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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Understanding the BMI Equation and How It Works
The BMI equation is a simple mathematical formula that helps measure body weight relative to height. In the UK, the standard BMI formula divides your weight in kilograms by your height in metres squared. Understanding this calculation is important because it helps you assess whether your weight falls within a healthy range according to NHS guidelines. While BMI is a useful screening tool, it has important limitations—it doesn't distinguish between muscle and fat, and individual health risks vary by ethnicity. Our free BMI calculator above does this calculation instantly, but knowing the formula helps you understand what the number actually means.
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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 standard BMI formula is:
- BMI = weight (kg) ÷ height (m)²
For example, if you weigh 70kg and are 1.75m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 BMI.
According to NHS BMI categories, this result falls within the healthy weight range. The NHS uses these standardised BMI range thresholds for all UK adults:
- 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 adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower overweight BMI threshold values due to higher health risks at the same BMI level. Use our free BMI calculator above to find your personal BMI instantly.
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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 Guidance and When to Seek Help
The NHS recognises that BMI is just one screening tool and has important BMI limitations. It doesn't account for muscle mass, bone density, or where fat is distributed around your body. The NHS therefore recommends considering waist circumference alongside BMI—men should aim for under 94cm (37 inches), women under 80cm (31.5 inches).
If your healthy BMI calculation shows you're outside the healthy range, or if you're concerned about your weight and health, the NHS recommends speaking to your GP. They can assess your individual circumstances, consider other health factors, and discuss weight management options suited to you. The NHS offers a tiered weight management pathway: from self-help and digital tools (Tier 1) through to specialist services and prescription weight loss medication (Tiers 3 and 4) for those who qualify.
- 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 correct BMI equation?
The BMI equation is: weight in kilograms divided by height in metres squared (BMI = kg ÷ m²). For example, someone weighing 75kg at 1.70m tall would calculate: 75 ÷ (1.70 × 1.70) = 75 ÷ 2.89 = 25.9 BMI, which falls in the overweight range according to NHS guidelines.
What is a healthy BMI according to the NHS?
The NHS defines a healthy BMI as 18.5 to 24.9 for most UK adults. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends a healthy range of 18.5 to 22.9 due to increased health risks at higher BMI levels in these populations.
Does the BMI formula work for everyone?
The BMI formula is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat, so athletes may have a high BMI despite low body fat. It's also less accurate during pregnancy, for elderly people with muscle loss, and for very tall or very short individuals. Your GP can provide personalised advice.
How do I use the BMI formula if I weigh in stone and pounds?
If your weight is in stone and pounds, first convert to kilograms (1 stone = 6.35kg). If you're 11 stone 4 pounds, that's 71.7kg. Then apply the standard BMI equation. Our free weight converter on this page converts between stone, pounds, and kilograms instantly.
What should I do if my BMI is outside the healthy range?
If your BMI calculation shows you're outside the 18.5–24.9 range, speak to your GP for personalised advice. They'll consider your overall health, waist measurement, and any weight-related health conditions. The NHS offers support through its weight management pathway, including lifestyle programmes and, in some cases, prescription weight loss medication.
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.