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
What Is the NHS Body Mass Calculator?
The NHS body mass calculator is a simple tool that measures your BMI (body mass index) — a ratio of your weight to height. It helps identify whether your weight falls into a healthy range according to NHS guidelines. BMI is calculated using a standard formula and categorised into ranges: underweight, healthy weight, overweight, and obese. While BMI is a useful screening tool, it's important to understand that it doesn't measure body fat percentage directly or account for muscle mass, so it works best alongside other health measures. You can calculate your personal BMI instantly using our free BMI calculator above.
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.
How NHS Body Mass Categories Work
The NHS uses five BMI categories to help assess weight-related health risk. A healthy BMI is between 18.5 and 24.9 for most UK adults. A BMI of 25 to 29.9 is classified as overweight, 30 to 34.9 as obese (Class I), 35 to 39.9 as obese (Class II), and 40 and above as severely obese (Class III).
Importantly, the NHS recommends lower BMI thresholds for adults of South Asian, Chinese, Black African, and Black Caribbean heritage — these groups have a higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels. For these populations, a healthy BMI range is 18.5 to 22.9, and overweight begins at 23.
The NHS formula is straightforward: BMI = weight (kg) ÷ height (m)². Use our free weight converter above to switch between stone, pounds, and kilograms so you can apply the calculation accurately.
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 Body Composition Context
BMI does not distinguish between muscle and fat — athletes or very muscular individuals may have a high BMI but low body fat percentage, while still being very healthy. This is why body composition measures like body fat percentage, waist circumference, and lean body mass provide important additional context.
The NHS recommends measuring waist circumference alongside BMI: over 94cm (37 inches) in men or 80cm (31.5 inches) in women indicates increased health risk. Visceral fat — fat stored around internal organs — is particularly linked to health problems, even in people with a normal BMI. For a full picture of your health, discuss your personal BMI result with your GP, especially if you have weight-related health concerns.
- 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 is the difference between BMI and body fat percentage?
BMI is a ratio of weight to height and doesn't measure actual fat. Body fat percentage measures the proportion of your weight that is fat tissue. Two people with the same BMI can have very different body fat percentages depending on muscle mass. Body fat percentage is more accurate for assessing health risk but requires special equipment to measure.
Can I use the NHS body mass calculator if I'm pregnant?
BMI is less reliable during pregnancy because weight gain is expected and necessary. If you're pregnant, discuss healthy weight management with your midwife or GP rather than relying on standard BMI categories. Your healthcare team can provide personalised guidance.
Is there an NHS body mass calculator tool online?
Yes — the NHS provides a free BMI calculator on the NHS Every Mind Matters website and through your GP's practice. MyWeightCheck also provides a free BMI calculator above that uses NHS categories and includes the ethnicity-adjusted thresholds recommended by the NHS.
What should I do if my BMI falls in the overweight or obese range?
First, discuss your result with your GP — BMI is just one health indicator. Your GP can assess other factors like blood pressure, blood sugar, and family history. If weight management is recommended, the NHS offers tiered support: self-help resources (Tier 1), GP-led programmes (Tier 2), specialist weight management services (Tier 3), and bariatric surgery if appropriate (Tier 4).
Do children use the same NHS BMI calculator as adults?
No — children have different BMI calculations. The NHS uses age and sex-specific growth charts (UK90 reference) for children under 18, not adult BMI categories. If you're checking a child's weight, use the NHS child BMI calculator or ask your GP for a health assessment.
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.