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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Calorie Deficit for Weight Loss Calculator
A calorie deficit is the most fundamental requirement for weight loss — you need to consume fewer calories than your body burns. The NHS recommends losing weight at a rate of 0.5 to 1 kg per week, which typically requires a deficit of around 500 kcal per day. Understanding your personal calorie needs and how to create a sustainable deficit is essential for effective weight management. This guide explains how to calculate your target, use a calorie deficit calculator, and combine it with other weight loss approaches for UK adults.
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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.
How to Calculate Your Calorie Deficit
Your daily calorie need (also called Total Daily Energy Expenditure or TDEE) depends on your age, sex, height, weight, and activity level. The NHS and most UK health professionals use the Mifflin-St Jeor equation to estimate baseline metabolic rate, then multiply by activity factors to get TDEE. A typical adult woman needs 1,800–2,000 kcal daily; a typical adult man needs 2,500–3,000 kcal daily — but individual needs vary significantly.
- Sedentary lifestyle: Multiply baseline by 1.2
- Light activity (1–3 days exercise): Multiply by 1.375
- Moderate activity (3–5 days): Multiply by 1.55
- Very active (6–7 days): Multiply by 1.725
Once you know your TDEE, subtract 500 kcal to lose approximately 0.5 kg per week, or 750 kcal to lose around 1 kg per week. The NHS recommends not exceeding 1 kg per week without medical supervision, as faster loss risks muscle loss and nutrient deficiencies. Use our free BMI calculator above to assess your current weight and health range, then determine how much weight loss is appropriate for your individual circumstances.
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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.
Combining Calorie Deficit with Other Weight Loss Approaches
A calorie deficit alone is effective, but combining it with increased physical activity, improved nutrition quality, and behavioural changes increases success. Regular aerobic exercise and strength training preserve muscle during weight loss and improve metabolic health. For adults in the UK seeking weight management support, the NHS offers tier 1 and tier 2 programmes including free digital tools, community schemes, and primary care-based behaviour change support. For some adults, prescription weight loss medication may be recommended as part of a comprehensive approach. Medications such as GLP-1 injections are prescription-only and require clinical assessment by a GP or specialist prescriber. Track your progress using our free progress tracker, and combine dietary changes with movement for sustainable, long-term results.
- 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 eat per day to lose weight?
The amount depends on your personal TDEE and goal rate of loss. NHS guidance recommends a deficit of 500 kcal per day (for approximately 0.5 kg per week loss) or up to 750 kcal per day (for approximately 1 kg per week). Most adults eating 1,500–1,800 kcal daily will achieve safe weight loss, but this must be individually assessed by your GP or a registered dietitian.
Is a calorie deficit the only way to lose weight?
Calorie deficit is the fundamental requirement for weight loss, but sustainability depends on overall lifestyle factors including food quality, physical activity, sleep, and stress management. Evidence shows that combining calorie deficit with regular exercise, improved nutrition, and behavioural support increases long-term success compared to calorie restriction alone.
Can I use a calorie deficit calculator with GLP-1 weight loss medication?
Yes. GLP-1 medications like Mounjaro and Wegovy work alongside calorie deficit and lifestyle changes. These prescription-only injections may help reduce appetite and support feeling fuller on smaller portions, making it easier to maintain a calorie deficit. Always use such medications under medical supervision and continue dietary awareness alongside treatment.
How quickly will I lose weight with a 500 kcal deficit?
The NHS recommends expecting to lose approximately 0.5 kg (just over 1 lb) per week with a 500 kcal daily deficit. However, weight loss is not always linear — some weeks you may lose more, others less, due to factors including water retention, hormonal changes, and muscle gain from exercise.
What's the difference between calorie deficit and low-calorie diets?
A calorie deficit is created by eating fewer calories than you burn — a personalised, sustainable approach. Low-calorie diets (800–1,200 kcal per day) are very restrictive and should only be followed under medical supervision. The NHS emphasises sustainable deficits (500–750 kcal daily) over extreme restriction for long-term weight management.
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.