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
What Does BMI Mean?
BMI stands for body mass index — a measurement that uses your weight and height to assess whether you're in a healthy weight range. The NHS uses BMI as a screening tool to help identify weight-related health risks in adults. While BMI has limitations and doesn't tell the whole story about your health, it's a useful starting point for understanding where your weight falls on the NHS health scale. This guide explains what BMI means, how the NHS interprets it, and what to do if you're concerned about yours.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 What They Mean
The NHS uses five BMI categories to classify adult weight status. According to NHS guidelines, a healthy BMI falls between 18.5 and 24.9 — this is the range where your health risks from weight are lowest. A BMI of 25.0 to 29.9 is classified as overweight, which increases your risk of developing weight-related conditions. A BMI of 30.0 or above is classified as obese, and the NHS further divides this into three classes based on severity.
- Underweight: BMI below 18.5 — may indicate insufficient nutrition
- Healthy weight: BMI 18.5–24.9 — lowest health risk from weight
- Overweight: BMI 25.0–29.9 — increased risk of type 2 diabetes and heart disease
- Obese (Class I): BMI 30.0–34.9 — significant health risks
- Obese (Class II): BMI 35.0–39.9 — higher health risks
- Severely obese (Class III): BMI 40.0+ — very high health risks
Use our free BMI calculator above to find your personal BMI instantly and see which NHS category you fall into. Important note: For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — a healthy BMI is 18.5–22.9, and overweight starts at 23.0.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 NHS Guidance
The NHS recognises that BMI is a useful screening tool but has significant limitations. BMI does not distinguish between muscle and fat — athletes may have a high BMI but low body fat, which BMI doesn't account for. BMI also doesn't show where on your body fat is stored, and carrying weight around your stomach carries different health risks than weight distributed elsewhere.
The NHS therefore recommends using BMI alongside other measurements, particularly waist circumference. Men with a waist over 94cm (37 inches) and women over 80cm (31.5 inches) have increased health risk, even with a healthy BMI. If you're concerned about your health or weight, your GP can offer personalised advice beyond BMI alone and discuss whether weight management support would benefit you.
- 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.
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by your height in metres squared (BMI = weight ÷ height²). For example, someone weighing 70kg with a height of 1.75m would have a BMI of approximately 22.9. You don't need to calculate it manually — use our free BMI calculator above for gradual improvement.
What's the difference between BMI and a healthy weight range?
BMI is a single number that places you in a category. A healthy weight range is the span of weights that corresponds to a healthy BMI (18.5–24.9) for your specific height. For example, at 1.73m tall, a healthy weight range is approximately 55–74kg (8st 8lbs–11st 8lbs). Your GP can advise on what's healthy for you personally.
Why does the NHS use different BMI categories for some ethnic groups?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean backgrounds have higher risk of type 2 diabetes and heart disease at lower BMI levels. The NHS therefore uses lower thresholds for these groups — a healthy BMI is 18.5–22.9 rather than 18.5–24.9. This reflects actual health risk, not a different standard.
What should I do if my BMI is in the overweight or obese range?
If your BMI is above 25, the NHS recommends speaking with your GP. Your doctor can assess your individual health, discuss any weight-related health conditions, and suggest appropriate support. This might include lifestyle advice, referral to a weight management programme, or in some cases, prescription treatment alongside diet and exercise.
Is BMI accurate for elderly people or those with high muscle mass?
No — BMI is less accurate for elderly adults (who lose muscle with age) and for muscular or athletic individuals. The NHS acknowledges these limitations. If you're concerned about your health despite a high BMI, discuss this with your GP. They can use additional assessments like waist circumference and blood tests to give you a fuller health picture.
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.