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
Understanding Water Weight in Your Weight Loss Journey
When you start a weight management programme in the UK—whether through lifestyle changes, diet, or prescription medication like Mounjaro or Wegovy—much of your initial weight loss comes from water, not fat. Water weight fluctuates naturally due to hormones, sodium intake, exercise, and hydration levels. Understanding the difference between water weight and true fat loss helps you stay motivated and realistic about your progress. Our free BMI calculator and progress tracker help you monitor your overall health journey beyond daily scale readings.
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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 Water Weight Affects Your Weight Loss
When you reduce calories, change your carbohydrate intake, or begin weight management medication like GLP-1 injections, your body sheds water along with fat. This is why people often lose several kilograms in the first week or two—much of this is water, not fat tissue. Typical water weight loss accounts for 1–2 kg in the first few days of a reduced-calorie diet or when starting prescription weight loss medication.
- Carbohydrate restriction causes rapid water loss (carbs retain water in muscle)
- Sodium reduction lowers fluid retention
- Exercise increases water loss through sweat
- Hormonal changes (especially in women) affect water retention throughout the cycle
- Medications like Mounjaro and Wegovy slow stomach emptying, which can initially affect hydration and water weight
True fat loss is slower—typically 0.5–1 kg per week on a sustainable calorie deficit. Use our free progress tracker to log your weight over several weeks and spot the real trend beneath daily fluctuations.
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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 Prescription Weight Loss & Water Weight
If you're considering prescription weight loss medication available on the NHS or privately, understanding water weight helps set realistic expectations. Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 weight management injections licensed in the UK—clinical trials showed average weight loss of 15–21% over several months alongside diet and activity changes. Initial rapid loss includes water weight, but sustained loss over weeks and months is primarily fat.
To find out if you're eligible for prescription weight management through the NHS, speak to your GP about tier 3 specialist weight management services in your area. For private prescribing, a UK-licensed prescriber will assess your BMI and health history. Calculate your BMI instantly using our free BMI calculator to understand your starting point, and try our GLP-1 weight loss calculator to estimate potential results at different doses.
- 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 all my initial weight loss just water weight?
No. On a calorie deficit, roughly 25–30% of initial weight loss is water, with the rest from fat and a small amount of muscle. This is why the first week shows dramatic loss—after that, the rate slows to the sustainable 0.5–1 kg per week recommended by the NHS.
How long does water weight loss last?
Initial water weight loss typically peaks within the first 2–7 days of dietary change or starting medication. After that, you may regain 1–2 kg of water if you increase carbs or sodium again. True fat loss continues steadily over weeks and months.
Will I regain water weight if I stop my diet or medication?
Yes, some water weight often returns when you resume normal eating or stop medication—this is normal and not fat regain. However, sustainable weight loss requires ongoing lifestyle changes or continued medication as prescribed by your GP or prescriber.
Does Mounjaro or Wegovy cause water weight gain?
No. Both medications may reduce initial bloating due to slower stomach emptying, which can temporarily affect water balance. Any weight loss with these medications is primarily fat loss sustained over months, as shown in clinical trials.
How do I know if I'm losing real fat, not just water weight?
Track weight over 4–6 weeks rather than daily. True fat loss shows a downward trend despite daily fluctuations. Clothing fit, energy levels, and reduced waist circumference are also reliable signs. Use our free progress tracker to log weekly averages and spot the true pattern.
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.