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 Do We Calculate Mass in Health and Weight Management?
Mass refers to the amount of matter in your body, measured in kilograms (kg), stone, or pounds (lbs) in UK health contexts. Understanding how we calculate and measure mass is essential for assessing your health, calculating your BMI, and tracking weight management progress. Whether you're preparing for a health assessment with your GP or exploring weight loss options, knowing how mass is measured and what it means for your health is the first step toward informed decision-making.
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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 Mass Measurement in UK Health Practice
Mass calculation in UK health services primarily uses kilograms (kg) as the standard unit, though body weight is often expressed in stone and pounds for personal context. Your mass is typically measured using calibrated scales at your GP surgery, clinic, or pharmacy—digital scales are most accurate. The measurement is straightforward: you stand on the scales, and the device registers your body weight in the selected unit.
Why mass matters for health: Your mass is a key input for calculating BMI (body mass index), which healthcare professionals use as a screening tool for weight-related health risks. The formula is simple: BMI = weight (kg) ÷ height (m)². Use our free BMI calculator above to check your personal BMI instantly and see where you fall on the NHS health scale. Mass is also used to assess healthy weight ranges, calculate medication doses (including prescription weight loss treatments like Mounjaro or Wegovy), and monitor progress during weight management programmes.
- Accurate mass measurement requires calibrated scales and consistent measurement times (typically morning, after using the toilet, before eating)
- Mass fluctuates daily due to hydration, food intake, hormones, and time of day—which is why weekly or monthly tracking is more meaningful than daily weigh-ins
- Convert your weight between kg, stone, and lbs instantly using the free weight converter on this page
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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.
Mass, BMI, and Healthy Weight Assessment
Mass alone doesn't tell the full story of health. Two people at the same weight but different heights will have different BMIs. Similarly, two people with the same BMI may have different body compositions—muscle weighs more than fat, so athletes may have a high mass but low body fat percentage. This is why NHS guidance emphasises that mass should be considered alongside height, age, ethnicity, and where fat is stored (waist circumference).
For UK adults, the NHS defines healthy mass ranges based on BMI. Adults of South Asian, Chinese, Black African, or Black Caribbean heritage have lower recommended thresholds due to higher health risk at lower BMI levels. Track your weight loss journey using our free progress tracker, which helps you monitor changes in mass over time alongside other health metrics. If you're considering prescription weight loss medication, your prescriber will use your current mass to calculate eligibility and appropriate doses.
- 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 the difference between mass and weight?
In everyday UK health contexts, "mass" and "weight" are used interchangeably—both refer to the heaviness of your body measured in kg, stone, or lbs. Technically, weight is the force gravity exerts on mass, but for health purposes, they mean the same thing. Your GP will refer to your weight or body mass when discussing health assessments.
How often should I measure my mass for weight management?
NHS guidance suggests weighing yourself once a week at the same time, preferably in the morning before eating. Daily weighing can be demotivating due to natural fluctuations from hydration and food intake. Weekly measurement gives a more accurate picture of genuine progress over time.
Why is mass important for calculating BMI?
BMI = weight (kg) ÷ height (m)² is the standard screening tool used by NHS professionals to assess health risk. Your mass is one half of this calculation; combined with your height, it determines whether you fall into the healthy, overweight, or obese category according to NHS thresholds.
Does muscle mass affect how we calculate health risk?
Yes. BMI doesn't distinguish between muscle and fat, so muscular individuals (athletes) may have a high BMI despite low body fat. This is why your GP may consider waist circumference, fitness level, and overall health alongside BMI when assessing risk.
How is mass used to determine prescription weight loss eligibility?
Your current mass determines your BMI, which is the primary criterion for eligibility for prescription weight loss treatments like Mounjaro or Wegovy in the UK. You must have a BMI of 30 or above, or 27–29.9 with weight-related health conditions, to be considered for treatment.
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.