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 Causes Weight Loss? Understanding the Science
Weight loss happens when your body uses more energy than you consume — creating what's called a calorie deficit. This can occur through dietary changes, increased physical activity, medical conditions, or prescription medications like GLP-1 injections. Understanding the mechanisms behind weight loss helps you make informed decisions about managing your weight effectively. In the UK, the NHS recommends sustainable weight management approaches tailored to your individual circumstances and health profile.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Weight Loss Happens in Your Body
Weight loss fundamentally occurs when there's a calorie deficit — your body expends more energy than it takes in. When this happens, your body taps into stored energy (fat) to meet its needs, resulting in weight reduction.
The main causes of weight loss include:
- Dietary changes: Reducing calorie intake through smaller portions or lower-calorie foods
- Increased activity: Exercise and movement burn additional calories
- Metabolic factors: Your basal metabolic rate (BMR) determines calories burned at rest
- Hormonal regulation: Hormones like GLP-1 naturally signal fullness and regulate appetite
- Prescription medications: GLP-1 medications like Wegovy mimic natural appetite-regulating hormones
The NHS recommends a sustainable deficit of approximately 500 kcal per day, which typically results in losing around 0.5–1 kg per week. Use our free BMI calculator above to understand your current weight status in the context of NHS guidelines.
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.
Prescription Weight Loss Medication in the UK
GLP-1 receptor agonists are prescription-only medications that work by activating natural appetite-regulating pathways. Wegovy (semaglutide) is licensed in the UK for weight management and may help reduce hunger and cravings. In clinical trials, Wegovy supported an average weight loss of approximately 15% body weight — though individual results vary significantly.
These medications are not a replacement for lifestyle changes. Instead, they work alongside diet and physical activity as part of a comprehensive weight management approach. Access in the UK is through NHS specialist services or private prescribing following clinical assessment. Try our GLP-1 weight loss calculator to explore potential outcomes at different doses, and use our weight converter to track progress in both metric and imperial units.
- 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's the difference between weight loss and fat loss?
Weight loss includes loss of fat, muscle, and water. Fat loss specifically targets stored fat. For sustainable health, you want to lose fat while preserving muscle, which requires adequate protein intake and regular resistance exercise alongside a calorie deficit.
How quickly should I lose weight?
The NHS recommends losing 0.5–1 kg (1–2 lbs) per week for sustainable results. Faster weight loss is possible with medical supervision but increases the risk of muscle loss and nutritional deficiencies. Aim for steady, consistent progress.
Can you lose weight without changing your diet?
Theoretically yes — exercise alone can create a calorie deficit — but it's inefficient. Most successful weight management combines dietary changes with increased physical activity. Prescription weight loss medication works best when combined with both lifestyle changes.
What causes sudden weight loss without trying?
Unintentional weight loss can signal medical conditions including thyroid problems, type 2 diabetes, depression, or medication side effects. If you're losing weight without dieting or increased exercise, consult your GP for assessment and advice.
Do prescription weight loss injections work without diet changes?
Prescription medications like Wegovy may help reduce appetite, but they work best alongside reduced-calorie eating and physical activity. Clinical trials showed weight loss occurred when medication was combined with lifestyle support, not medication alone.
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.