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
800 Calories a Day Weight Loss and UK Medication Options
An 800-calorie-a-day diet is considered a very low calorie diet (VLCD) and should only be followed under medical supervision in the UK. While gradual weight management is possible with such restricted intake, it carries risks including muscle loss, nutritional deficiencies, and metabolic changes. For adults eligible for prescription weight management, medications like Mounjaro and Wegovy offer a safer, medically supported approach to weight loss alongside lifestyle changes. This guide explains VLCD safety, UK-licensed medication options, and how to access NHS weight management services.
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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 Prescription Weight Loss Medications and How They Compare
For adults with a BMI of 30 or above (or 27+ with weight-related health conditions), several prescription weight loss medications are available in the UK. These work differently from extreme calorie restriction and are designed to be used alongside a reduced-calorie diet and physical activity—not instead of proper nutrition.
- Mounjaro (tirzepatide) — dual GIP and GLP-1 receptor agonist; clinical trials showed average weight loss of approximately 15–21% body weight at maximum dose (15 mg weekly)
- Wegovy (semaglutide) — GLP-1 receptor agonist; average weight loss around 15% body weight at maintenance dose (2.4 mg weekly)
- Saxenda (liraglutide) — GLP-1 receptor agonist; daily injection option with average weight loss of 5–10% body weight
All are prescription-only and require a clinical assessment. Individual results vary significantly. Use our free GLP-1 weight loss calculator above to explore projected outcomes at different doses, or calculate your personal BMI to understand your eligibility.
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 vs Private Access and Getting Started in the UK
NHS route: Available through tier 3 specialist weight management services after GP referral. Access varies by region; waiting lists can be lengthy in some areas. NICE has approved tirzepatide (Mounjaro) for use within NHS services for eligible adults.
Private route: Available through registered online prescribing services following a clinical assessment. No NHS referral required; typically faster access than NHS services.
Getting started: Speak with your GP about your weight management goals and ask about available options in your area. You can also use our BMI calculator on this page to check your personal BMI and understand where you sit within NHS eligibility criteria. A licensed prescriber will assess whether prescription medication is 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.
Is an 800-calorie diet safe?
An 800-calorie diet is classified as a very low calorie diet (VLCD) and should only be followed under strict medical supervision. It carries risks of muscle loss, nutrient deficiencies, gallstones, and metabolic slowdown. The NHS recommends weight loss of 0.5–1 kg per week, which typically requires a deficit of 500 kcal/day, not 800. Always consult your GP before attempting any extreme calorie restriction.
How much weight can I lose with GLP-1 medication?
Clinical trial data shows average weight loss of 14–21% body weight depending on the medication and dose used. However, individual results vary significantly. Mounjaro at maximum dose (15 mg) showed approximately 21% average loss; Wegovy at 2.4 mg showed around 15%. Weight loss is achieved alongside reduced-calorie diet and physical activity—medication is not a standalone solution.
Who is eligible for prescription weight loss medication in the UK?
Adults with a BMI of 30 kg/m² or above are generally eligible. Those with a BMI of 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease, sleep apnoea) may also qualify. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at lower BMI thresholds due to higher health risk at similar weights. Your GP or prescriber will assess eligibility.
Can I get weight loss medication on the NHS?
Yes. NICE has approved tirzepatide (Mounjaro) for use within NHS tier 3 specialist weight management services. Access is through GP referral to your local specialist service. Availability and waiting times vary significantly by region (ICB). Your GP can tell you what's available locally. Private prescription options are also available without NHS referral.
How do Mounjaro and Wegovy differ?
Mounjaro is a dual GIP and GLP-1 receptor agonist (once-weekly, max 15 mg), while Wegovy is a GLP-1 agonist only (once-weekly, max 2.4 mg). In clinical trials, Mounjaro showed slightly higher average weight loss (~21% vs ~15%), but individual responses vary. Both require prescription and clinical assessment. Your prescriber will advise which suits you based on your health history. ```
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.