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 — The Science Behind Losing Weight
Weight loss happens when your body burns more calories than it consumes — this is called a calorie deficit. But the mechanisms behind weight loss are more complex than simple maths. Your metabolism, hormones, diet quality, physical activity, and even stress levels all play a role. Whether you're using lifestyle changes, prescription weight loss medication, or both, understanding what drives weight loss helps you manage your health more effectively. Let's explore the key factors that cause weight loss and how to achieve it sustainably.
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 Actually Works — The Calorie Deficit Explained
Weight loss occurs when energy intake is less than energy expenditure — the fundamental principle known as a calorie deficit. Your body needs energy to function: to breathe, maintain body temperature, repair tissues, and move. This energy requirement is called your Basal Metabolic Rate (BMR). When you consume fewer calories than your BMR plus daily activity burns, your body accesses stored fat for energy, resulting in weight loss.
The NHS recommends a deficit of approximately 500 kcal per day, which typically results in losing around 0.5 kg per week — a sustainable and safe rate. This can be achieved through diet reduction, increased physical activity, or both combined.
However, weight loss is not purely linear. Factors affecting it include:
- Age — metabolism typically slows with age
- Sex — men generally have higher metabolic rates than women
- Muscle mass — more muscle burns more calories at rest
- Genetics — individual variation in metabolism is significant
- Sleep and stress — poor sleep and high stress can slow weight loss
- Hormonal factors — thyroid function, insulin sensitivity, and other hormones affect how your body stores and burns fat
Try our free BMI calculator above to establish your baseline, then use our weight loss projection calculator to see how different approaches may impact your weight over time.
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.
How Prescription Weight Loss Medication Causes Weight Loss
Prescription weight loss medication works by targeting hormone pathways that regulate appetite and food intake. GLP-1 medications like Wegovy (semaglutide) and Mounjaro (tirzepatide) activate natural hormones in your gut that signal fullness to your brain. This reduces hunger, decreases cravings, and helps you feel satisfied with smaller portions — making it easier to maintain a calorie deficit without feeling constantly hungry.
Clinical trials show these medications support meaningful weight loss: Wegovy users lost approximately 15% of body weight on average, and Mounjaro users achieved 15–clinically studied weight loss — though individual results vary significantly. These medications are prescription-only and require clinical assessment. They're used alongside reduced-calorie diet and physical activity, not instead of them. Your GP or a licensed prescriber can assess whether prescription weight loss medication is suitable for your health profile and 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.
What is the main cause of weight loss?
The primary cause is a calorie deficit — when you consume fewer calories than your body burns. This forces your body to use stored fat for energy. A deficit of around 500 kcal per day typically results in losing approximately 0.5 kg per week according to NHS guidance.
Can weight loss happen without diet changes?
Yes, but it's slower. Increasing physical activity alone creates a calorie deficit, though combining both diet and exercise is more effective. Your BMI and health context matter — use our BMI calculator to understand your personal starting point.
How do weight loss injections cause weight loss?
Prescription injections like Wegovy and Mounjaro activate GLP-1 and GIP hormone pathways that reduce appetite and food cravings. This makes it easier to maintain a calorie deficit. They require a clinical assessment and prescription from a licensed healthcare professional.
Why does weight loss slow down over time?
Your body adapts to a calorie deficit, and as you lose weight, you need fewer calories to maintain your smaller body mass. Metabolism can also slow. This is why varying physical activity, focusing on protein intake, and managing sleep help sustain ongoing weight loss.
Is fast weight loss better than slow weight loss?
No. The NHS recommends losing 0.5–1 kg per week. Faster weight loss often results in muscle loss alongside fat loss, is harder to sustain, and can cause nutritional deficiencies. Our progress tracker helps you monitor sustainable weight loss without extreme measures.
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.