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 Is BMI and How Does It Relate to Your Body?
BMI (body mass index) is a measurement that compares your weight to your height to estimate whether you're at a healthy weight. It's calculated by dividing your weight in kilograms by your height in metres squared. The NHS uses BMI as a screening tool to assess health risk across the population, though it has important limitations. Understanding your body mass index helps you know whether your current weight falls within a healthy range for your height, and whether lifestyle changes or medical support might be beneficial. Our free BMI calculator above can instantly show you your personal BMI and where it falls within NHS guidelines.
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 Healthy Weight Ranges
The NHS divides body mass index into five main categories that apply to most UK adults:
- Underweight: BMI below 18.5 — may indicate insufficient weight for health
- Healthy weight: BMI 18.5 to 24.9 — the target range for most adults
- Overweight: BMI 25.0 to 29.9 — increased health risks
- Obese Class I: BMI 30.0 to 34.9 — higher health risks
- Obese Classes II and III: BMI 35.0+ — significantly elevated health risks
The bmi formula is simple: weight (kg) ÷ height (metres)². For example, someone weighing 70kg and 1.73m tall would have a BMI of approximately 23.4, falling in the healthy weight range. However, if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — a healthy bmi for these groups starts at 18.5 but extends only to 22.9, reflecting higher cardiovascular risk at standard weights.
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 Why Body Composition Matters
While bmi range uk guidance is useful, body mass index has significant limitations. It doesn't distinguish between muscle and fat — an athlete with high muscle mass may have a high BMI despite low body fat. It also ignores where fat is stored; excess weight around the waist carries greater health risk than weight distributed elsewhere. Age, bone density, and pregnancy also affect BMI accuracy. For a more complete picture of health, the NHS recommends combining BMI assessment with waist circumference measurement (over 94cm for men or 80cm for women suggests increased risk) and considering overall fitness and lifestyle. Your GP can help interpret your results in the context of your individual health, family history, and any existing health conditions.
- 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.
Why does the NHS use BMI if it has limitations?
BMI is a quick, inexpensive screening tool that works well for most of the population to identify health risk at a population level. It's not perfect for individuals, but combined with other measures like waist circumference and clinical judgment, it provides useful health guidance. Your GP can assess your personal health beyond BMI alone.
What's the difference between being overweight and obese according to BMI?
According to NHS guidelines, overweight is a BMI of 25.0 to 29.9, while obese starts at 30.0. The distinction matters because obesity carries significantly higher health risks including type 2 diabetes, heart disease, and certain cancers. Even modest weight loss from the obese range can improve health outcomes.
Should children use the same BMI categories as adults?
No — children's BMI is calculated the same way but interpreted differently using age and sex-specific growth charts (UK90 references). Adult categories don't apply. If you're concerned about a child's weight, your GP or health visitor can provide age-appropriate guidance using proper growth assessment tools.
How accurate is BMI for very muscular people?
BMI can overestimate health risk for very muscular individuals because it counts muscle the same as fat. A bodybuilder might have a BMI suggesting obesity while having very low body fat. Body composition analysis (DEXA scan or similar) gives a more accurate picture in these cases, but this is rarely needed for general health assessment.
Can I use BMI to check if I'm at a healthy weight for my height?
Yes — BMI is a reasonable starting point. Calculate your BMI using our free BMI calculator above, then check it against NHS categories. If your BMI is in the healthy range (18.5 to 24.9), weight is unlikely to be a major health concern. If outside this range, discuss next steps with your GP, who can consider your full 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.