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 We Calculate BMI: The Formula Explained
BMI (Body Mass Index) is calculated using a simple mathematical formula based on your weight and height. Understanding how we calculate BMI helps you interpret your result and makes sense of the healthy BMI ranges recommended by the NHS. Our free BMI calculator on this page does the maths instantly, but knowing the formula behind it gives you confidence in your result and helps you understand what your BMI really tells you about your health.
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.
The BMI Formula and How It Works
The BMI formula is straightforward: divide your weight in kilograms by your height in metres, squared. Mathematically, that's: BMI = weight (kg) ÷ height (m)²
For example, if you weigh 70kg and are 1.75m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 BMI. This falls comfortably in the healthy BMI range of 18.5 to 24.9 according to NHS guidelines.
Once calculated, your BMI result is compared against the NHS BMI categories:
- Below 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)
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds because these groups face higher health risks at lower weights. The healthy range for these groups is 18.5–22.9, and overweight begins at 23.0.
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 What Else Matters
While the BMI formula is simple and useful for screening, it has important limitations. BMI does not distinguish between muscle and fat — athletes may have a high BMI but low body fat. It also ignores where fat is stored; abdominal fat poses greater health risks than fat elsewhere. Waist circumference is equally important: men with waists over 94cm (37 inches) and women over 80cm (31.5 inches) have increased health risk, even if BMI is normal.
For a complete picture of your health, combine BMI with waist measurement, fitness level, blood pressure, and cholesterol levels. Your GP can assess these factors together. Use our free BMI calculator and weight converter above to track your metrics, and remember that individual health is unique — your GP can give you personalised guidance about what your BMI means for you specifically.
- 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 the BMI formula the same in the UK as other countries?
Yes, the BMI formula is universal: weight (kg) ÷ height (m)². However, the NHS BMI categories and thresholds are UK-specific. For example, the UK uses different cutoffs for South Asian, Chinese, and Black African heritage populations based on their higher health risks at lower BMI levels.
How accurate is the BMI formula for calculating body fat?
BMI is a screening tool only — it estimates overall weight status but does not measure body fat directly. Athletes, muscular individuals, and older adults with muscle loss may have inaccurate BMI results. Body composition analysis and waist circumference provide better body fat estimates.
Can I use pounds and inches instead of kilograms and metres?
Yes. The imperial formula is: BMI = (weight in pounds × 703) ÷ height (inches)². However, the NHS uses metric measurements. Our BMI calculator handles any unit instantly — just enter your weight and height in your preferred measurements.
What is a healthy BMI range for adults in the UK?
According to NHS guidelines, a healthy BMI is 18.5–24.9 for most UK adults. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is 18.5–22.9. Individual health factors vary — your GP can advise on what's healthy for you.
Should I use the same BMI categories if I'm pregnant or over 65?
No. Standard BMI categories are less accurate during pregnancy and for older adults. If you're pregnant, your GP will provide pregnancy-specific weight guidance. For over-65s, discuss healthy weight with your GP — muscle loss with age means BMI may not reflect health accurately.
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.