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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What BMI Is 21.8kg? UK Weight & Health Guide
21.8kg converts to 3 stone 6.3 pounds (3st 6.3lbs) in UK measurements. However, your BMI — which determines whether this weight is healthy — depends entirely on your height. At different heights, 21.8kg can fall anywhere from dangerously underweight to moderately overweight according to NHS guidelines. This guide shows you exactly what BMI 21.8kg gives at common adult heights, and what NHS category you fall into. Use our free BMI calculator above to find your exact BMI instantly.
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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.
21.8kg BMI at Different Heights
Your BMI at 21.8kg varies significantly depending on your height. Here are examples at common adult heights, using the standard BMI formula: weight (kg) divided by height (metres) squared.
- At 150cm (4ft 11in): BMI = 9.7 — dangerously underweight (normal range is 18.5–24.9)
- At 160cm (5ft 3in): BMI = 13.6 — severely underweight
- At 165cm (5ft 5in): BMI = 15.9 — underweight
- At 170cm (5ft 7in): BMI = 15.0 — underweight
- At 175cm (5ft 9in): BMI = 14.1 — underweight
- At 180cm (5ft 11in): BMI = 13.4 — underweight
At 21.8kg, BMI is consistently in the underweight range for adults of all typical heights. If this applies to you, speak with your GP about whether your weight is healthy for your individual circumstances, as very low weight can indicate nutritional or health concerns requiring support.
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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.
NHS BMI Categories & Health Context
The NHS defines healthy BMI as 18.5 to 24.9 for most adults. Below 18.5 is underweight and may indicate malnutrition, eating disorders, or underlying health conditions. BMI is a useful screening tool but has important limitations — it doesn't distinguish between muscle and fat, doesn't account for bone density, and may be less accurate for athletes, elderly adults, or those with significant muscle mass.
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends slightly different thresholds due to higher health risks at lower BMI levels. Regardless of your BMI, your GP can provide personalised advice about what's healthy for you based on your individual circumstances, medical history, and body composition.
- 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 21.8kg in stone and pounds?
21.8kg converts to 3 stone 6.3 pounds (3st 6.3lbs). To convert any weight to kg to stone, divide kilograms by 6.35029. Alternatively, use our free weight converter above for instant conversions between kg, stone, and pounds.
Is 21.8kg a healthy weight?
At 21.8kg, BMI is underweight across all typical adult heights. Whether this is healthy depends on your individual circumstances, age, medical history, and body composition. Contact your GP for personalised advice if you're concerned about your weight.
What BMI do I have at 21.8kg?
Your BMI depends on your height. At 160cm you'd have a BMI of 13.6; at 170cm you'd have a BMI of 15.0; at 180cm you'd have a BMI of 13.4. Use the BMI calculator on this page to find your exact BMI by entering your height and weight.
Why does BMI change with height?
BMI accounts for both weight and height because the same weight looks and feels different depending on how tall you are. A taller person carrying the same weight is distributed over a larger frame, resulting in a lower BMI. This is why BMI is always calculated using both measurements.
Should I be worried if my BMI is underweight at 21.8kg?
Underweight BMI can sometimes indicate nutritional deficiency, eating disorders, or underlying health conditions. However, some people naturally have lower body weight. Speak with your GP who can assess your individual health, medical history, and whether any investigation or support is needed.
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.