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 Body Mass Index (BMI) and What Does It Mean?
Body mass index (BMI) is a simple health screening tool that measures your weight in relation to your height. It's calculated by dividing your weight in kilograms by your height in metres squared. The NHS uses BMI to help assess whether adults are at a healthy weight, and to identify potential health risks associated with being underweight, overweight, or obese. BMI is widely used in UK healthcare but has important limitations — it doesn't account for muscle mass or where fat is stored on your body. Use our free BMI calculator above to find your personal BMI instantly and understand where you sit on the NHS health scale.
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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 Healthy Weight Ranges
The NHS recognises five main BMI categories for adults:
- Underweight: BMI below 18.5 — may indicate nutritional concerns
- Healthy weight: BMI 18.5 to 24.9 — associated with lowest health risks
- Overweight: BMI 25.0 to 29.9 — increased risk of weight-related health conditions
- Obese (Class I): BMI 30.0 to 34.9 — significantly elevated health risks
- Severely obese (Class II/III): BMI 35.0 and above — highest health risks
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds because these groups face higher cardiovascular and diabetes risk at the same BMI levels. For these populations, a healthy BMI range is 18.5 to 22.9, and overweight begins at 23.0. NICE guidance emphasises that while BMI is useful, waist circumference and overall health context matter equally — a GP should always consider individual circumstances.
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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.
NHS Weight Management Pathway and When to Seek Help
If your BMI indicates you're overweight or obese, the NHS offers a structured pathway to support. Most people start with Tier 1 self-help resources — digital tools like our free BMI calculator and weight converter, plus community programmes. Your GP can refer you to Tier 2 services for structured behaviour change support. For those with a BMI of 30 or above, or 27–29.9 with weight-related conditions like type 2 diabetes or high blood pressure, Tier 3 specialist weight management services may offer prescription medications including GLP-1 injections alongside lifestyle support. Speak to your GP if you're concerned about your weight or health — they can assess your individual situation and discuss options tailored to you.
- 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 according to the NHS?
The NHS defines a healthy BMI as 18.5 to 24.9 for most adults. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends a slightly lower range of 18.5 to 22.9 due to higher health risks at the same BMI levels.
Why are BMI categories different for certain ethnic groups?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean backgrounds have higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels compared to European populations. The NHS adjusted thresholds reflect this evidence to ensure equal health protection across populations.
What are the BMI limitations I should know about?
BMI doesn't distinguish between muscle and fat — athletes may have high BMI but low body fat percentage. It also doesn't account for where fat is stored (abdominal fat carries higher health risk), age-related muscle loss, or bone density. Your GP should always assess your overall health, not BMI alone.
Can I use BMI to check my healthy weight range?
Yes. Once you know your height in metres, multiply it by itself, then multiply by 18.5 and 24.9 to find your healthy weight range in kg. Our free BMI calculator above does this instantly — simply enter your weight and height to see where you fall on the NHS scale.
When should I contact my GP about my BMI or weight?
Contact your GP if your BMI is above 30, if you're experiencing weight-related health problems like breathlessness or joint pain, or if you're concerned about weight changes. They can assess your individual health and discuss support options, including referral to specialist services if appropriate.
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.