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 a Kcal Deficit Works for Weight Loss
A calorie (kcal) deficit is when you eat fewer calories than your body burns — the foundation of weight loss. To lose weight sustainably, the NHS recommends creating a deficit of around 500 kcal per day, which typically results in losing approximately 0.5 to 1 kg per week. This approach works because your body uses stored fat for energy when intake falls short of expenditure. However, the size of your deficit matters — too large and you risk muscle loss and fatigue, too small and progress stalls. Understanding how to create and maintain a safe kcal deficit is key to successful weight management in the UK.
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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.
The Science Behind Kcal Deficits and Weight Loss
Your body requires a certain number of calories daily to maintain current weight — this is your Total Daily Energy Expenditure (TDEE). TDEE depends on age, sex, height, weight, and activity level. When you consume fewer calories than your TDEE, your body accesses stored energy (fat) to make up the difference, resulting in weight loss.
- 500 kcal daily deficit: approximately 0.5 kg loss per week
- 750 kcal daily deficit: approximately 0.75 kg loss per week
- 1000 kcal daily deficit: approximately 1 kg loss per week
The NHS recommends a moderate deficit of 500–750 kcal per day for sustainable results. Larger deficits (1000+ kcal) can lead to muscle loss, nutrient deficiencies, and fatigue — and are only recommended under medical supervision. Use our free BMI calculator to assess your current weight status, then calculate your personalised TDEE to determine an appropriate deficit for your goals.
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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.
Safe Kcal Deficit Strategies and Practical Tips
Creating a kcal deficit doesn't require extreme restriction. Sustainable approaches combine modest calorie reduction with increased physical activity. Rather than cutting 500 kcal through diet alone, consider reducing food intake by 250 kcal and burning 250 kcal through activity — this balance supports better adherence and preserves muscle mass.
- Eat whole foods: vegetables, lean protein, whole grains, and healthy fats
- Reduce portion sizes gradually rather than cutting food groups entirely
- Track intake honestly — use a food app or food diary for the first few weeks
- Add 150 minutes of moderate activity per week (brisk walking, cycling)
- Aim for 0.5–1 kg weight loss per week — faster loss often rebounds
Track your progress over weeks and months, not days. Our free progress tracker helps log weight changes and identify patterns. If weight loss stalls after several weeks despite consistent effort, speak to your GP about support options including weight loss medication like GLP-1 injections.
- 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 a healthy kcal deficit for weight loss?
The NHS recommends a deficit of 500–750 kcal per day, resulting in 0.5–1 kg weight loss weekly. This balanced approach supports sustainable loss while preserving muscle and energy. Larger deficits (1000+ kcal) risk muscle loss and fatigue, and should only be attempted with medical supervision.
How do I calculate my daily kcal needs?
Your Total Daily Energy Expenditure (TDEE) depends on age, sex, height, current weight, and activity level. Online TDEE calculators provide estimates, or speak to a dietitian for personalised guidance. Once you know your TDEE, subtract 500–750 kcal to establish your target intake for sustainable weight loss.
Can I lose weight without counting calories?
Yes, many people lose weight through portion control, eating whole foods, and increasing activity without strict calorie counting. However, if progress slows, tracking intake for a few weeks helps identify where additional kcal savings are possible — most people underestimate portions and intake.
Is combining diet with activity better than diet alone?
Yes. Splitting the deficit between diet (250 kcal reduction) and activity (250 kcal burned) is often more sustainable than diet alone. It preserves muscle mass, supports better adherence, improves cardiovascular health, and makes the deficit feel less restrictive. The NHS recommends 150 minutes moderate activity weekly alongside dietary changes.
What if I'm not losing weight despite a kcal deficit?
Weight loss can plateau due to metabolic adaptation, underestimated intake, or underlying health factors. Review portions honestly, ensure consistency, and increase activity. If no progress after 4–6 weeks, consult your GP about support options, including structured diet programmes or prescription weight loss medication like GLP-1 injections, which may help when diet and activity alone aren't enough.
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.