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
What Is 17 x 5 Intermittent Fasting?
17 x 5 (also written as 17:5) is an intermittent fasting pattern where you fast for 17 hours and eat within a 5-hour window each day. This approach sits between the popular 16:8 method and more restrictive fasting schedules. Evidence suggests intermittent fasting may support weight management when combined with a healthy diet, though individual results vary. Like any significant dietary change, it's worth discussing with your GP before starting, particularly if you have existing health conditions or take regular medications.
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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.
How the 17 x 5 Fasting Pattern Works
In a 17:5 fasting window, you eat during a 5-hour period—for example, from 12pm to 5pm—and fast for the remaining 19 hours including overnight sleep. This gives you slightly more time to eat than the 16:8 method, which may make it easier to fit in two substantial meals.
- Fasting period: 17 consecutive hours with only water, black tea, or black coffee
- Eating window: 5 hours to consume your daily meals and snacks
- Flexibility: You can shift the window to suit your routine—morning eater or evening eater
- Daily repetition: Follow the same schedule every day for best results
Research on intermittent fasting suggests it may help create a calorie deficit, though the weight loss mechanism is primarily through eating fewer calories overall, not from the fasting itself. The NHS notes that sustainable weight loss typically requires a calorie deficit of around 500 kcal per day for approximately 0.5 kg per week 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.
Is 17 x 5 Safe and Effective for Weight Loss?
Intermittent fasting, including 17:5, is safe for most healthy adults when done properly. However, it's not suitable for everyone. Avoid this approach if you're pregnant, breastfeeding, have a history of eating disorders, take medications requiring food, or have certain medical conditions like type 2 diabetes (unless supervised by your GP).
Evidence shows intermittent fasting can support weight loss in the short term, with results comparable to traditional calorie restriction when total calorie intake is similar. Success depends on what you eat during your 5-hour window—choosing nutrient-dense, whole foods matters more than the fasting pattern itself. Using the free BMI calculator and progress tracker on this page can help you monitor your starting point and track changes over time.
- 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.
Is 17 x 5 fasting better than 16:8 for weight loss?
Both patterns may support similar weight loss results because the primary factor is total calorie intake, not fasting duration. 17:5 simply offers a slightly longer eating window, which some people find easier to maintain. The "best" method is whichever you can stick to consistently.
What can I drink during the 17-hour fasting period?
Water is always fine. Black tea and black coffee without milk, cream, or sugar are also acceptable. Avoid calorie-containing beverages, sweetened drinks, and milk-based coffee as these can break your fast and trigger an insulin response.
Should I take supplements during my fasting window?
Most vitamins and supplements should be taken during your eating window with food for best absorption. Check the label or ask your GP or pharmacist about any medications or supplements you take regularly, as some require food.
Can I do 17 x 5 if I have type 2 diabetes?
Intermittent fasting can affect blood sugar control and medication timings. Do not start 17:5 fasting without consulting your GP first, particularly if you take diabetes medications like insulin or sulphonylureas. Your GP can advise whether it's safe for your situation.
How long does it take to see weight loss results from 17 x 5?
Initial changes may appear within 1-2 weeks, though this often reflects water loss. Meaningful fat loss typically takes 4-8 weeks at a sustainable rate of 0.5-1 kg per week. Use our free progress tracker to log weekly weight and spot trends over time.
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.