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 Retention And Weight Loss
Water retention is a common reason why the scales don't move, even when you're losing fat. Your body can hold several kilograms of fluid due to hormones, diet, exercise, and medications — temporarily masking genuine weight loss progress. Understanding this difference helps you stay motivated and track your health accurately using measurements beyond just body weight alone.
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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.
UK Prescription Weight Management Options
If you're struggling with weight management and water retention is affecting your progress, several prescription-only weight loss medications are available in the UK for eligible adults. These include Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) — all licensed for weight management through the NHS or privately.
- Mounjaro: Dual GIP and GLP-1 receptor agonist, once-weekly injection. Average clinical weight loss: 15–21% of body weight over 72 weeks.
- Wegovy: GLP-1 receptor agonist, once-weekly injection. Average clinical weight loss: approximately 15% of body weight.
- Saxenda: GLP-1 receptor agonist, daily injection. Effective for sustained weight management.
These medications may help reduce appetite, support feeling fuller after smaller meals, and assist with consistent weight management — though individual results vary significantly. Try our free GLP-1 weight loss calculator to see potential outcomes at different doses, and use our BMI calculator to check your eligibility range.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
NHS vs Private Access And Next Steps
NHS access: Available through specialist tier 3 weight management services following a GP referral. Access varies by region and waiting lists can be substantial. Your GP can advise on local services.
Private access: Available through GPhC-registered online prescribing services following a clinical assessment. No NHS referral needed, but costs typically range from £129–£320 monthly depending on dose.
Next step: Speak with your GP about whether prescription weight management is appropriate for your situation. Be honest about water retention concerns — they can advise whether this is a natural fluctuation or a sign of an underlying condition. Your GP can also assess your BMI, health history, and eligibility for treatment options available in the UK.
- 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.
How much weight can water retention add to the scales?
Water retention can add 1–3 kg (2–7 lbs) to your weight temporarily. Hormonal changes, high sodium intake, intense exercise, and some medications can increase fluid retention. This typically normalises within a few days and doesn't represent fat loss or gain.
Will Mounjaro or Wegovy help with water retention?
GLP-1 and GIP/GLP-1 medications work by reducing appetite and supporting consistent weight management — they don't specifically treat water retention. However, by supporting stable eating patterns and reducing sodium intake (often easier with reduced appetite), they may help manage fluid balance indirectly.
Should I weigh myself daily if I have water retention issues?
No. The NHS recommends weighing yourself once weekly, ideally on the same day and time, to avoid confusion from daily water fluctuations. Weekly measurements give a clearer picture of true weight trends. Use our free progress tracker to log and visualise your journey over time.
What's the difference between water retention and actual weight loss?
Water retention is temporary fluid held in tissues, usually lasting days. Actual weight loss is reduction in fat and muscle mass over weeks. Track both: weigh weekly, measure waist circumference, and note how clothes fit. This gives a complete health picture beyond the scales.
Can I be eligible for prescription weight loss medication if I have water retention?
Yes. Eligibility is based on BMI (30+, or 27+ with weight-related health conditions) and individual health assessment — not on water retention status. Your GP or prescriber will assess your suitability. Use our BMI calculator to check your eligibility range and discuss options with your healthcare provider.
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.