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 Do a BMI Test — Step-by-Step Guide
A BMI (Body Mass Index) test is a simple measurement that compares your weight to your height to assess whether you're in a healthy weight range. You can calculate your BMI in just a few seconds using a straightforward formula, or use our free BMI calculator above to get an gradual improvement. This guide explains exactly how to do a BMI test yourself, what your result means according to NHS guidelines, and why BMI is a useful but limited health screening tool for UK adults.
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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 BMI — The Formula and Three Easy Steps
Calculating your BMI requires just two pieces of information: your weight in kilograms and your height in metres. The BMI formula is: weight (kg) ÷ height (m)². Here's how to do a BMI test manually in three steps:
- Step 1: Measure your weight in kg (or convert to kg if using stone and pounds)
- Step 2: Measure your height in metres — convert cm to metres by dividing by 100
- Step 3: Divide your weight by your height squared (height × height)
For example, if you weigh 70kg and are 1.75m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 BMI. According to NHS guidelines, this result falls in the healthy weight range (18.5–24.9). Use our BMI calculator on this page for an instant calculation, or try our weight converter if you need to convert between stone, pounds, and kilograms first.
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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.
Understanding Your BMI Result — NHS Categories and What They Mean
Once you've calculated your BMI, compare it to the NHS BMI categories to see what your result means. A BMI below 18.5 is underweight, 18.5–24.9 is a healthy weight, 25.0–29.9 is overweight, 30.0–34.9 is obese (Class I), 35.0–39.9 is obese (Class II), and 40.0 or above is severely obese (Class III). If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds: a healthy BMI range of 18.5–22.9 and overweight starting at 23.0. Your GP can discuss what your specific BMI means for your health and whether any action is needed, especially if other health factors are involved.
- 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.
Can I do a BMI test at home without equipment?
Yes. You only need to know your weight and height. Weigh yourself on bathroom scales and measure your height using a tape measure against a wall, or use your height from a recent GP appointment. Then use the BMI formula or our free calculator above to get your result instantly.
What's the difference between doing a BMI test myself and at my GP?
The calculation is identical. Your GP will use the same BMI formula using your measured weight and height. A GP appointment is useful if you want personalised health advice about your BMI result, especially if you have other health concerns or weight-related conditions.
Is BMI an accurate measure of health?
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat — athletes may have a high BMI despite low body fat. It also doesn't account for where fat is stored (waist circumference matters too) or individual health factors. Your GP can advise on whether BMI alone is enough or if other measurements are needed.
My BMI is 27 — am I overweight?
According to NHS guidelines, a BMI of 27 falls in the overweight category (25.0–29.9). However, if you're of South Asian or Black African heritage, you may be advised to aim for a lower BMI. Speak to your GP about whether your weight affects your health, especially if you have other risk factors like high blood pressure or type 2 diabetes.
Can I use the BMI test to check if my child is a healthy weight?
Adult BMI categories don't apply to children — they use different age and sex-specific centile charts. Use the NHS Healthy Weight Calculator for children instead, or ask your GP or health visitor for guidance on your child's healthy weight range.
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.