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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What Is the MM Equation for Body Composition?
The MM equation is a formula used to estimate muscle mass percentage from body weight and body fat percentage. It's one of several methods fitness professionals and healthcare practitioners use in the UK to assess body composition — the ratio of muscle to fat in your body — rather than relying on BMI alone. This approach provides more detailed insight into whether weight changes reflect muscle gain, fat loss, or both, making it useful alongside our free BMI calculator and weight converter for understanding your overall health picture.
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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.
How the MM Equation Works
The MM equation uses a simple two-step calculation:
- Step 1: Calculate fat mass in kg: body weight × (body fat percentage ÷ 100)
- Step 2: Calculate muscle mass in kg: body weight − fat mass
- Convert to percentage: (muscle mass ÷ body weight) × 100
For example, a person weighing 80 kg with 25% body fat has 20 kg of fat mass and 60 kg of muscle mass — a muscle mass percentage of 75%. The equation assumes all non-fat mass is muscle, though technically this includes bones, organs, and water. NHS guidance acknowledges that body composition assessment is more nuanced than BMI alone, particularly for athletes or older adults experiencing natural muscle loss. The accuracy of the MM equation depends entirely on the accuracy of the initial body fat percentage measurement.
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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.
Accuracy, Limitations and When to Use It
The MM equation is only as accurate as the body fat measurement it's based on. Methods like DEXA scans, bioelectrical impedance analysis (BIA), or skinfold calipers can all introduce error. The equation also oversimplifies: it groups muscle, bone, organs, and water as a single "fat-free mass" category. For weight management and health assessment in the UK context, the equation works best as part of a broader picture. Consider using it alongside waist circumference, which the NHS recognises as a key indicator of cardiovascular risk independent of BMI. For personalised nhs health advice about your body composition and weight management strategy, consult your GP or a registered dietitian who can recommend the most appropriate assessment methods for your individual needs.
- 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 MM equation and BMI?
BMI measures weight relative to height only, without distinguishing muscle from fat. The MM equation attempts to separate muscle mass from fat mass, giving a more detailed body composition picture. However, BMI remains the NHS standard screening tool for weight-related health risk, whilst the MM equation is more specialised and requires a body fat measurement first.
How accurate is the MM equation?
Accuracy depends on the body fat percentage measurement used as input. If your body fat percentage is measured using DEXA (gold standard), the MM equation is reasonably reliable. If estimated using bioelectrical impedance or equations, accuracy may be lower. The equation also simplifies body composition by treating all non-fat tissue as muscle, which isn't entirely precise.
Can I use the MM equation to track my fitness progress?
Yes — if you measure body fat percentage consistently using the same method, the MM equation can help track whether weight changes reflect muscle gain or fat loss. This is useful for fitness goals. However, for general health and weight management in the UK, the NHS recommends monitoring waist circumference and BMI alongside lifestyle changes rather than relying solely on muscle mass percentages.
Should I use the MM equation instead of my GP's advice about healthy weight?
No. The MM equation is a specialist body composition tool, not a substitute for medical assessment. Your GP uses BMI, waist circumference, and your individual health history to advise on healthy weight. If you're interested in body composition for fitness or specific health reasons, discuss this with your GP — they can recommend appropriate assessment methods and refer you to specialist services if needed.
Does the MM equation account for bone density and organ weight?
No — the MM equation assumes all non-fat mass is muscle, but this category also includes bones, organs, water, and connective tissue. For a more detailed breakdown, you would need advanced imaging like DEXA scans. For most people managing weight and health in the UK, the MM equation is less relevant than understanding overall BMI, body fat percentage, and waist circumference together.
Ready to take control of your weight loss journey?
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