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
Average Weight Loss First Week On Mounjaro — What To Expect
Most people starting Mounjaro experience minimal weight loss in the first week — typically 0 to 2 lbs (0 to 1 kg). This is normal and expected. Initial weight loss is often fluid loss rather than fat loss, as your body adjusts to the medication. The significant weight loss comes later as you progress through dose escalations. Understanding what constitutes healthy weight loss and how it relates to your BMI helps set realistic expectations for your weight management journey.
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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 First Week Weight Loss Is Modest — And What That Means For Your BMI
In clinical trials, Mounjaro showed an average weight loss of around 15–21% of body weight over 72 weeks — not in the first week. During week one, most users experience minimal weight reduction, typically 0–1 kg, because your body is still adjusting to the 2.5 mg starting dose. Any early weight loss is usually fluid rather than fat loss. This is completely normal and does not indicate the medication isn't working. Your BMI will not shift meaningfully in week one, so comparing your starting BMI to your week-one BMI is not useful. Weight loss accelerates over weeks 4–8 as your appetite decreases and you maintain a calorie deficit. For context, your healthy BMI range (18.5–24.9 according to NHS guidelines) translates to specific weight ranges depending on your height — use our free BMI calculator to see your personalised healthy weight target.
Clinically BackedWeight Loss
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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 Track Real Progress Beyond First Week Weight Loss
Rather than focusing on week-one changes, track progress weekly or fortnightly — you'll see clearer patterns from week 3 onwards. Keep a record of your weight using our free progress tracker to identify trends rather than single-day fluctuations. Most people see meaningful BMI reduction after 4–6 weeks on Mounjaro when dose-related appetite suppression fully kicks in. Remember that weight loss varies significantly between individuals based on diet, exercise, starting weight, and metabolism. If you're concerned about your progress or side effects, discuss with your prescriber — they may adjust your dose or provide additional support. Your healthy weight range (based on NHS BMI guidelines) is your target, not a specific week-one outcome.
- 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.
Will I lose weight in my first week on Mounjaro?
Most people experience minimal weight loss in week one — typically 0–1 kg — as the body adjusts to the starting 2.5 mg dose. Any loss is usually fluid rather than fat. Meaningful weight loss typically begins from week 3 onwards as appetite suppression increases.
What does week-one weight loss mean for my BMI?
Week-one weight loss won't meaningfully change your BMI. Your BMI is calculated from your overall weight and height — single-week changes are too small to shift your BMI category. Focus on longer-term progress instead.
How much weight should I expect to lose overall on Mounjaro?
Clinical trials showed an average weight loss of 15–21% of body weight over 72 weeks, depending on dose and adherence. Individual results vary significantly. Your prescriber can discuss realistic expectations based on your starting weight.
When does significant weight loss on Mounjaro typically start?
Most people notice appetite reduction and weight loss acceleration from weeks 3–8 as they progress through dose escalations. By week 12, meaningful fat loss is usually evident. Patience during the initial weeks is important.
How can I track my weight loss progress accurately on Mounjaro?
Use our free progress tracker to log your weight weekly or fortnightly — this reveals trends better than daily weigh-ins. Calculate your BMI using our BMI calculator to monitor movement towards your healthy weight range. Discuss progress with your prescriber at check-ins.
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.