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 Calculate BMI — The Formula Explained
The BMI (Body Mass Index) formula is a simple calculation used to assess whether an adult's weight is healthy for their height. The BMI calculation divides weight in kilograms by height in metres squared — this gives you a single number that the NHS uses to screen for underweight, healthy weight, overweight, and obesity. Understanding the BMI formula helps you interpret your result and what it means for your health.
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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 How to Calculate It
The BMI formula in metric units (used in the UK) is:
- BMI = weight (kg) ÷ height (m)²
To calculate your BMI, first convert your height to metres. For example, if you are 1.73 metres tall and weigh 70kg, your calculation would be: 70 ÷ (1.73 × 1.73) = 70 ÷ 2.99 = 23.4. This BMI of 23.4 falls in the healthy BMI range of 18.5–24.9 according to NHS guidelines. The healthy BMI threshold starts at 18.5 and extends to 24.9 for adults of most backgrounds, though the NHS BMI categories differ slightly for South Asian, Chinese, Black African, and Black Caribbean adults. Use our free BMI calculator above to compute your result instantly without a UK pharmacy calculation.
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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 Categories and NHS Ranges
Once you have calculated your BMI using the formula, the NHS places your result into categories. An overweight BMI threshold begins at 25.0, whilst obesity starts at 30.0. The BMI categories UK standard divides results into: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25.0–29.9), obese class I (30.0–34.9), obese class II (35.0–39.9), and severely obese (40.0 and above). However, BMI limitations mean this formula does not account for muscle mass, bone density, or where fat is stored — so it is a screening tool, not a diagnostic one. Your GP can provide personalised guidance about what your BMI category means for your individual 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.
What's the difference between the metric and imperial BMI formulas?
The metric formula (used in the UK) divides weight in kg by height in metres squared. The imperial version divides weight in pounds by height in inches squared, then multiplies by 703. Both produce the same BMI result — the metric version is simply easier to calculate because the NHS uses kilograms and metres.
Is BMI accurate for everyone?
BMI is a useful screening tool but has limitations. It does not distinguish between muscle and fat, so athletes may have a high BMI despite low body fat. It also does not account for age, gender, ethnicity differences, or where fat is stored. Waist circumference and other measures provide additional health context.
What BMI calculation do I need if I'm of South Asian or Black African heritage?
The NHS recommends lower BMI thresholds for adults of South Asian, Chinese, Black African, and Black Caribbean backgrounds because these groups have higher cardiovascular and diabetes risk at lower BMI levels. For these groups, healthy BMI is 18.5–22.9 instead of 18.5–24.9.
Can I calculate my BMI if I'm pregnant?
BMI calculations during pregnancy use the same formula but are interpreted differently. Pre-pregnancy BMI is more relevant. If you're pregnant, your GP or midwife can provide personalised weight guidance, as standard BMI categories do not apply during pregnancy.
What's a healthy weight range for my height using the BMI formula?
To find your healthy weight range, multiply 18.5 by your height in metres squared for the lower bound, and 24.9 by the same figure for the upper bound. For example, at 1.73m, healthy weight is 55.4–74.5kg (8st 10lbs to 11st 10lbs). Use our weight converter and BMI calculator above to check your personal range instantly.
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.