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 Compute Body Mass Index
Body mass index (BMI) is computed using a simple mathematical formula based on your weight and height. It's a standard health screening tool used by the NHS and GPs across the UK to assess whether an adult's weight falls within a healthy range. Computing your BMI takes just seconds — divide your weight in kilograms by your height in metres, squared. Understanding how to compute BMI helps you make informed decisions about your health, and our free BMI calculator above makes this instant and effortless.
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 UK Categories
To compute BMI, use this metric formula: BMI = weight (kg) ÷ height (m)². For example, if you weigh 75kg and are 1.73m tall, your BMI would be 75 ÷ (1.73 × 1.73) = 25.1. Once you've computed your BMI, the NHS classifies it into 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 thresholds — healthy weight starts at 18.5 and overweight begins at 23.0, reflecting higher health risks at lower body mass index values in these populations.
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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.
BMI Limitations and Body Composition
While BMI is useful for screening, it has important limitations. BMI does not distinguish between muscle and fat percentage — athletes and very muscular individuals may compute a high BMI yet have low body fat. This is why body composition matters too. Waist circumference is also important: men with waists over 94cm and women over 80cm face increased health risk, regardless of BMI. Your GP can assess your overall health beyond BMI alone. Complementary measures like lean body mass assessment and visceral fat measurement provide a fuller picture of your health status.
- 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 does my computed BMI actually mean for my health?
BMI is a screening tool, not a diagnosis. A healthy BMI (18.5–24.9 according to NHS standards) suggests lower risk of weight-related diseases like type 2 diabetes and cardiovascular disease. However, BMI alone doesn't account for fitness level, muscle mass, or where fat is stored. Your GP can interpret your BMI within your full health context.
How do I compute BMI if I use stone and pounds instead of kilograms?
Convert to kilograms first, then apply the formula. One stone equals 6.35kg, and one pound equals 0.454kg. Alternatively, use our free weight converter above to switch between units instantly, then use the BMI calculator to compute your result immediately.
Is the BMI formula the same for women and men?
Yes, the BMI formula is identical for both sexes: weight (kg) ÷ height (m)². However, healthy body fat percentages differ between men and women. Men typically have lower healthy body fat (6–17%), whilst women have higher healthy ranges (14–24%). BMI categories are the same, but body composition assessment may differ.
Why does the NHS recommend different BMI thresholds for some ethnic groups?
South Asian, Chinese, Black African, and Black Caribbean populations face higher risk of type 2 diabetes and heart disease at lower BMI values. The NHS therefore recommends lower thresholds — overweight begins at BMI 23.0 rather than 25.0 for these groups. This reflects genetic and metabolic differences in disease risk.
What's the difference between computing BMI and assessing body composition?
BMI is a weight-to-height ratio only. Body composition measures the proportion of fat, muscle, bone, and water in your body. A high BMI might indicate obesity, or simply high muscle mass. A normal BMI doesn't assures healthy body composition. Professional body fat assessment provides more detailed health information than BMI alone.
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.