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 Fat Free Mass and Why It Matters in the UK
Fat free mass (FFM) is the total weight of everything in your body except fat — including muscle, bone, water, and organs. Understanding your fat free mass is important for assessing overall body composition and health, which goes beyond BMI alone. The NHS recognises that body composition matters more than weight alone for predicting health outcomes. This UK health guide explains what fat free mass is, how it differs from body fat percentage, and why it's a useful measure alongside BMI for understanding your weight and wellness.
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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 Fat Free Mass Differs from BMI and Body Fat
BMI measures total weight relative to height, but it does not distinguish between muscle and fat. Two people with the same BMI can have very different body compositions — one might be muscular with low body fat, while another might have high body fat and less muscle. This is why fat free mass is a more detailed measure of body composition.
Fat free mass includes lean tissue: skeletal muscle, organs, bone density, water, and connective tissue. A higher fat free mass relative to your total weight means you have more muscle and less fat, which is generally associated with better metabolic health, bone strength, and functional fitness. NHS guidance acknowledges that preserving and building muscle mass is important for long-term health, particularly as we age. You can calculate your BMI using our free BMI calculator above to establish your baseline, then assess your body composition for a fuller picture of your health status.
- Fat free mass typically accounts for 70–90% of total body weight in healthy adults
- Muscle tissue is metabolically active — it burns calories even at rest
- As we age, fat free mass naturally declines unless we maintain physical activity
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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.
Measuring Fat Free Mass — UK Tools and GP Guidance
Several methods can estimate fat free mass, though accuracy varies. DEXA scans (dual-energy x-ray absorptiometry) are the gold standard and are available through NHS services for specific clinical reasons. Bioimpedance analysis (BIA) scales estimate body composition by measuring how electrical current passes through tissue, but accuracy depends on hydration levels and device quality. Skinfold calipers measure subcutaneous fat and calculate body composition, though they require trained assessment.
Your GP can advise on whether a formal body composition assessment is appropriate for your health goals. For practical tracking, you can use weight converter tools alongside progress tracking to monitor changes in total weight and estimate body composition trends over time. Our free weight converter helps you track weight changes in your preferred UK measurements (stone, pounds, or kilograms), which supports informed conversations with your healthcare team about weight management and fitness progress.
- 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 fat free mass and fat mass?
Fat free mass is everything except fat — muscle, bone, organs, and water. Fat mass is only body fat. Together they make up your total body weight. A healthy body composition balances both; high fat free mass relative to fat mass is generally associated with better health outcomes and metabolic function.
Why does fat free mass matter more than total weight?
Two people at the same weight can have very different health profiles depending on their muscle-to-fat ratio. Muscle is denser than fat, so someone muscular might weigh more but be healthier. Fat free mass reflects metabolic health and functional fitness better than total weight alone, which is why the NHS recognises body composition as important alongside BMI.
How can I increase my fat free mass?
Resistance training and strength exercises are the most effective ways to build muscle (fat free mass). Combined with adequate protein intake and calorie management, regular strength work preserves and builds lean tissue. This is particularly important as we age — the NHS encourages muscle-strengthening activities at least twice weekly for adults.
Can the NHS measure my body composition?
Some NHS services, particularly specialist weight management services (tier 3), may assess body composition. DEXA scans are available through the NHS but typically only for clinical reasons such as osteoporosis screening. Ask your GP whether a body composition assessment would be appropriate for your health situation.
Is a high fat free mass always better?
Generally yes, but context matters. Very high fat free mass with very low body fat requires significant training and dietary discipline. For most people, a healthy balance is the goal — sufficient muscle to support metabolic health, bone strength, and function, combined with moderate body fat levels appropriate for your age and health status.
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.