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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Understanding Weight Gain — UK Health Information
Weight gain happens gradually over time and is influenced by many factors including diet, physical activity, metabolism, genetics, and medical conditions. If you're concerned about gaining weight, it's important to understand the science behind it and know what steps the NHS recommends for healthy weight management. This UK health guide explains the causes of weight gain and practical approaches supported by evidence-based health advice.
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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.
What Causes Weight Gain?
Weight gain occurs when you consume more calories than your body uses for energy. However, it's not just about calories — many other factors influence how your body stores fat and manages weight:
- Calorie imbalance: Eating more than your body burns is the primary mechanism, typically requiring a surplus of around 3,500 kcal to gain 0.5 kg (1 lb)
- Genetics: Your genes influence metabolism, appetite regulation, and where your body stores fat
- Hormones: Thyroid function, cortisol (stress hormone), insulin, and oestrogen all affect weight regulation
- Medical conditions: Hypothyroidism, polycystic ovary syndrome (PCOS), depression, and type 2 diabetes can increase weight gain
- Medications: Some antidepressants, corticosteroids, and diabetes medications can cause weight gain as a side effect
- Age: Metabolism naturally slows with age — resting metabolic rate decreases by approximately 2–8% per decade after age 30
- Sleep and stress: Poor sleep and chronic stress increase cortisol and ghrelin (hunger hormone), promoting weight gain
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 Guidance for Healthy Weight Management
The NHS recommends a practical, sustainable approach to managing your weight. Rather than gradual weight management, the goal is steady, maintainable change that you can sustain long-term. Losing weight at a rate of 0.5 to 1 kg per week is considered safe and sustainable. This typically requires a calorie deficit of around 500 kcal per day, which can be achieved through a combination of reduced-calorie eating and increased physical activity.
Check your current BMI using our free BMI calculator above to understand whether your weight falls within a healthy range for your height. If you're concerned about weight gain or have underlying health conditions, your GP can provide personalised health advice and discuss options including referral to NHS weight management services, structured behaviour change programmes, or — if clinically appropriate — prescription weight management medication.
- 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.
Why am I gaining weight even though I'm not eating much?
Weight gain can happen for reasons beyond calorie intake, including hormonal imbalances (thyroid, PCOS, menopause), certain medications, medical conditions like depression or type 2 diabetes, insufficient sleep, chronic stress, or simply changes in metabolism with age. If you're gaining weight unexpectedly, your GP can investigate underlying causes through blood tests and health assessment.
Is it possible to gain weight from eating healthy foods?
Yes — even healthy foods contain calories. If you eat large portions of nutrient-dense foods like nuts, whole grains, avocado, or olive oil, you can exceed your daily calorie needs and gain weight. Weight gain depends on total calories consumed, not just food quality. Use our free weight converter and BMI calculator to track your weight progress over time.
What's the difference between gaining fat and gaining muscle?
Fat is stored energy; muscle is metabolically active tissue. Muscle weighs more than fat, so you can gain weight from building muscle while losing fat — your weight might increase but your BMI and body composition improve. A combination of strength training and adequate protein helps build muscle while losing fat.
How quickly can someone gain weight?
Weight gain rate varies by individual and calorie surplus. Generally, gaining 0.5–1 kg per week is realistic with a significant calorie surplus (around 500–1,000 kcal above daily needs). Rapid weight gain can occur during illness, hormonal changes, or medication side effects — discuss sudden changes with your GP.
At what point should I be concerned about weight gain?
If your BMI exceeds 25 (overweight) or 30 (obese) according to NHS guidelines, or if you're experiencing weight-related health problems like high blood pressure, type 2 diabetes, or joint pain, it's worth discussing with your GP. Early intervention through lifestyle changes or — if appropriate — structured weight management support can prevent further health complications.
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.