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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Calories for Weight Loss Formula: UK Guide
To lose weight effectively, you need to create a calorie deficit — consuming fewer calories than your body burns. The standard formula involves calculating your daily calorie needs, then reducing intake by 500 calories per day to lose approximately 0.5 kg weekly, according to NHS guidance. However, prescription weight management options are now available in the UK for eligible adults, which can support weight loss alongside dietary changes. This guide explains the calorie formula, how it works, and what medication support you might access through your GP.
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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.
Understanding the Calorie Deficit Formula
The foundation of weight loss is calorie deficit — burning more energy than you consume. Your Basal Metabolic Rate (BMR) is the number of calories your body burns at rest. You can estimate this using the Mifflin-St Jeor formula:
- Men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age) – 161
Next, multiply your BMR by your activity level (sedentary = 1.2, lightly active = 1.375, moderately active = 1.55, very active = 1.725) to get your Total Daily Energy Expenditure (TDEE). A deficit of 500 kcal per day typically results in 0.5 kg weekly loss. For safer, sustainable weight management in the UK, the NHS recommends losing no more than 1 kg per week. Our free BMI calculator can help you assess your current weight status and healthy targets.
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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.
Prescription Weight Loss Options in the UK
For adults who meet eligibility criteria, prescription weight loss medications are now available through the NHS and private prescribers. Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 receptor agonists that support weight management by regulating appetite and promoting fullness. These are prescription-only medicines requiring a clinical assessment. To access prescription weight loss medication through the NHS, you'll need referral to a specialist tier 3 weight management service — ask your GP if you're eligible. Privately, licensed prescribers can assess eligibility without NHS referral. Try our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and use our progress tracker to monitor your journey alongside lifestyle changes and any prescribed treatment.
- 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 500-calorie deficit rule for weight loss?
A daily deficit of 500 calories typically results in losing approximately 0.5 kg (1 lb) per week. This assumes your calorie calculation is accurate and you maintain the deficit consistently. However, individual results vary based on metabolism, muscle mass, and adherence. The NHS recommends sustainable loss of 0.5–1 kg weekly rather than rapid loss.
How do I calculate my daily calorie needs for weight loss?
First calculate your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor formula based on your age, sex, height, and weight. Multiply your BMR by your activity level factor (1.2–1.725) to get your Total Daily Energy Expenditure (TDEE). Then subtract 500 calories from TDEE for a 0.5 kg weekly loss. A GP or dietitian can provide personalised guidance.
Are prescription weight loss injections available on the NHS in the UK?
Yes, Mounjaro and Wegovy are approved for NHS use through specialist tier 3 weight management services, though regional availability varies significantly and waiting lists can be lengthy. Your GP can refer you if you meet eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). Private access is also available through licensed UK prescribers.
How do calorie formulas compare to GLP-1 weight loss medication?
Calorie deficit formulas provide the foundation for weight loss through diet alone. GLP-1 medications like Mounjaro and Wegovy support this process by reducing appetite and promoting satiety, making it easier to maintain a calorie deficit. They're used alongside dietary changes, not instead of them. Clinical trials show average weight loss of 15–21% body weight over 68 weeks.
What is the difference between BMI and calorie-based weight loss targets?
BMI indicates whether your current weight is healthy for your height, while calorie deficit describes how to lose weight safely. Use our free BMI calculator to identify your healthy weight range, then use the calorie formula to plan your daily intake. For South Asian, Chinese, Black African, or Black Caribbean adults, the NHS recommends lower BMI thresholds due to higher health risks at lower weights.
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.