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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Obesity Classification By BMI — NHS Guide
Obesity classification by BMI is the standard way the NHS assesses weight-related health risk in UK adults. The NHS uses specific BMI categories to identify when excess weight may increase your risk of serious health conditions. Understanding where your BMI falls helps you know whether your weight is within a healthy range and when to speak with your GP. This guide explains the NHS BMI categories, how they're calculated, and what they mean for your health.
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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 Classification
The NHS BMI formula divides your weight in kilograms by your height in metres squared (kg ÷ m²). Your result falls into one of five NHS BMI categories that indicate weight-related health risk:
- Underweight: BMI below 18.5 — may indicate insufficient nutrition or underlying health issues
- Healthy weight: BMI 18.5–24.9 — associated with lowest health risk
- Overweight: BMI 25.0–29.9 — increased risk of type 2 diabetes and heart disease
- Obese (Class I): BMI 30.0–34.9 — significantly increased health risks
- Severely obese (Class II and III): BMI 35.0+ — substantial health risks requiring medical attention
The overweight BMI threshold of 25 means even modest excess weight can indicate increased health risk. However, BMI has important limitations — it doesn't distinguish muscle from fat and may be less accurate for very muscular individuals or older adults. Use our free BMI calculator above to find your personal BMI instantly and see which NHS category you fall into.
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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 structured weight management pathway with four tiers of support. Tier 1 includes self-help using tools like our free weight converter and BMI calculator, plus community programmes. If you're struggling, your GP can refer you to Tier 2 (primary care weight management services) or specialist Tier 3 services where prescription weight loss medication may be considered. NICE guidance recommends discussing weight management with your GP if your BMI is 25 or above with weight-related health conditions, or 30 or above regardless of conditions. Your GP can assess your individual health risk and discuss appropriate support, including lifestyle changes, structured programmes, or referral to specialist services where medically suitable.
- 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 BMI formula used by the NHS?
The NHS BMI formula is weight (kg) ÷ height (m)². For example, someone weighing 70kg who is 1.73m tall calculates: 70 ÷ (1.73 × 1.73) = BMI of 23.4, which falls in the healthy weight range. You can calculate your own BMI instantly using our free BMI calculator on this page.
Are BMI categories the same for all ethnic groups in the UK?
No. The NHS recommends lower BMI thresholds for adults of South Asian, Chinese, Black African, and Black Caribbean heritage because these groups face higher risk of type 2 diabetes and heart disease at lower BMI levels. A healthy BMI for these groups is 18.5–22.9 rather than 18.5–24.9. Always discuss your personal risk with your GP.
What BMI qualifies for NHS weight loss medication?
Generally, a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions (such as type 2 diabetes, high blood pressure, or sleep apnoea) may qualify for consideration by specialist NHS services. However, access varies significantly by region and specialist availability. Your GP can assess your eligibility and refer you if appropriate.
What are the limitations of BMI classification?
BMI doesn't distinguish between muscle and fat, so athletes may have a high BMI despite low body fat. It's also less accurate for elderly people, pregnant women, and those with significant muscle mass. Waist circumference measurement is often used alongside BMI for a more complete health picture. Your GP can discuss your individual risk factors.
When should I speak to my GP about my BMI?
Contact your GP if your BMI is 25 or above and you have weight-related health conditions, or if your BMI is 30 or above regardless of other factors. Also speak to your GP if you're struggling to manage your weight, experiencing weight gain despite healthy habits, or have concerns about your health. Your GP can provide personalised advice and support.
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.