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
36 Hour Fast Weight Loss — What You Need to Know
A 36 hour fast is a form of intermittent fasting where you abstain from food for 36 hours. While some people use extended fasting to lose weight, evidence suggests that sustainable weight loss in the UK is best achieved through a combination of dietary changes and, where appropriate, professional support. For eligible adults, prescription weight management options including GLP-1 medications like Mounjaro and Wegovy are available through the NHS or private prescribing. This guide explains how extended fasting compares to evidence-based approaches and what medication options may be available to you.
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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.
Intermittent Fasting and Weight Loss — What Evidence Shows
A 36 hour fast falls within the intermittent fasting category — a pattern of eating and fasting. Research shows intermittent fasting can support modest weight loss, typically similar to traditional calorie restriction when total calories eaten are comparable.
- Sustainability: Extended fasts can be challenging long-term. Many people find shorter fasting windows (16:8 or 5:2) more sustainable.
- Muscle loss risk: Very extended fasts without adequate protein intake may increase muscle loss alongside fat loss.
- NHS perspective: The NHS Weight Loss Plan emphasises sustainable calorie deficit through diet and activity, not specific fasting protocols.
- Individual variation: Weight loss response to fasting varies significantly based on genetics, metabolism, and adherence.
For prescription weight management in the UK, evidence-based GLP-1 medications offer clinical trial data showing 15–21% average body weight loss when combined with lifestyle changes.
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.
UK Prescription Weight Loss Options — Eligibility and Support
If you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions, you may be eligible for prescription weight loss medication under NHS or private care.
- Mounjaro (tirzepatide): Dual GIP/GLP-1 agonist. Average weight loss 15–21% body weight in trials. Once-weekly injection.
- Wegovy (semaglutide): GLP-1 agonist. Average weight loss approximately 15% body weight. Once-weekly injection.
- NHS route: Referral to tier 3 specialist weight management service. Availability varies by region.
- Private route: Online prescription from UK-licensed prescriber. Costs approximately £129–£320/month depending on dose.
Check your BMI using our free BMI calculator, then discuss options with your GP or a licensed prescriber to see what's appropriate for you.
- 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 weight in 36 hours without eating?
Yes, you will lose some weight during a 36 hour fast, but much of this is water weight and glycogen depletion, not fat loss. True fat loss requires a sustained calorie deficit over weeks and months. A single 36 hour fast is unlikely to produce meaningful long-term weight loss without ongoing dietary changes.
Is a 36 hour fast safe?
For most healthy adults, occasional short-term fasting is safe. However, extended fasts aren't appropriate for people with diabetes, eating disorders, during pregnancy, or while breastfeeding. Always consult your GP before attempting prolonged fasts, especially if you take medications or have existing health conditions.
How does intermittent fasting compare to GLP-1 weight loss medication?
Intermittent fasting relies on reducing eating windows. GLP-1 medications (like Mounjaro and Wegovy) work by reducing appetite and supporting feeling fuller. Clinical trials show GLP-1 medications produce 15–21% average weight loss. Both work best combined with a healthy diet and physical activity. Individual results vary significantly.
What's the NHS eligibility for prescription weight loss treatment?
You may be eligible if your BMI is 30 or above, or 27–29.9 with weight-related conditions like type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Chinese, Black African, and Black Caribbean adults have lower thresholds. Contact your GP to ask about referral to specialist weight management services in your area.
How quickly will I lose weight with medication versus fasting?
Sustainable weight loss is typically 0.5–1 kg per week. GLP-1 medications in clinical trials showed average weight loss of 15–21% over 60–72 weeks, working alongside diet and exercise. Extended fasting alone is difficult to maintain long-term. Combining a reduced-calorie diet with appropriate medication (if eligible) and activity produces the most consistent results.
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.