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 a BMI of 42 Mean?
A BMI of 42 falls into the obese (Class II) category according to NHS guidelines. This BMI range indicates an increased health risk and means your weight may be affecting your wellbeing. The good news is that support is available through the NHS and other routes. Understanding your BMI is the first step towards better health. Use our free BMI calculator above to check your personal BMI and see where you stand, or use the weight converter to see your weight in different units.
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 Your Health
The NHS BMI categories classify adults into five groups based on their height and weight. A BMI of 42 places you in the obese Class II category (BMI 35.0–39.9 falls into Class I; 40 and above is Class III/severe obesity). According to the NHS BMI range, a healthy BMI is 18.5 to 24.9. At a BMI of 42, you are significantly above this range.
It's important to understand the bmi formula behind this: BMI = weight (kg) ÷ height (m)². However, BMI has important bmi limitations — it doesn't account for muscle mass, bone density, or where fat is stored. Your GP can provide personalised interpretation of your BMI in the context of your overall health. Having a healthy bmi reduces risk of type 2 diabetes, heart disease, stroke, and certain cancers.
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.
NHS Weight Management Pathway and Your Options
The NHS weight management pathway has four tiers. At BMI 42, you typically qualify for support through Tier 2 (primary care) or Tier 3 (specialist weight management services). Your GP can refer you to a specialist weight management service in your area, where you'll receive support including dietary advice, physical activity programmes, and potentially prescription weight loss medication if appropriate following a clinical assessment.
Access to prescription weight loss treatment varies by region, and waiting lists can be lengthy. Some people also access treatment privately. Your GP is the best starting point — they can discuss all available options with you and help you decide what approach suits your circumstances and health needs.
- 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 health risks come with a BMI of 42?
A BMI of 42 significantly increases your risk of type 2 diabetes, heart disease, high blood pressure, stroke, sleep apnoea, and certain cancers including breast and bowel cancer. Even modest weight loss of 5–10% can reduce these risks meaningfully.
How can I calculate my healthy weight range?
Use our free BMI calculator above to find your healthy weight range for your height. A healthy BMI is 18.5–24.9, so multiply your height (in metres) squared by 18.5 and 24.9 to find your target range. Your GP can also help you set realistic goals.
Can the NHS help me lose weight at BMI 42?
Yes. Speak to your GP, who can refer you to your local weight management service (Tier 2 or 3). Services vary by region but typically include behaviour change support, dietary advice, exercise programmes, and potentially prescription medication like Mounjaro or Wegovy following clinical assessment.
What is the difference between BMI categories and actual health risk?
BMI is a screening tool, not a diagnosis. A BMI of 42 indicates increased health risk, but your individual risk also depends on family history, fitness level, where you carry weight, and existing health conditions. Your GP can assess your personal risk profile.
Should I try to lose weight quickly with a BMI of 42?
The NHS recommends losing weight at 0.5–1 kg per week for sustainable results. gradual weight management can lead to muscle loss and nutrient deficiency. Speak to your GP or a dietitian about a safe, structured approach tailored to your 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.