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 Percentage of Body Weight Loss
Calculating your percentage body weight loss is simple maths that helps you track progress beyond just the scales. This guide shows you the exact formula, real examples using UK measurements, and how percentage loss connects to health outcomes — especially important if you're considering prescription weight management options available through the NHS or private healthcare 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.
The Formula and Real UK Examples
The percentage body weight loss formula is straightforward: (Starting weight – Current weight) ÷ Starting weight × 100 = % weight loss.
Real UK example: If you weigh 95 kg (15 stone) and lose 9.5 kg (1 stone 8 lbs), your percentage loss is (95 – 85.5) ÷ 95 × 100 = clinically studied weight loss.
Why percentage matters: losing 10 kg sounds impressive, but a 10 kg loss means different things for different people. A 10 kg loss is 10% for someone who weighs 100 kg, but 15% for someone who weighs 67 kg. Percentage loss accounts for your starting point, making it a fairer way to measure progress and compare your results to clinical trial data. You can track your progress using our free progress tracker above.
- Even modest percentage losses (5–10%) show measurable health improvements
- Clinical trials for prescription weight loss medications report results as percentage loss, not absolute kg
- NHS weight loss guidelines focus on sustainable, gradual percentage reduction
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.
Prescription Weight Loss Options in UK
Three prescription-only weight management medications are licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Clinical trials show these support average percentage losses of 15–21% body weight over several months, alongside reduced-calorie diet and increased physical activity.
Eligibility: UK adults with a BMI of 30 or above, or 27 or above with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea), may be eligible. Access is through NHS specialist weight management services (tier 3) or private prescribers following clinical assessment. Our GLP-1 weight loss calculator can help you estimate potential percentage loss at different dose levels. Your GP can discuss whether prescription weight loss medication is suitable for your individual situation.
Two routes exist for accessing prescription weight loss medication in the UK. NHS route: referral to your local tier 3 specialist weight management service via your GP, though waiting lists vary significantly by region. Private route: online consultation with a UK-licensed prescriber, typically faster, available without NHS referral. Whichever route you choose, a prescriber will assess your health, BMI, and medical history before issuing a prescription. Use our free BMI calculator to check your starting BMI — this forms the basis for calculating your percentage loss once treatment begins.
- 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.
What counts as significant percentage weight loss?
The NHS considers even a 5% reduction in body weight meaningful — it can lower blood pressure, improve blood sugar control, and reduce joint strain. A 10% loss is considered major. Clinical trials for prescription weight loss medications show 15–21% average loss, though individual results vary significantly.
How do I calculate percentage weight loss if I use stone and pounds?
Convert to a single unit first. If you weigh 15 stone 4 lbs (starting) and lose to 13 stone 10 lbs (current), convert both to kg: 97 kg to 86.6 kg. Then use the formula: (97 – 86.6) ÷ 97 × 100 = 10.7% loss. Our weight converter makes this instant.
Why do medication trials report percentage loss instead of kg?
Percentage loss is fairer for comparison — it accounts for different starting weights. Losing 15 kg means different things for a 120 kg person (12.5% loss) versus a 70 kg person (21% loss). Percentage standardises the results, making trial outcomes comparable.
Can I reach the percentage losses shown in clinical trials?
Clinical trial results (15–21% for Mounjaro and Wegovy) are averages achieved with medication, diet changes, and increased physical activity. Your individual result depends on your starting point, adherence, lifestyle changes, and personal metabolism. Always discuss realistic expectations with your prescriber.
Is losing weight quickly better than losing it slowly in percentage terms?
No. The NHS recommends losing 0.5–1 kg per week for sustainable results and to preserve muscle. This typically equals 2–5% loss per month. Faster percentage losses (usually under medical supervision only) increase muscle loss risk. Sustainable, steady progress is healthier long-term.
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.