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 To Get Your BMI Using The Formula
The BMI formula is a simple calculation that divides your weight by your height squared. Understanding the formula helps you work out your own BMI manually or use it with confidence. This guide explains both the metric and imperial versions, plus how to interpret your result using NHS BMI categories for UK adults. You can also use our free BMI calculator above for gradual improvement, but knowing the formula itself is useful for understanding what your BMI actually means.
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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 BMI Formula Explained Step-By-Step
The metric BMI formula is the standard used by the NHS and works like this:
- BMI = weight (kg) ÷ height (m)²
For example, if you weigh 70kg and are 1.75 metres tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 (healthy weight).
The imperial BMI formula is used with pounds and inches:
- BMI = (weight in lbs × 703) ÷ height (inches)²
For example, 154 lbs at 5ft 9in (69 inches): (154 × 703) ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7 (healthy weight). Both formulas give the same result — the metric version is easier for UK measurements. Use our weight converter above if you need to switch between kg, stone, and pounds.
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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.
BMI Ranges And What Your Result Means
According to NHS guidelines, BMI results fall into these BMI categories:
- Below 18.5 = Underweight
- 18.5–24.9 = Healthy weight
- 25.0–29.9 = Overweight
- 30.0–34.9 = Obese (Class I)
- 35.0–39.9 = Obese (Class II)
- 40.0+ = Severely obese (Class III)
If you are of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds. Your healthy weight range at a given height is: lower bound = 18.5 × height(m)² and upper bound = 24.9 × height(m)². Our BMI calculator can instantly work this out for you.
- 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 do I need to square the height in the BMI formula?
Height is squared to account for the fact that weight increases three-dimensionally with height. Squaring the height ensures that taller people aren't penalised in the calculation. This relationship between weight and height squared is why the BMI formula works across all heights.
Is the BMI formula accurate for everyone?
BMI is useful as a screening tool but has important limitations. It doesn't distinguish between muscle and fat — athletes may have a high BMI despite low body fat. It also doesn't account for where fat is stored (waist circumference matters too) or variations in bone density. Your GP can provide personalised health advice beyond BMI.
What's the difference between the metric and imperial BMI formulas?
The metric formula (kg ÷ m²) uses kilograms and metres, standard for NHS and UK healthcare. The imperial formula uses pounds and inches, multiplied by 703 to give equivalent results. In the UK, metric is preferred, but both give identical BMI values.
Can I use the BMI formula to calculate my healthy weight range?
Yes. To find your healthy weight range (BMI 18.5–24.9), multiply your height in metres squared by 18.5 (lower bound) and 24.9 (upper bound). For example, at 1.73m: lower = 18.5 × 2.99 = 55.4 kg (8st 10lbs), upper = 24.9 × 2.99 = 74.5 kg (11st 10lbs).
Does BMI change with age or gender?
The BMI formula is the same for all adults regardless of age or gender. However, body composition naturally changes with age — muscle decreases and fat increases. NHS guidance also differs for children, where age and sex-specific centile charts apply instead of adult BMI categories.
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.