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
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BMI Chart Formula and How to Calculate Your BMI
The BMI formula is a simple calculation used by the NHS to assess whether an adult's weight is in a healthy range. BMI (Body Mass Index) divides your weight in kilograms by your height in metres squared. It's one of the most widely used screening tools in UK healthcare, though it has important limitations. Understanding the BMI formula helps you interpret your results and know where you fall on the NHS BMI categories chart. This guide explains how the calculation works, what the ranges mean, and when to seek help from your GP.
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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.
The BMI Formula and NHS Categories Explained
The BMI formula is: BMI = weight (kg) ÷ height (m)²
To calculate your BMI, convert your height to metres, square that number, then divide your weight in kilograms by the result. For example, if you weigh 70kg and are 1.75m tall: 70 ÷ (1.75 × 1.75) = 22.9 BMI.
The NHS uses these BMI range categories for adults:
- Under 18.5 = Underweight
- 18.5–24.9 = Healthy weight
- 25.0–29.9 = Overweight
- 30.0–34.9 = Obese (Class I)
- 35.0–39.9 = Obese (Class II)
- 40.0+ = Severely obese (Class III)
However, the NHS recognises that BMI has limitations—it doesn't distinguish between muscle and fat, and doesn't account for where fat is stored. Use our free BMI calculator above to find your personal BMI instantly alongside these reference categories.
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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.
NHS Weight Management Support and When to Seek Help
If your BMI falls outside the healthy range (18.5–24.9), the NHS recommends speaking with your GP about your weight and health. Your GP can offer personalised advice and refer you to NHS weight management services if appropriate. The NHS operates a four-tier pathway: tier 1 is self-help and community support; tier 2 is primary care interventions; tier 3 is specialist multidisciplinary services where prescription medications may be considered; and tier 4 is bariatric surgery for severe obesity. Even a modest 5–clinically studied weight loss can improve blood pressure, blood sugar control, and reduce disease risk. Use the weight converter tool on this page to understand your weight in different units (stone, pounds, kilograms).
- 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's the difference between BMI calculated using the metric formula versus imperial?
The metric BMI formula divides kg by height (m)². The imperial version divides weight in pounds by height in inches squared, then multiplies by 703. Both give the same result—the NHS uses the metric version. Our BMI calculator handles the conversion automatically for you.
Do the NHS BMI categories apply to everyone in the UK?
No. The NHS recommends lower BMI thresholds for adults of South Asian, Chinese, Black African, and Black Caribbean heritage because these groups have higher cardiovascular and diabetes risk at the same BMI. For these groups, healthy weight is 18.5–22.9 and overweight starts at 23.0. Ask your GP which thresholds apply to you.
Is BMI the only thing my GP will consider about my weight?
No. Your GP will also look at waist circumference, family history, existing health conditions, and overall fitness. BMI is a screening tool, not a diagnostic measure. A healthy waist circumference is under 94cm (37in) for men and 80cm (31.5in) for women. Your GP can give you a complete health assessment.
Why does the BMI formula use height squared?
Height is squared because BMI is designed to account for the fact that taller people naturally carry more weight. Squaring height ensures that BMI changes proportionally with both weight and height, making it comparable across different body sizes.
Can I use the BMI formula for children?
No. Children's BMI uses age and sex-specific centile charts because they're still growing. Adult BMI categories don't apply to children under 18. Use the NHS Healthy Weight Calculator for children, or ask your GP for guidance on your child's weight.
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.