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
Fat Loss vs Weight Loss — Understanding the Difference
When you step on the scales, you're measuring total body weight — but that number doesn't tell you whether you've lost fat, muscle, or water. Fat loss and weight loss are not the same thing, and understanding the difference is crucial for sustainable health management. This UK health guide explains what matters for your wellbeing and how to track genuine progress beyond the number on the scales.
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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.
What's the Difference Between Fat Loss and Weight Loss?
Weight loss is a decrease in total body weight from any source — fat, muscle, water, or glycogen. It's what your scales measure. Fat loss is specifically the reduction of body fat tissue whilst preserving muscle mass. This is what actually improves your health.
The distinction matters because:
- You can lose weight whilst gaining fat (if you lose muscle through crash dieting)
- You can lose fat whilst staying the same weight (if you gain muscle through exercise)
- gradual weight management often includes significant muscle loss, which slows metabolism
- Fat loss with preserved muscle gives sustainable, long-term health benefits
According to NHS guidance, healthy weight management involves losing fat whilst maintaining or building muscle mass through a combination of reduced-calorie intake and increased physical activity. Our free BMI calculator and weight converter can help you track your starting point, but body composition tells a more complete health story than BMI alone.
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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.
How to Achieve Fat Loss Rather Than Just Weight Loss
The best approach to fat loss combines three elements: a moderate calorie deficit (around 500 kcal per day for roughly 0.5 kg weekly loss), resistance exercise to preserve muscle, and adequate protein intake.
Crash dieting causes gradual weight management but much of it comes from muscle — leaving you with a higher body fat percentage despite a lower number on the scales. NHS guidance recommends losing weight at 0.5–1 kg per week for sustainable fat loss.
For those exploring prescription weight management options, medications like Wegovy (semaglutide) are designed to support fat loss through appetite regulation alongside lifestyle changes. Our GLP-1 weight loss calculator can help you understand potential outcomes with medical support. Track your progress using our free progress tracker to monitor both weight and how you feel — energy, fitness, and clothing fit often improve before the scales shift significantly.
- 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.
Can you lose weight without losing fat?
Yes. gradual weight management often includes significant muscle and water loss alongside fat loss. Crash diets can result in losing 50% muscle and 50% fat, which is unhealthy. This is why NHS guidance recommends gradual weight loss (0.5–1 kg per week) combined with resistance exercise to preserve muscle mass.
Why is muscle mass important for weight management?
Muscle tissue burns more calories at rest than fat tissue. Preserving muscle maintains your metabolic rate, making it easier to sustain weight loss long-term. Losing muscle slows metabolism, making future weight loss harder and weight regain more likely.
How do I know if I'm losing fat or just water weight?
Weight fluctuates daily due to hydration, hormones, and food intake (1–2 kg swings are normal). True fat loss appears as consistent downward trends over 2–4 weeks, plus improvements in how clothes fit, energy levels, and how you feel. Body measurements (waist, hips) often decrease before scales reflect fat loss.
Does BMI distinguish between fat loss and weight loss?
No. BMI calculates weight divided by height squared — it doesn't distinguish muscle from fat. This is why BMI has limitations. Body composition analysis, waist circumference, and how you feel are better indicators of health progress. Use our free BMI calculator as a screening tool, but don't rely on it alone.
Can prescription weight loss medication help with fat loss specifically?
Weight loss medications like Wegovy work by reducing appetite and supporting a calorie deficit. Combined with physical activity and adequate protein, they can help achieve fat loss rather than muscle loss. Clinical trials showed weight loss results, but individual outcomes vary and must be combined with lifestyle changes for sustainable fat loss.
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.