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
What Is Regular Body Mass Index?
Regular body mass index (BMI) is a standard measurement that compares your weight to your height to assess whether you're in a healthy weight range. In the UK, the NHS uses BMI as a screening tool to identify potential health risks linked to excess weight. It's calculated by dividing your weight in kilograms by your height in metres squared. While BMI has limitations — it doesn't account for muscle mass or body composition — it remains a useful first step in understanding your health. Use our free BMI calculator above to check your personal BMI instantly.
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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.
How BMI Is Calculated and Categorised
The standard BMI formula is straightforward: weight (kg) divided by height (metres) squared. For example, someone weighing 70kg who is 1.73m tall would have a BMI of 23.4.
The NHS classifies regular BMI into five categories:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese (Class I): BMI 30.0–34.9
- Obese (Class II): BMI 35.0–39.9
- Severely obese (Class III): BMI 40.0 and above
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 — because these populations face higher health risks at lower BMI levels. Use our weight converter to switch between kg, stone, and pounds if you prefer imperial measurements.
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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 Body Composition Matters
Regular BMI is a useful screening tool but has important limitations. It doesn't distinguish between muscle and fat — athletes or very muscular individuals may have high BMI but low body fat percentage, while others may have a healthy BMI yet carry excess visceral fat around organs. BMI also doesn't account for age, bone density, or how fat is distributed.
This is why complementary measures matter: waist circumference (men over 94cm = increased risk), waist-to-height ratio (healthy is below 0.5), and lean body mass assessment provide a fuller picture. Your GP can assess your overall health using multiple factors beyond BMI alone, giving you personalised guidance tailored to your individual circumstances.
- 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's the difference between regular BMI and other BMI measures?
Regular BMI uses the standard formula for adults aged 18+. Children use age and sex-specific BMI centile charts instead. Some clinicians also use ethnic-adjusted BMI thresholds for specific populations to account for health risks at lower weights.
Is a BMI of 25 actually overweight if I feel healthy?
A BMI of 25 falls in the overweight category according to NHS guidelines, but this doesn't automatically mean you're unhealthy. BMI is a screening tool, not a diagnosis. Factors like fitness, muscle mass, family history, and blood pressure matter too. Your GP can assess your individual health picture.
Why do South Asian adults have different BMI thresholds?
Research shows South Asian, Chinese, Black African, and Black Caribbean populations develop type 2 diabetes and cardiovascular disease at lower BMI levels than European populations. The NHS recommends lower thresholds for these groups to identify risk earlier and support timely intervention.
Can I have a healthy BMI but unhealthy body composition?
Yes. Someone with a healthy BMI might have high body fat percentage and low lean body mass, whilst an athlete might have high BMI due to muscle. This is why body composition — measured by body fat percentage, visceral fat, and waist circumference — provides important additional context beyond BMI alone.
How often should I check my BMI?
If you're trying to reach a healthy weight, checking monthly is reasonable. If you're maintaining a stable weight, annual checks at your GP appointment are sufficient. Rapid changes warrant a check sooner. Remember, weight fluctuates daily due to water retention, food intake, and hormones — focus on trends over weeks, not daily variations.
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.