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
How to Calculate Body Mass Index
Body mass index (BMI) is a simple measurement that uses your weight and height to assess whether you're in a healthy weight range. Calculating your BMI takes just seconds — it's the first step many people take when reviewing their health. You can use our free BMI calculator above for an gradual improvement, or follow the simple formula yourself. Understanding your BMI category according to NHS guidelines helps you and your GP identify any health risks linked to excess weight, although BMI is just one piece of the body composition picture and doesn't account for muscle mass or where fat is stored on your body.
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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.
The 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 1.75m tall, your calculation is 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 BMI.
According to NHS guidelines, 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)
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds: healthy weight is 18.5–22.9, and obesity begins at 27.5 due to higher health risks at lower BMI levels in these populations.
Clinically BackedWeight Loss
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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 Why Body Composition Matters
While BMI is a useful screening tool, it has important limitations. BMI does not distinguish between lean body mass (muscle) and fat — athletes or very muscular individuals may have a high BMI yet low body fat percentage. Body composition tells you what percentage of your weight is fat versus muscle, which is more clinically meaningful. Your body fat calculator and waist circumference measurement provide additional context. For example, someone with a healthy BMI but high visceral fat (fat stored around organs) may face health risks. The NHS recommends checking both your BMI and waist measurement: men over 102cm or women over 88cm are at higher health risk. Use the weight converter on this page to check your weight in stone if you prefer imperial units.
- 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 the exact BMI formula for UK measurements?
BMI = weight (kg) ÷ height (m)². For example, 70kg and 1.75m tall = 70 ÷ 3.06 = 22.9 BMI. Use our free calculator above for gradual improvement without a UK pharmacy calculation.
Is a BMI of 25 considered overweight in the UK?
Yes, according to NHS guidelines, a BMI of 25.0–29.9 falls into the overweight category. However, if you're of South Asian or Black African heritage, the threshold is lower — 23.0–27.4 is considered overweight.
Why does BMI not account for muscle mass?
BMI measures total weight divided by height, so it cannot distinguish muscle from fat. Muscular people may have high BMI but low body fat percentage. Body composition analysis gives more detail about whether your weight is healthy for you.
What should I do if my BMI is above 30?
A BMI above 30 suggests obesity. Speak to your GP about your individual health risks, weight loss options, and whether lifestyle changes or medical support would help. Your GP can assess whether prescription weight management treatment is appropriate for you.
Does BMI apply differently for children?
Yes, children's BMI uses age and sex-specific centile charts, not adult categories. The NHS Healthy Weight Calculator has a separate tool for children. Always consult your GP or health visitor about a child's weight.
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.