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
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How to Calculate Percent Weight Loss in the UK
Calculating your weight loss as a percentage of your starting weight is one of the most useful ways to track progress, especially when using prescription weight management medication in the UK. Unlike absolute weight loss in kilograms or stone, percentage weight loss shows meaningful progress regardless of your starting point. This guide explains how to calculate percent weight loss and explores UK-licensed prescription options available through the NHS and private prescribing, helping you understand what medication-supported weight loss might achieve if you're eligible.
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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-Licensed Prescription Weight Management Options
Three prescription medications are currently licensed in the UK for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Clinical trials show average weight loss results:
- Mounjaro: approximately 15–clinically studied weight loss over 72 weeks
- Wegovy: approximately clinically studied weight loss over 68 weeks
- Saxenda: approximately 5–clinically studied weight loss over 56 weeks
You may be eligible for prescription weight loss medication if you have a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply (BMI 27.5+). Individual results vary significantly — these figures represent clinical trial averages alongside reduced-calorie diets and increased physical activity. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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 Access vs Private Prescription in the UK
NHS access to prescription weight loss medication is through specialist tier 3 weight management services — not standard GP prescription. You'll need a GP referral and assessment, though availability and waiting times vary significantly by region. Private prescription is available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber, without NHS referral, typically within days.
Whichever route you choose, speak with your GP first about whether prescription weight management is appropriate for your health. Your GP can assess your BMI, health conditions, and current medications to determine eligibility. Use our free BMI calculator to check your starting point, then discuss options with your healthcare team. Remember that medication is most effective alongside lifestyle changes — diet and physical activity remain essential.
- 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 do I calculate percent weight loss?
Subtract your current weight from your starting weight to find total loss, then divide by starting weight and multiply by 100. Example: Started at 100kg, now 85kg. Loss = (100 − 85) ÷ 100 × 100 = 15%. Use our free progress tracker to log weight and calculate percentage automatically.
What percentage weight loss is considered significant?
Even 5–clinically studied weight loss brings measurable health benefits including lower blood pressure, improved blood sugar control, and reduced joint pain. Clinical trials show that GLP-1 medications achieve 15–21% loss on average, though individual results vary significantly.
How long does it take to see percent weight loss results with prescription medication?
Mounjaro and Wegovy show weight loss within the first 4 weeks for many people, with continued loss over 12–16 weeks as doses increase. Peak results typically appear around 16–20 weeks. Results depend on dose, diet, activity, and individual metabolism.
Am I eligible for prescription weight loss medication on the NHS?
Eligibility depends on BMI (30+) or BMI 27+ with health conditions, plus regional availability. Your GP can assess you and refer to specialist weight management services if appropriate. Private prescription is available through online clinical assessment without NHS referral.
Should I calculate weight loss as percentage or kilograms?
Both are useful. Percentage weight loss shows meaningful progress at any starting weight, making it ideal for comparison. Kilograms or stone show absolute change. Track both using our free progress tracker for complete perspective.
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.