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 Many Calories Do You Need To Lose Weight?
To lose weight, you need to consume fewer calories than your body uses — this is called a calorie deficit. The NHS recommends a deficit of around 500 calories per day, which typically leads to a loss of approximately 0.5 to 1 kg per week. However, the exact number of calories you need depends on your age, sex, height, current weight, activity level, and metabolism. This guide explains how calorie deficit works, what the NHS recommends, and how prescription weight management options available in the UK can support your goals alongside dietary changes.
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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 Calorie Deficit And Weight Loss
A calorie deficit occurs when you consume fewer calories than your body burns each day. Your body then uses stored energy (fat and muscle) to make up the difference, resulting in weight loss. The NHS recommends a deficit of approximately 500 kcal per day for sustainable weight loss of around 0.5 to 1 kg per week.
To calculate your personal calorie needs, you first need to estimate your basal metabolic rate (BMR) — the calories your body burns at rest — plus activity calories. Use our free BMI calculator above to check your current weight status, then consider: reducing portion sizes, choosing lower-calorie alternatives, and increasing physical activity. Very low-calorie diets (under 800 kcal daily) should only be undertaken with medical supervision.
- Safe rate: 0.5–1 kg per week
- Typical deficit needed: 500 kcal per day
- Duration: Weight loss is a gradual process — results take weeks and months
For adults in the UK, weight management tools like our free progress tracker can help you log food intake and monitor results over time.
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.
Prescription Weight Loss Support Available In The UK
For eligible adults, the NHS and private prescribers offer prescription weight management medications that work alongside calorie reduction and exercise. These include Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) — all GLP-1 or dual-mechanism weight loss injections licensed in the UK.
These medications may help reduce appetite and support feeling fuller after meals, making it easier to maintain a calorie deficit. Eligibility typically requires a BMI of 30 or above (or 27+ with weight-related health conditions like type 2 diabetes or high blood pressure). Our GLP-1 weight loss calculator can help you explore potential outcomes with prescription support. Access is available through NHS specialist weight management services or private online prescribers following a clinical assessment.
- 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?
This depends on your age, sex, height, current weight, and activity level. The NHS recommends a calorie deficit of around 500 kcal per day, which typically results in losing 0.5–1 kg per week. Your GP can provide personalised guidance based on your health profile. Never eat fewer than 1,200 kcal daily without medical supervision.
Is a very low-calorie diet safe for weight loss?
Very low-calorie diets (under 800 kcal daily) are generally not recommended without medical supervision and can lead to nutrient deficiencies and muscle loss. The NHS advises aiming for sustainable weight loss of 0.5–1 kg per week through moderate calorie reduction combined with increased physical activity.
How can prescription weight loss medication help with a calorie deficit?
Medications like Mounjaro and Wegovy are GLP-1 receptor agonists that may help reduce appetite and support feeling fuller after smaller meals. This makes it easier to maintain a calorie deficit without feeling deprived. They work alongside diet and exercise, not instead of them, and require a prescription following clinical assessment.
Am I eligible for prescription weight loss medication in the UK?
Adults with a BMI of 30 or above, or 27 or above with weight-related health conditions (such as type 2 diabetes, high blood pressure, or sleep apnoea), may be eligible. Eligibility is assessed individually by your GP or a licensed prescriber. Contact your GP to discuss whether prescription weight management might be suitable for you.
How does the weight loss calculator on MyWeightCheck help with calorie planning?
Our free GLP-1 weight loss calculator helps eligible adults estimate potential weight loss outcomes with prescription medication support. It provides personalised projections based on your current weight, height, and the medication dose. Combined with our BMI calculator and progress tracker, these tools help you plan and monitor your weight management journey.
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.