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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How to Check Your Body Mass
Checking your body mass means understanding not just your weight, but how that weight is distributed and whether it poses health risks. In the UK, the standard method is calculating your BMI (Body Mass Index) alongside body composition measures like body fat percentage and lean body mass. Your GP uses these measurements to assess overall health. This guide explains what body mass means, how to measure it accurately, and why it matters for your health in the UK context.
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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 Calculate and Understand Body Mass
Body mass index (BMI) is the primary measure used by the NHS. It's calculated by dividing your weight in kilograms by your height in metres squared: BMI = weight (kg) ÷ height (m)². Our free BMI calculator above does this instantly—just enter your weight and height.
BMI 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 South Asian, Chinese, Black African, and Black Caribbean adults, the NHS recommends lower thresholds starting at 23.0 for overweight. However, BMI alone doesn't distinguish between muscle and fat, which is why body composition analysis—measuring your actual body fat percentage and lean body mass—provides a more complete picture of your health.
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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.
Why BMI Has Limits and What Else Matters
BMI is a useful screening tool but has important limitations. It doesn't account for muscle mass, so athletes may have a high BMI but low body fat percentage. It also ignores where fat is stored—visceral fat (around organs) poses greater health risks than subcutaneous fat (under the skin).
A healthy body fat percentage is roughly 25–31% for women and 18–24% for men. Waist circumference is also crucial: the NHS recommends keeping it below 94cm (37in) for men and 80cm (31.5in) for women. Use our weight converter to track changes in metric or imperial measurements, and monitor progress over time using our free progress tracker alongside lifestyle changes like diet and activity.
- 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 body mass and body weight?
Body mass is your total weight (what a scale shows), while body composition analysis breaks that into muscle, fat, bone, and water. Two people of the same weight can have very different body compositions. BMI estimates health risk from body mass alone, but measuring body fat percentage gives a clearer picture of your actual fitness and health.
Can you have a healthy BMI but unhealthy body composition?
Yes. Someone with a healthy BMI may have high body fat if they have low muscle mass. Conversely, athletes can have high BMI but low body fat percentage. This is why the NHS now recommends considering both BMI and waist circumference together, not BMI alone.
How do I measure body fat percentage at home?
Common home methods include bioelectrical impedance scales, skinfold calipers, and tape measurements. These are less accurate than DEXA scans or hydrostatic weighing (available privately), but they're convenient for tracking trends over time. Your GP can advise on the most accurate option for you.
Why does the NHS use different BMI thresholds for some ethnic groups?
Research shows that people from South Asian, Chinese, Black African, and Black Caribbean backgrounds have higher health risks from obesity and type 2 diabetes at lower BMI levels. The NHS therefore recommends lower BMI thresholds (starting at 23.0 for overweight) for these groups to catch health risks earlier.
Should I focus on BMI or body fat percentage?
Ideally, check both. BMI is quick and free to calculate, but body fat percentage and waist circumference give a more complete health picture. If your GP is concerned about your weight, they'll consider all these measures alongside your overall health, family history, and any existing conditions. ```
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.