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
Three Week Weight Loss — What's Realistic?
Losing weight over three weeks depends on your starting weight, diet, activity level, and whether you're using prescription medication. The NHS recommends losing no more than 0.5 to 1 kg per week for sustainable results, which means a realistic three-week loss is 1.5 to 3 kg. gradual weight management is possible but often comes with muscle loss and difficulty maintaining results long-term. This guide covers realistic expectations, safe approaches, and prescription weight management options available in the UK.
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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.
Realistic Three-Week Weight Loss Expectations
The amount of weight you can lose in three weeks depends on several factors. A calorie deficit of 500 kcal per day typically results in approximately 0.5 kg weekly loss — meaning three weeks would give roughly 1.5 kg loss. Larger deficits produce faster results but increase the risk of muscle loss, nutrient deficiency, and difficulty sustaining the approach long-term.
Factors affecting three-week weight loss:
- Starting weight — heavier individuals lose faster initially
- Diet quality — calorie deficit is most important, but nutrient density matters
- Physical activity — exercise supports fat loss while preserving muscle
- Hydration and sleep — both affect metabolic rate and hunger hormones
- Medication — GLP-1 weight loss medications can significantly increase results
- Individual metabolism — varies by age, genetics, and health conditions
The NHS recommends focusing on sustainable lifestyle changes rather than rapid loss, as most gradual weight management is regained within weeks or months.
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.
Prescription Weight Management in the UK
GLP-1 weight loss medications such as Wegovy (semaglutide) and Mounjaro (tirzepatide) can increase weight loss results over three weeks when used alongside diet and exercise. In clinical trials, participants on these medications lost significantly more weight than those on diet alone. However, these are prescription-only medicines requiring a clinical assessment to determine eligibility and suitability.
Try our GLP-1 weight loss calculator to estimate potential weight loss at different doses. Access to prescription weight loss treatment in the UK is through NHS specialist weight management services or private prescribing following an online clinical assessment. Your GP can advise on NHS options, which may involve waiting lists depending on your region.
- 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.
Can you lose 5kg in three weeks safely?
Losing 5 kg in three weeks requires a very large calorie deficit (approximately 1,200 kcal daily), which is not recommended without medical supervision as it increases muscle loss and nutritional risks. The NHS recommends 0.5–1 kg weekly for sustainable results. If you're considering gradual weight management, consult your GP first.
What's the fastest safe weight loss rate?
The NHS recommends losing 0.5 to 1 kg per week (2 to 4 kg monthly). This can be achieved through a 500–1,000 kcal daily calorie deficit combined with physical activity. Faster loss is possible with medical support, but increases risks of muscle loss, gallstones, and nutritional deficiency.
Do weight loss medications help in three weeks?
Yes. GLP-1 medications like Wegovy and Mounjaro may increase weight loss over three weeks when used alongside diet and exercise. Clinical trials showed average losses of 1–2% body weight weekly on these medications. Results vary individually — your prescriber will assess suitability based on your BMI and health.
How do I track weight loss progress accurately?
Use our free progress tracker to log weekly weight changes, dietary habits, and activity levels. Weigh yourself at the same time each week (morning, after toilet, before eating). Weight naturally fluctuates by 0.5–2 kg daily due to fluid, food, and hormones — focus on weekly trends, not daily changes.
Should I use a very low calorie diet for three weeks?
Very low calorie diets (under 800 kcal daily) should only be used under medical supervision. While they produce rapid results, they increase risks of muscle loss, nutrient deficiency, gallstones, and difficulty sustaining weight loss. Your GP or a specialist weight management service can assess whether this approach is appropriate for you.
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.