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
Calorie Calculator and Weight Loss Medication in the UK
A calorie deficit is the foundation of weight loss, but calculating the right amount can be complex. In the UK, adults struggling with weight management have access to prescription weight loss medications alongside dietary changes. This guide explains how calories relate to weight loss and what prescription options are available through the NHS and private routes. Understanding both approaches helps you make informed decisions with your GP about the best path for sustainable weight management.
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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-Licensed Prescription Weight Loss Medications
Three prescription medications are currently licensed in the UK for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Mounjaro and Wegovy work as GLP-1 receptor agonists (with Mounjaro also activating GIP), which help reduce appetite and support feeling fuller after eating. Clinical trials showed Mounjaro helped participants lose an average of 15–21% of their body weight, whilst Wegovy achieved approximately 15% average weight loss — though individual results vary significantly.
These medications are prescription-only and require a clinical assessment to establish eligibility. Adults with a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure, may be eligible. Weight loss with these medicines works best alongside a reduced-calorie diet and increased physical activity — they support behaviour change rather than replacing lifestyle effort.
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.
Getting Prescription Weight Loss Treatment in the UK
NHS access: Referral to a specialist tier 3 weight management service is typically required, though availability varies significantly by region. Waiting lists can be lengthy in many areas. Private access: Licensed prescribers offer consultations through GPhC-registered online pharmacies, with treatment available without NHS referral.
Your GP is the first point of contact. They can assess whether prescription weight loss medication is suitable for you based on your individual health, medications, and weight management history. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current health category. A prescriber will then determine the right option during your consultation.
NHS access: Referral to a specialist tier 3 weight management service is typically required, though availability varies significantly by region. Waiting lists can be lengthy in many areas. Private access: Licensed prescribers offer consultations through GPhC-registered online pharmacies, with treatment available without NHS referral.
Your GP is the first point of contact. They can assess whether prescription weight loss medication is suitable for you based on your individual health, medications, and weight management history. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current health category. A prescriber will then determine the right option during your consultation.
- 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 calorie deficit do I need for weight loss?
The NHS recommends a deficit of approximately 500 kcal per day, which typically results in losing 0.5 kg (1 lb) per week. Individual needs vary based on age, metabolism, activity level, and current weight. A GP or dietitian can provide personalised guidance. Even a 5–10% reduction in body weight can bring meaningful health benefits.
How do prescription weight loss medications fit with calorie counting?
GLP-1 medications like Mounjaro and Wegovy support weight loss by reducing appetite and helping you eat smaller portions — they work alongside a reduced-calorie diet, not instead of it. They help make calorie deficit maintenance more manageable by reducing cravings and food preoccupation.
Am I eligible for prescription weight loss medication on the NHS?
You may be eligible if you have a BMI of 30 or above, or 27 or above with weight-related health conditions (diabetes, high blood pressure, sleep apnoea). South Asian, Chinese, Black African, and Black Caribbean adults may have access at lower BMI thresholds. Your GP can assess your individual eligibility.
What's the difference between Mounjaro and Wegovy?
Both are once-weekly injections for weight management. Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1), whilst Wegovy (semaglutide) activates one (GLP-1). Clinical data shows Mounjaro achieved slightly higher average weight loss (15–21% vs 15%), though results vary. Your prescriber will recommend the best option for you.
How quickly will I lose weight on prescription medication?
Weight loss typically begins within the first few weeks but becomes more noticeable over 8–12 weeks as doses increase. Average weight loss in clinical trials was achieved over 68–72 weeks. Individual progress varies based on dose, adherence, diet, and activity levels. Your prescriber will monitor your progress regularly.
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.