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
How Calorie Deficits Work for Weight Loss
A calorie deficit is the foundation of weight loss — it means consuming fewer calories than your body burns. The NHS recommends losing weight at a sustainable rate of 0.5 to 1 kg per week, which typically requires a deficit of around 500 calories per day. Understanding how calories and weight loss work together helps you set realistic goals and choose the right approach for your health, whether that's through diet, exercise, prescription weight loss medication, or a combination of methods. Use our free BMI calculator above to check where you stand, then explore what calorie deficit strategy suits your lifestyle best.
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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.
Understanding Calorie Deficits and Weight Loss
A calorie deficit occurs when you burn more calories than you consume. Your body then uses stored energy (primarily fat) to make up the difference, resulting in weight loss over time.
- 500 kcal daily deficit = approximately 0.5 kg weight loss per week
- 1000 kcal daily deficit = approximately 1 kg weight loss per week
- Sustainable rate: NHS recommends no faster than 1 kg per week for long-term health
To calculate your personal calorie needs, you can use the Mifflin-St Jeor formula based on your age, height, weight, and activity level. Most adults require between 1800–2500 kcal daily. A moderate deficit of 500 kcal per day is sustainable and avoids muscle loss. Very gradual weight management (faster than 1.5 kg per week) increases the risk of nutritional deficiency and loss of lean muscle mass — this is why NHS weight management guidance emphasises gradual, steady progress. For structured UK weight loss support, the NHS offers tier 1 and tier 2 programmes including digital tools and community weight management services.
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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 Supports Calorie Deficits
Prescription weight loss medications like GLP-1 receptor agonists (such as semaglutide in Wegovy or tirzepatide in Mounjaro) can help create a calorie deficit by reducing appetite and supporting feelings of fullness. These are prescription-only medicines available through NHS specialist services or private prescribing following clinical assessment.
Clinical trials showed that Wegovy helped participants lose an average of around 15% body weight alongside dietary changes, whilst Mounjaro supported approximately 15–clinically studied weight loss — individual results vary considerably. These medications work alongside, not instead of, lifestyle changes. Try our GLP-1 weight loss calculator to estimate potential outcomes at different doses, and use our free progress tracker to monitor your weight loss journey over time.
- 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 should I cut daily to lose weight safely?
The NHS recommends creating a deficit of around 500 kcal per day, resulting in approximately 0.5 kg weight loss per week. This is sustainable and helps preserve muscle mass. Very large deficits (over 1000 kcal daily) should only be done under medical supervision and may increase the risk of nutritional deficiency and muscle loss.
Can I lose weight faster with a larger calorie deficit?
Losing faster than 1–1.5 kg per week increases risks of muscle loss, nutrient deficiency, and metabolic slowdown. The NHS emphasises sustainable, gradual weight loss. If you're struggling with a standard deficit, your GP can refer you to specialist services or discuss prescription weight loss options.
What is the relationship between calories and GLP-1 weight loss medication?
GLP-1 medications like Wegovy help by reducing appetite, making it easier to maintain a calorie deficit naturally. They don't replace the need for dietary changes — they support it. Clinical trials combined medication with reduced-calorie diets and physical activity. Always follow prescriber guidance alongside lifestyle modifications.
How do I calculate my personal calorie needs for a deficit?
Use the Mifflin-St Jeor formula: multiply your basal metabolic rate by your activity level factor (sedentary = 1.2, moderate = 1.55, very active = 1.9). Most UK adults need 1800–2500 kcal daily. A 500 kcal reduction creates the standard deficit. Your GP or a registered dietitian can help personalise this.
Will a calorie deficit alone lead to weight loss, or do I need exercise?
A calorie deficit alone can drive weight loss, but combining diet with physical activity preserves muscle mass, improves cardiovascular health, and supports long-term sustainability. The NHS recommends both dietary changes and increased activity for best results and general fitness benefits.
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.