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
How to Get the BMI Formula and Calculate Your BMI
The BMI formula is a simple calculation used by the NHS to assess whether an adult's weight falls within a healthy range. BMI (body mass index) uses your weight and height to estimate whether you may be at increased health risk. In metric, the BMI formula is: weight in kilograms divided by height in metres squared (kg ÷ m²). In imperial, it's weight in pounds multiplied by 703, then divided by height in inches squared. The NHS recommends using this calculation as part of routine health checks. However, it's important to understand that BMI has limitations — it doesn't account for muscle mass or body composition. Use our free BMI calculator above to instantly compute your personal BMI and see where you fall on the NHS categories.
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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 standardised BMI ranges to assess health risk for UK adults. These categories are based on extensive research and NICE guidance:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9
- Overweight: BMI 25.0 to 29.9
- Obese (Class I): BMI 30.0 to 34.9
- Obese (Class II): BMI 35.0 to 39.9
- Severely obese (Class III): BMI 40.0 and above
If you are of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds, as these groups face higher health risks at lower weights. For these populations, a healthy BMI is 18.5 to 22.9, and overweight begins at 23.0. This tailored guidance reflects modern understanding of ethnic variation in health risk. Your GP can discuss what your personal BMI means for your health and whether any action is needed.
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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.
When to Seek NHS Help and Weight Management Support
If your BMI falls outside the healthy range, the NHS offers support through its weight management pathway. Most people start at tier 1 with self-help tools, community programmes, and the free NHS Weight Loss Plan. If you need additional support, your GP can refer you to tier 2 services, which offer behaviour change programmes and structured advice. For those with a BMI of 30 or above, or 25–29.9 with weight-related health conditions, specialist tier 3 services are available — these may include prescription weight management medications. Tier 4 services (bariatric surgery) are considered for severe obesity with significant health complications. Availability varies by region. Contact your GP to discuss which tier of support is right for 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.
What is the exact BMI formula used by the NHS?
The metric formula is: weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.73m tall: 70 ÷ (1.73 × 1.73) = 70 ÷ 2.99 = BMI of 23.4. In imperial: (weight in lbs × 703) ÷ height (inches)². The NHS uses the metric formula in clinical practice.
Is a BMI of 25 considered overweight according to NHS guidance?
Yes. According to NHS guidelines, a BMI of 25.0 to 29.9 is classified as overweight. A healthy BMI range is 18.5 to 24.9. However, BMI does not account for muscle mass, so athletes may have a higher BMI without excess body fat. Your GP can provide personalised interpretation.
Do NHS BMI categories differ for different ethnic groups?
Yes. The NHS recommends lower BMI thresholds for people of South Asian, Chinese, Black African, and Black Caribbean heritage because these groups have higher cardiovascular and metabolic disease risk at lower BMI levels. For these groups, overweight starts at BMI 23.0 and obesity at 27.5. Your GP can advise on thresholds relevant to your background.
Can I use the BMI formula if I'm pregnant?
BMI is less reliable during pregnancy because weight gain is expected and includes the baby, placenta, and fluids. The NHS advises pregnant women to discuss healthy weight gain with their midwife or GP rather than relying on BMI. Post-pregnancy, BMI becomes a useful tool again after weight has stabilised.
What are the main limitations of the BMI formula?
BMI does not distinguish between muscle and fat — muscular people may have high BMI but low body fat. It ignores where fat is stored (waist circumference matters). BMI is less accurate for very tall or short people, the elderly (due to muscle loss), or those with significant muscle development. Use BMI alongside waist measurement and GP assessment for best health insight.
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.