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
Muscle Calculator — Understanding Body Composition in the UK
A muscle calculator helps you estimate your lean muscle mass and body fat percentage — information that BMI alone cannot provide. Unlike a standard BMI calculator, which only uses height and weight, a muscle calculator considers factors like age, sex, and fitness level to give you a more complete picture of your body composition. This is particularly useful in the UK where many people rely solely on BMI for health assessment, missing important insights into whether weight comes from muscle or fat. Understanding your muscle mass can help guide your health and fitness decisions alongside NHS guidance.
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 UK Muscle Calculators Work
Muscle calculators use various equations to estimate your lean body mass — the weight of everything in your body except fat. The most common methods in UK health guidance include the Katch-McArdle formula and Devine formula, both recognised by NHS fitness services and personal trainers across the UK.
These calculators typically ask for:
- Age and sex
- Height and weight
- Waist circumference or body fat percentage (if known)
- Activity level or fitness experience
From this data, the calculator estimates your muscle mass in kilograms and provides a body fat percentage. This is more informative than BMI alone, which doesn't distinguish between muscle (heavy) and fat (lighter). For example, an athletic person might have a BMI in the "overweight" range but actually have excellent body composition with high muscle mass. Use our free BMI calculator above to compare your BMI alongside these composition insights.
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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.
UK Health Context — Why Muscle Mass Matters
The NHS emphasises that body composition is more clinically relevant than weight alone for assessing health risk. Muscle is metabolically active — it burns calories at rest, supports bone density, and protects against injury and age-related decline.
According to UK health guidance, healthy muscle ranges vary by age and sex:
- Adult men: typically 30–40% of body weight as muscle
- Adult women: typically 20–30% of body weight as muscle
- Older adults: maintaining muscle becomes even more critical for mobility and independence
If you're tracking weight loss or fitness progress, use our free progress tracker to log both weight and body composition changes over time. Combined with our weight converter tool to switch between stone, lbs, and kilograms, you'll have a complete picture of your health journey. Your GP can also discuss healthy muscle targets specific to your age and individual circumstances.
- 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.
Is a muscle calculator more accurate than BMI?
A muscle calculator provides different information than BMI. BMI is a screening tool based on height and weight only; a muscle calculator estimates body composition. Neither is perfect — muscle calculators use formulas that estimate rather than measure directly. For the most accurate assessment, bioelectrical impedance analysis (BIA) or DEXA scans are available through some UK private health providers. Your GP can advise on what's most appropriate for your health goals.
What is a healthy muscle percentage for UK adults?
Healthy muscle percentage varies by age and sex. Adult men typically have 30–40% of body weight as muscle; adult women 20–30%. As we age, maintaining muscle becomes more important for mobility and bone health. UK health guidance emphasises that some muscle loss is normal with age, but regular strength training helps preserve it. Consult your GP for personalised targets based on your age and health status.
Can I use a muscle calculator if I'm overweight or obese?
Yes. Muscle calculators work for all BMI ranges. In fact, they're particularly useful for overweight and obese individuals because they help distinguish between muscle and fat. Someone with a high BMI might have significant muscle mass alongside excess fat. Understanding your composition helps you set realistic fitness goals and track meaningful progress beyond the scale alone.
How does muscle mass affect weight loss?
Muscle weighs more than fat per unit volume, so building muscle during weight loss can slow scale progress even though your body is improving. This is why tracking body composition and using tools like our progress tracker is valuable — you might lose fat while gaining or maintaining muscle, which is a positive health outcome. Combine strength training with a calorie deficit for optimal results, and discuss your approach with your GP.
Where can I find more UK health guidance on body composition?
The NHS Health and Wellbeing service provides guidance on healthy weight, body composition, and fitness. Your GP can refer you to NHS weight management services or local fitness programmes if needed. MyWeightCheck offers free tools including a BMI calculator, weight converter, and progress tracker to support your health journey alongside professional NHS guidance.
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.