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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BMI in SI Units: NHS Standards for UK Adults
BMI stands for Body Mass Index and is measured in SI units as kilograms per square metre (kg/m²). This is the standard measurement system used by the NHS and healthcare professionals across the UK. Whether you're checking your weight health or understanding your BMI range, knowing how SI units work is essential for interpreting NHS guidance and health recommendations accurately.
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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.
NHS BMI Categories and Healthy Weight Ranges
The BMI formula in SI units is straightforward: divide your weight in kilograms by your height in metres squared (weight ÷ height²). The NHS uses consistent BMI categories for all UK adults:
- Underweight: BMI below 18.5 kg/m²
- Healthy weight: BMI 18.5–24.9 kg/m²
- Overweight: BMI 25.0–29.9 kg/m²
- Obese (Class I): BMI 30.0–34.9 kg/m²
- Obese (Class II): BMI 35.0–39.9 kg/m²
- Severely obese (Class III): BMI 40.0 kg/m² and above
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds due to higher health risks at lower weights: healthy weight is 18.5–22.9 kg/m², and overweight begins at 23.0 kg/m². The healthy BMI range of 18.5–24.9 kg/m² is where the NHS advises most adults should aim, as this correlates with the lowest health risks. Use our free BMI calculator above to check your personal BMI instantly in SI units.
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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 Weight Management Pathway and When to Seek Help
The NHS provides a four-tier weight management pathway. Tier 1 involves self-help through digital tools and community programmes. Tier 2 includes primary care interventions where your GP can discuss weight health and healthy lifestyle changes. Tier 3 offers specialist multidisciplinary weight management services with structured programmes and, where appropriate, prescription medications. Tier 4 covers bariatric surgery for severe obesity with significant health complications.
If your BMI is above the healthy range, speak to your GP. They can assess whether you need support and determine which tier of the NHS pathway is appropriate for you. Your GP will also consider other health factors beyond BMI, such as waist circumference and weight-related conditions like type 2 diabetes or high blood pressure.
- 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 does SI units mean in the context of BMI?
SI units stands for Système International d'Unités (International System of Units). For BMI, this means kilograms per square metre (kg/m²). It's the metric standard used globally and by the NHS. Your weight in kg is divided by your height in metres squared to calculate BMI in SI units.
Why does the NHS use SI units instead of imperial measurements?
The NHS uses SI units (metric) because they are the international standard in healthcare and clinical practice. Metric measurements are precise, consistent, and easily comparable across healthcare settings. However, many UK adults also use stone and pounds informally — our weight converter helps you switch between both systems easily.
Is there a difference between BMI in SI units and BMI calculated in pounds and inches?
No — the BMI value is the same regardless of which measurement system you use. Whether you calculate using kg/m² or pounds/inches, a BMI of 25 kg/m² is identical to a BMI calculated from imperial units. The SI formula is simply the standard clinical method.
What BMI in SI units should I aim for according to NHS guidance?
The NHS recommends a healthy BMI range of 18.5–24.9 kg/m² for most UK adults. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is 18.5–22.9 kg/m². Your GP can advise on a target BMI based on your individual health profile and any weight-related health conditions.
Does BMI in SI units account for muscle mass?
No — BMI does not distinguish between muscle and fat. Athletes or very muscular individuals may have a high BMI despite low body fat. BMI is a screening tool, not a complete health assessment. Your GP may also consider waist circumference, lifestyle factors, and overall health when evaluating your weight health.
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.