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
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Weight Loss Calories Calculator: Understanding Calorie Deficits in the UK
Understanding how many calories you need to lose weight is fundamental to successful weight management in the UK. A calorie deficit — consuming fewer calories than your body burns — is the core principle behind sustainable weight loss. The NHS recommends losing weight at a rate of 0.5 to 1 kg per week, which typically requires a deficit of around 500 calories per day. However, for eligible adults, prescription weight loss medications are now available through the NHS and privately, offering additional support alongside dietary changes. This guide explains how calorie deficits work, how to calculate your needs, and what prescription 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.
Prescription Weight Loss Options Available in the UK
If you're struggling with weight management through diet and exercise alone, several prescription weight loss medications are now licensed in the UK for eligible adults:
- Mounjaro (tirzepatide) — a once-weekly GLP-1 and GIP receptor agonist injection that may help reduce appetite and support feeling fuller after meals. Clinical trials showed participants lost an average of 15–21% of their body weight alongside a reduced-calorie diet and increased physical activity.
- Wegovy (semaglutide) — a once-weekly GLP-1 receptor agonist injection. Research demonstrated average weight loss of around 15% of body weight in clinical trials.
- Saxenda (liraglutide) — a once-daily GLP-1 injection, also licensed for weight management in the UK.
All three are prescription-only medicines and require a clinical assessment to determine eligibility. Weight loss medications work best when combined with a reduced-calorie diet and increased physical activity — they are not replacements for lifestyle changes. Try our free GLP-1 weight loss calculator to see projected outcomes at different doses if you're considering prescription weight loss support.
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: How to Access Weight Loss Medication
NHS access: Weight loss medications are available through NHS specialist tier 3 and tier 4 weight management services. You'll need a referral from your GP, and availability varies significantly by region. Waiting times can be substantial in many areas. Ask your GP whether your local weight management service offers prescription medication options.
Private access: Available through UK-licensed prescribers via online consultation, followed by prescription dispensing through a GPhC-registered pharmacy. This route doesn't require NHS referral and is typically faster. Costs vary depending on medication and dose.
First step: Speak with your GP about whether prescription weight loss treatment is suitable for you. They can assess your BMI, health conditions, and medications. Use our free BMI calculator and weight converter to understand your current weight status before your appointment.
- 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.
How many calories per day do I need to lose weight?
The NHS recommends a calorie deficit of around 500 kcal per day for sustainable weight loss of approximately 0.5 kg (1 lb) per week. Your total daily calorie need depends on your age, sex, height, current weight, and activity level. A GP or registered dietitian can calculate your personal requirements. Very low calorie diets (under 800 kcal/day) should only be followed under medical supervision.
Are prescription weight loss injections available on the NHS?
Yes. Mounjaro, Wegovy, and Saxenda are all licensed in the UK and available through NHS specialist weight management services. Access requires a GP referral to your local tier 3 service and is subject to eligibility criteria and regional availability. Waiting lists vary significantly by area.
What is the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist only. Clinical trials showed slightly higher average weight loss with Mounjaro (15–21% vs 15%), but individual results vary. Both are once-weekly injections. Your prescriber will assess which is more suitable for you.
Can I get weight loss medication privately without seeing my GP?
Yes. Private prescribers can issue prescriptions for weight loss medications following an online consultation. However, it's advisable to inform your GP that you're considering or using prescription weight loss medication, as they can monitor your progress and adjust any other medications you're taking.
What is the recommended rate of weight loss?
The NHS recommends losing 0.5 to 1 kg (1 to 2 lbs) per week for sustainable weight loss. This typically requires a calorie deficit of around 500 kcal per day. Faster weight loss is possible but may increase muscle loss and is not recommended without medical supervision. Use our progress tracker to monitor your weight loss sustainably.
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.