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
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Body Mass Index Converter Explained
A body mass index converter helps you calculate and understand your BMI quickly using your weight and height. BMI is a screening tool that uses your weight in kilograms and height in metres to determine whether you fall into a healthy weight range according to NHS guidelines. It's useful for spotting health patterns, though it doesn't account for muscle mass, body fat percentage, or lean body mass distribution. Our free BMI calculator above lets you convert your measurements instantly and see where you fall within NHS weight categories. Individual results vary based on age, fitness level, and personal health factors.
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 a Body Mass Index Converter Works
A body mass index converter uses a simple mathematical formula to calculate your BMI from your weight and height. The metric formula is: BMI = weight (kg) ÷ height (m)². For example, if you weigh 70 kg and are 1.75 metres tall, your BMI would be 70 ÷ (1.75 × 1.75) = 22.9, which falls in the healthy weight range.
According to NHS guidelines, BMI categories are:
- 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 people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds. For these groups, a healthy BMI is 18.5–22.9, and overweight starts at 23.0. This reflects higher cardiovascular and diabetes risk at lower BMI levels for these populations. Use our free weight converter above to switch between kg, stone, and pounds if you need to work in different units.
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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 Context
A body mass index converter is a useful screening tool, but it has important limitations. BMI does not distinguish between muscle and fat — athletes with high muscle mass may have an elevated BMI but low body fat percentage. BMI also doesn't account for where fat is stored; visceral fat around organs carries greater health risks than subcutaneous fat beneath the skin. For accurate body composition assessment, measures like body fat percentage, lean body mass, and waist circumference provide additional context.
The NHS recommends checking your waist measurement alongside BMI: men over 94 cm (37 inches) and women over 80 cm (31.5 inches) face increased health risk. Try our BMI calculator on this page for a personal assessment, then speak with your GP if you have concerns about your weight or 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 BMI and body fat percentage?
BMI is a weight-to-height ratio that doesn't distinguish muscle from fat. Body fat percentage measures the actual proportion of fat tissue in your body, giving a clearer picture of body composition. Athletes may have high BMI but low body fat. A body composition assessment using body fat percentage is more accurate for fitness contexts, while BMI is useful for population-level health screening.
Does a body mass index converter work for children?
No. Children use age and sex-specific growth charts called centiles, not adult BMI categories. The NHS uses UK90 growth reference charts for children. If you're checking a child's weight, use the NHS Healthy Weight Calculator for children, which accounts for growth patterns and development.
Why do different ethnicities have different BMI thresholds?
Research shows that South Asian, Black African, Caribbean, and Chinese populations face higher risks of type 2 diabetes and cardiovascular disease at lower BMI levels than other groups. The NHS acknowledges this by recommending thresholds of 18.5–22.9 for these populations, compared to 18.5–24.9 for others. This ensures fairer, more personalised health assessment.
Should I use kg or stone for BMI calculation?
Either works — a body mass index converter handles both. In the UK, you can enter weight in stone and pounds, or kilograms; the calculator will convert automatically. The NHS often uses kilograms clinically, but many UK adults find stone and pounds more familiar. Use whichever units feel natural to you.
What else should I check alongside my BMI?
For a complete picture, also measure your waist circumference and assess body fat percentage. Check your blood pressure, cholesterol, and blood sugar levels — these matter even if your BMI is healthy. Your GP can provide personalised guidance on what other health markers matter for you specifically.
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.