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 Do You Check Your Body Mass Index?
Checking your body mass index (BMI) is straightforward and takes just two measurements: your weight and height. BMI is a simple screening tool used by the NHS to assess whether an adult's weight falls within a healthy range. In this guide, we'll walk you through exactly how to calculate your BMI, what your result means, and why it matters for your health. You can calculate it manually using a simple formula, or use our free BMI calculator above for an gradual improvement.
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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.
How to Calculate Your BMI — The Formula and Steps
BMI is calculated using a straightforward formula based on your weight and height. The metric formula is: weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.73m tall, your BMI would be 70 ÷ (1.73 × 1.73) = 23.4.
To calculate manually:
- Measure your weight in kilograms (or convert using our weight converter if you know your weight in stone and pounds)
- Measure your height in metres
- Multiply your height by itself (height²)
- Divide your weight by this number
- Your result is your BMI
According to NHS guidelines, a healthy BMI for most adults is between 18.5 and 24.9. However, adults of South Asian, Chinese, Black African, or Black Caribbean heritage have a lower healthy threshold: 18.5 to 22.9. Use our free BMI calculator above to check your personal BMI instantly — it's quicker and removes calculation errors.
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 Categories and What Your Result Means
Once you've calculated your BMI, you can compare it to the NHS categories. A BMI below 18.5 is underweight; 18.5–24.9 is healthy weight; 25–29.9 is overweight; and 30 or above is obese (with further grades for severe obesity). These categories apply to most UK adults, but remember that BMI has important limitations. It doesn't distinguish between muscle and fat — athletes may have a high BMI but low body fat percentage. It also doesn't account for where fat is stored, which matters for health risk. For a more complete picture, consider measuring your waist circumference or discussing your body composition with your GP, especially if you're concerned about your health.
- 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.
Is BMI the only way to assess if I'm a healthy weight?
No. BMI is a useful screening tool, but it has limitations. It doesn't account for muscle mass or body composition. Your waist circumference, body fat percentage, and overall fitness level are also important. Your GP can assess your individual health risk and recommend additional measures if needed.
Can I use the same BMI categories if I'm of South Asian or Black African heritage?
No. The NHS recommends lower BMI thresholds for adults of South Asian, Chinese, Black African, and Black Caribbean backgrounds because these groups face higher health risks at lower BMI levels. A healthy BMI for these populations is 18.5–22.9, not 18.5–24.9.
What if my BMI is high but I feel fit and healthy?
High BMI doesn't always mean poor health, especially if you have significant muscle mass. However, BMI is still a useful screening indicator. Speak with your GP about your overall health, including fitness level, diet, and any family history of weight-related conditions. They can advise whether your weight poses any health risks.
How often should I check my BMI?
There's no set rule. If you're monitoring your weight, checking BMI monthly or quarterly is reasonable. If you're tracking weight loss progress, you might check weekly, though remember that BMI alone doesn't capture changes in body composition or muscle gain alongside fat loss.
Can I use BMI calculations for children?
No. Children and teenagers use age and sex-specific BMI percentile charts, not adult categories. If you're concerned about a child's weight, speak with your GP or health visitor, who can assess growth and development properly using NHS growth reference charts.
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.