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
Understanding the Mass Body Index Formula — UK Context
The mass body index (BMI) formula is a simple calculation used by the NHS to assess whether an adult's weight is healthy for their height. The standard formula divides weight in kilograms by height in metres squared. In the UK, BMI is a key screening tool for identifying underweight, healthy weight, overweight, and obese categories. Understanding how BMI is calculated and what your result means is important for making informed health decisions. This guide explains the formula, NHS guidance, and how to interpret your personal BMI score.
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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 to Calculate BMI — The Formula Explained
The BMI formula is straightforward: divide your weight in kilograms by your height in metres, squared. The metric equation is:
- BMI = weight (kg) ÷ height (m)²
For example, if you weigh 70kg and are 1.75 metres tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 BMI. Use our free BMI calculator above to find your result instantly without a UK pharmacy calculation. The NHS uses this formula as a standard screening tool in primary care, though it's important to note that BMI does not distinguish between muscle and fat. If you prefer imperial measurements (stone and pounds), convert your weight using our weight converter, then apply the same formula. Most UK health professionals use the metric version, but understanding both systems helps you navigate health guidance more effectively.
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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 BMI Categories — What Your Score Means
The NHS defines healthy weight as a BMI between 18.5 and 24.9 for most UK adults. A BMI below 18.5 is classed as underweight; 25.0–29.9 is overweight; 30.0–34.9 is obese (Class I); 35.0–39.9 is obese (Class II); and 40.0 or above is severely obese (Class III). However, for adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds—healthy weight starts at 18.5 but overweight begins at 23.0 and obese at 27.5. This reflects higher health risks in these populations at lower BMI levels. Your GP can discuss what your personal BMI means for your health and whether any action is needed. Remember that BMI is a screening tool only and doesn't account for muscle mass, bone density, or fat distribution.
- 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 the BMI formula different in the UK compared to other countries?
No, the BMI formula is the same worldwide: weight (kg) ÷ height (m)². However, the NHS uses different category thresholds for people of certain ethnic backgrounds, recognising that health risks vary by ethnicity at the same BMI level. The calculation itself is universal.
Can I use the BMI formula with stone and pounds?
Yes, but it's more complex. Convert stone and pounds to kilograms first, then apply the standard formula. Alternatively, use our weight converter to switch between units instantly, then use our BMI calculator to avoid a UK pharmacy calculation errors.
Why doesn't BMI account for muscle mass?
The BMI formula only measures weight relative to height—it cannot distinguish between muscle, fat, bone, and water. Athletes may have a high BMI but low body fat. This is why your GP may also consider waist circumference and other health markers alongside BMI.
What if my BMI falls between two categories?
BMI categories have specific ranges (e.g., 25.0–29.9 for overweight). If your result falls within a range, you're in that category. For example, a BMI of 25.5 means you're in the overweight category, not on the border. Your GP can give personalised guidance.
Should I use the BMI formula if I'm pregnant or elderly?
BMI is less reliable during pregnancy and for elderly adults (who may have lost muscle mass with age). If you're pregnant, discuss healthy weight management with your midwife or GP. For elderly adults, your GP may use additional assessments alongside BMI.
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.