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
Intermittent Fasting for Fast Weight Loss
Intermittent fasting (IF) involves eating within set time windows and fasting for extended periods — typically 16:8 (16 hours fasting, 8-hour eating window) or 5:2 (eating normally 5 days, restricting calories 2 days). Research suggests IF can support weight loss, though the mechanism works through calorie deficit rather than fasting itself. The NHS considers IF a viable approach for some people, though sustainable weight management requires lasting lifestyle changes alongside any method you choose.
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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.
How Intermittent Fasting Supports Weight Loss
Intermittent fasting works primarily by creating a calorie deficit — the total calories consumed during eating windows are typically lower than if eating throughout the day. Clinical evidence shows IF produces similar weight loss to standard calorie restriction when total energy intake is equivalent, meaning the fasting window itself isn't "magical" but rather a tool for eating fewer calories overall.
- 16:8 method: Fast overnight plus 8 additional hours; eat lunch and dinner within an 8-hour window. Average weight loss around 3–4 kg over 8–12 weeks when combined with a reduced-calorie diet
- 5:2 method: Eat normally Monday–Friday, restrict to 500–600 kcal on two non-consecutive days. NHS guidance supports this approach as sustainable for some individuals
- Metabolic effects: Extended fasting may slightly increase fat burning, though the overall calorie deficit remains the primary driver of weight loss
For most people, IF results in weight loss of approximately 0.5–1 kg per week — the same rate the NHS recommends for sustainable 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.
Intermittent Fasting vs Other Weight Loss Methods
Intermittent fasting is effective for weight management but is not inherently "faster" than calorie restriction alone — weight loss depends on total calories consumed, not meal timing. Other prescription weight loss options available in the UK may offer different support: GLP-1 medications like Wegovy reduce appetite directly, helping people eat fewer calories automatically. The fastest clinically proven weight loss combines medication with dietary changes — evidence shows GLP-1 weight loss injections support average weight loss of 15–20% body weight alongside lifestyle changes.
Use our free BMI calculator to check your current health status, and our GLP-1 weight loss calculator to estimate potential outcomes if prescription support is suitable for you. Consider discussing your preferred approach with your GP to identify what works best for your circumstances.
- 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 intermittent fasting faster than regular calorie reduction?
No — both methods produce similar weight loss when total calories are the same. Weight loss speed depends on calorie deficit, not meal timing. The NHS recommends losing 0.5–1 kg per week using either approach. Success depends on which method you can sustain long-term.
What does the NHS say about intermittent fasting?
The NHS recognises IF as a valid weight management approach for some people, particularly the 5:2 method. However, it's not suitable for everyone — pregnant women, those with eating disorder history, or certain medical conditions should avoid IF. Discuss with your GP before starting.
Can I combine intermittent fasting with weight loss medication?
Potentially, though this requires discussion with your prescriber. GLP-1 medications (like Wegovy) already reduce appetite, so careful monitoring is needed to ensure adequate nutrition. Always inform your GP about both IF and any medications you're considering.
How much weight can I lose with intermittent fasting?
Average loss is 0.5–1 kg per week with IF, similar to standard calorie restriction. Total loss depends on your starting weight, how long you follow IF, and overall calorie deficit. Our weight loss calculator can project potential outcomes based on your circumstances.
Is intermittent fasting safe long-term?
Short-term IF (8–12 weeks) is safe for most healthy adults. Long-term safety data is limited. Some people experience fatigue, difficulty concentrating, or muscle loss. The NHS advises consulting your GP before starting IF, particularly if you have diabetes, take medications, or have other health conditions.
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.