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
When Do You Start To See Weight Loss?
Weight loss timelines vary significantly between individuals, but most people begin noticing changes within 2–4 weeks of making lifestyle changes or starting prescription weight management medication. Several factors influence how quickly you'll see visible results, including your starting weight, metabolism, diet changes, and whether you're using medication like Wegovy or Mounjaro. The NHS recommends losing weight at a sustainable rate of 0.5–1 kg per week, which means realistic expectations are key to long-term success. Understanding what to expect helps you stay motivated and recognise progress, even when the scales move slowly.
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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.
Weight Loss Timelines: What the Evidence Shows
Research on weight loss medication and lifestyle changes shows that visible results typically appear at different stages. Within the first 1–2 weeks, many people experience initial water weight loss and reduced appetite—though this may not be immediately visible. By weeks 2–4, clothes often feel looser and friends may notice subtle changes. Significant measurable weight loss on the scales usually becomes evident by week 4–8.
Clinical trials of Wegovy (semaglutide), a GLP-1 weight loss injection licensed in the UK, showed that participants lost an average of 5–10% of body weight within the first 12–16 weeks, with greater loss at higher doses. However, individual results vary considerably. Mounjaro (tirzepatide), another prescription weight management option, demonstrated similar or slightly greater weight loss in trials. For lifestyle-only approaches, visible changes typically appear within 3–4 weeks if you're maintaining a consistent calorie deficit and increasing physical activity.
Use our free GLP-1 weight loss calculator to estimate potential weight loss timelines at different doses, or track your own progress with our progress tracker as you work towards your goals.
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.
Factors That Influence How Quickly You'll See Results
Several factors determine whether weight loss becomes visible quickly or gradually. Starting weight matters significantly—people with higher starting weights often see faster initial results. Metabolism, which varies by age, sex, muscle mass, and genetics, affects how quickly your body processes a calorie deficit. Consistency is crucial: steady adherence to dietary changes produces more reliable results than sporadic efforts.
Your diet composition affects both weight loss and how you feel while losing it. Physical activity supports faster weight loss, though exercise alone without dietary change produces modest results. Age and hormonal factors (particularly for women) can slow visible progress. If you're using prescription weight management like Wegovy or Mounjaro, your dose level and how your body responds to the medication influence timelines. Water retention, inflammation, and hormonal fluctuations can mask weight loss on the scales even when fat loss is occurring.
- 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 see weight loss within the first week?
Possibly, but initial weight loss in the first few days is usually water weight, not fat loss. Most people won't see significant fat loss results until week 2–4. If you're starting prescription medication like Wegovy, you may notice reduced appetite immediately, but visible weight loss typically takes longer. Stay patient—fast initial results often don't reflect true fat loss.
How much weight should I expect to lose per week?
The NHS recommends losing 0.5–1 kg (1–2 lbs) per week as a safe, sustainable rate. This equates to a calorie deficit of approximately 500–1,000 kcal per day. Faster weight loss is possible with medication or supervised low-calorie diets, but slower, steady loss is generally more sustainable long-term. Use our BMI calculator to determine your healthy weight range first.
Does prescription weight loss medication make results visible faster?
Yes, clinical trials show that GLP-1 medications like Wegovy and dual GIP/GLP-1 medications like Mounjaro support faster weight loss than lifestyle alone. In the STEP trials, Wegovy users lost an average of 5–10% of body weight within 12–16 weeks. However, even with medication, results take time—you'll likely notice appetite reduction before visible weight changes.
What if I'm not seeing weight loss after 4 weeks?
If you've been consistent with lifestyle changes or started medication, discuss this with your GP. Slow or absent weight loss can indicate thyroid issues, medication interactions, or metabolic factors. Review your calorie intake and physical activity levels. Remember that body composition changes (gaining muscle, losing fat) can mean the scales don't move even though your body is changing.
How can I track progress if the scales aren't moving?
Weight is only one measure of progress. Track body measurements (waist, hips, chest), how clothes fit, energy levels, and how you feel. Before-and-after photos over 4–8 weeks often reveal visible changes the scales don't capture. Our free progress tracker helps you log multiple metrics beyond just weight to see your full picture of change.
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.