy weight range.
How do I calculate my BMI?
The BMI formula is: weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.73m tall, your BMI = 70 ÷ (1.73 × 1.73) = 23.4. Use our free BMI calculator on this page for gradual improvement in seconds.
Why does the NHS use BMI to assess weight?
BMI is a quick, non-invasive screening tool that helps identify whether weight may increase health risks. The NHS uses it alongside other measures like waist circumference and blood tests to build a complete picture of your health. However, BMI alone doesn't account for muscle mass or fat distribution.
Is BMI accurate for athletes or very muscular people?
No. BMI does not distinguish between muscle and fat, so athletes and very muscular individuals may have a high BMI despite low body fat. In these cases, body composition measures and waist circumference are more accurate indicators of health risk.
What is the BMI overweight threshold in the UK?
According to NHS guidelines, overweight begins at a BMI of 25.0. A BMI between 25.0 and 29.9 is classified as overweight. From 30.0 onwards, the classifications move into obese categories. Your GP can discuss what these ranges mean for your personal health.
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.