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?
Body Mass Index (BMI) is a simple measure of body weight relative to height, used by the NHS and health professionals across the UK to assess whether an adult falls within a healthy weight range. It's calculated by dividing your weight in kilograms by your height in metres squared. BMI is a useful screening tool for identifying potential health risks linked to excess weight, though it has important limitations — it doesn't distinguish between muscle and fat, and doesn't account for where fat is stored on your body. Use our free BMI calculator above to find your personal BMI instantly.
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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.
How to Interpret Your BMI Calculator Results
When you calculate your BMI using the calculator on this page, your result will fall into one of five NHS categories. For most UK adults, a BMI below 18.5 indicates being underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 means overweight, 30 to 34.9 is obese (Class I), and 35 and above is severe obesity.
However, if you are of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds: a healthy range is 18.5 to 22.9, overweight starts at 23, and obesity begins at 27.5 — because these populations face higher cardiovascular and diabetes risk at lower BMI levels.
- Women and men use the same BMI formula — there is no gender-specific calculation
- Age matters less than you might think — the same categories apply to all adults, though muscle naturally decreases with age
- Your result is a snapshot — BMI changes as your weight changes, so track it over time using our free progress tracker for meaningful context
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 vs Body Fat Percentage — Which Matters More?
Body fat percentage and body composition provide a more detailed picture than BMI alone. While BMI tells you whether your overall weight is healthy for your height, body fat percentage reveals how much of that weight is fat versus lean muscle. A very muscular athlete might have a high BMI but low body fat, meaning they are actually in excellent health. Conversely, someone with a normal BMI might carry excess visceral fat (dangerous fat around organs), which BMI cannot detect.
The NHS recommends checking waist circumference alongside BMI for a fuller picture: men should aim below 94cm (37 inches), women below 80cm (31.5 inches). For a complete body composition assessment, consider both metrics — use our weight converter and BMI calculator together to establish your baseline, then monitor both your weight and waist measurement over time.
- 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.
Is BMI accurate for everyone?
BMI is a useful screening tool for most adults but has limitations. It doesn't account for muscle mass, bone density, age-related muscle loss, or ethnicity-specific health risks. Athletes may have high BMI but low body fat. Always discuss your personal health with your GP for context specific to you.
What's the difference between BMI and body fat percentage?
BMI measures weight relative to height only. Body fat percentage shows the actual composition of your body — how much is fat versus muscle. Someone can have a healthy BMI but high body fat percentage, or a high BMI with low body fat if very muscular. Both metrics together give a clearer picture.
Why are BMI thresholds different for some ethnic groups?
Research shows that South Asian, Chinese, Black African, and Black Caribbean populations have higher risk of type 2 diabetes and heart disease at lower BMI levels. The NHS therefore recommends lower BMI thresholds for these groups to catch health risks earlier.
Can I lose weight if my BMI is already healthy?
Yes. A healthy BMI doesn't assures good body composition or health. If your BMI is normal but you carry excess body fat (especially around the abdomen), reducing fat through diet and exercise can improve your health — even if your BMI stays the same.
How often should I check my BMI?
Checking monthly is reasonable if you're actively managing your weight. The NHS Weight Loss Plan suggests weighing yourself weekly to track progress. Use our free progress tracker to log your weight and BMI over time — this reveals trends better than single measurements.
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.