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 Loss
Water weight loss refers to losing water retained in your body rather than fat loss. Your weight naturally fluctuates day-to-day due to hydration, sodium intake, hormones, and exercise — these changes are temporary water shifts, not sustainable weight management. Understanding the difference between water weight and true fat loss is crucial for setting realistic weight loss goals and staying motivated during your health journey. Most rapid initial weight loss from dieting or exercise is water, not fat. True fat loss happens gradually through consistent calorie deficit combined with physical activity.
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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.
Why Your Weight Fluctuates — Water vs Fat Loss
Your body weight changes constantly, and most day-to-day fluctuations are water weight, not fat loss. Here's why:
- Sodium intake: High-sodium foods cause your body to retain water. A salty meal can add 0.5–1 kg of water weight overnight.
- Carbohydrates: Each gram of carbohydrate binds approximately 3 grams of water. After a high-carb meal, weight increases temporarily.
- Exercise: Muscle damage during workouts causes inflammation and water retention — you may weigh more after starting exercise even if losing fat.
- Hormones: Oestrogen and progesterone fluctuations affect water retention, particularly in the week before menstruation.
- Hydration status: Dehydration reduces weight temporarily; rehydration increases it again.
True fat loss requires a consistent calorie deficit. The NHS recommends losing 0.5–1 kg per week for sustainable results — this is primarily fat loss, not water.
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.
Real Weight Loss vs Water Fluctuations
To distinguish real progress from temporary water weight changes, track patterns over 2–4 weeks rather than daily weigh-ins. Use our free weight converter and BMI calculator to monitor your actual health metrics alongside weight. A realistic approach: expect weight loss of 0.5–1 kg per week. In the first 1–2 weeks of dieting, you may lose 2–3 kg due to water loss and glycogen depletion — this is normal but unsustainable. After this initial phase, weight loss should slow to a steadier pace reflecting genuine fat loss. For sustainable weight management in the UK, combine dietary changes with increased physical activity, monitor your BMI, and consult your GP if you're concerned about rapid weight changes.
- 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 water weight loss real progress?
Water weight loss is real in that you genuinely weigh less, but it's temporary. It reflects changes in hydration and glycogen stores, not fat loss. True progress is sustainable fat loss of 0.5–1 kg per week, which the NHS recommends as safe and achievable.
How much water weight can you lose in a day?
Water weight can fluctuate 1–2 kg daily depending on sodium intake, exercise, hormones, and hydration. This is completely normal. Don't be alarmed by daily variations — focus on weekly or monthly trends instead.
Does drinking more water help you lose weight?
Drinking adequate water supports overall health and may slightly increase calories burned during digestion. However, water itself has no calories. Weight loss requires a calorie deficit. Proper hydration is important for sustainable weight management but isn't a weight loss solution on its own.
How long does it take to lose actual fat weight?
Real fat loss typically becomes visible after 2–4 weeks of consistent calorie deficit and exercise. The NHS recommends 0.5–1 kg per week as a safe, sustainable rate. Use our BMI calculator to track genuine health progress beyond just weight numbers.
Can GLP-1 weight loss medication help with water retention?
GLP-1 medications like Wegovy support weight loss through appetite regulation and reduced calorie intake, resulting in genuine fat loss alongside initial water loss. Individual results vary. Prescription weight loss medication requires clinical assessment and works best alongside lifestyle changes and proper hydration.
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.