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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Understanding the MM Equation for UK Health
The MM equation (Mifflin-St Jeor) is the gold standard formula NHS nutritionists and health professionals use to estimate your basal metabolic rate—the calories your body burns at rest. This equation is more accurate than older methods and accounts for age, sex, weight, and height. Understanding your metabolic rate is essential for effective weight management and forms the foundation of any sustainable diet plan. Our UK health information breaks down how this calculation works and why it matters for your health goals.
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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.
How the MM Equation Works
The Mifflin-St Jeor equation calculates your basal metabolic rate (BMR)—the minimum calories your body needs daily to function at rest. The formula differs for men and women:
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Once you have your BMR, multiply it by an activity factor (typically 1.2 to 1.9) to find your total daily energy expenditure (TDEE)—the actual calories you burn daily. This calculation is far more reliable than basic formulas and aligns with NHS guidance on weight management. Knowing your TDEE helps you determine the calorie deficit needed for sustainable weight loss of approximately 0.5–1 kg per week, as recommended by NHS health advice.
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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.
Putting the MM Equation Into Practice
Once calculated, your BMR guides realistic weight management targets. A typical calorie deficit of 500 kcal daily (based on your TDEE) creates a loss of roughly 0.5 kg weekly—the safe rate endorsed by healthy weight UK standards. For example, if your TDEE is 2,500 kcal, eating 2,000 kcal daily produces sustainable results without extreme restriction. Our free BMI calculator complements metabolic calculations by showing where your current weight sits on NHS guidelines, while our weight converter helps you track progress in stone, pounds, or kilograms. Always consult your GP if planning significant dietary changes, especially if you have existing health conditions affecting metabolism.
- 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.
Why is the MM equation more accurate than older formulas?
The Mifflin-St Jeor equation was developed from modern research on actual metabolism and is validated across diverse age groups and weights. Older Harris-Benedict formulas overestimated calorie needs by up to 15%. NHS health professionals prefer MM because it accounts for metabolic changes with age and produces results closer to actual energy expenditure, making it ideal for weight management planning.
How do I use my BMR to lose weight?
Calculate your BMR using the MM equation, then multiply by your activity level (sedentary = 1.2, moderate = 1.5, active = 1.75) to find TDEE. Create a calorie deficit by eating 300–500 kcal below TDEE daily. This creates safe, sustainable loss of 0.5–1 kg weekly, aligned with NHS guidance. Never eat below your BMR without medical supervision.
Does the MM equation work for everyone?
The equation works well for most adults but has limitations. It doesn't account for muscle mass, metabolism changes from dieting, certain medications, or medical conditions like thyroid disease. Athletes may have higher actual calorie needs than calculated. Your GP can provide personalised advice if you have health conditions affecting metabolism.
What's the difference between BMR and TDEE?
BMR (basal metabolic rate) is calories burned at complete rest—what the MM equation calculates directly. TDEE (total daily energy expenditure) includes BMR plus calories burned through activity. To lose weight, you create a deficit below your TDEE, not your BMR. Both figures are essential for accurate weight management planning.
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.