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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12.3 Lbs to Kg — Is This a Healthy Weight?
12.3 pounds equals 5.58 kilograms using the standard kilogram to stone conversion. However, whether this weight is healthy depends entirely on your height — there's no single "healthy weight" that applies to everyone. The NHS uses BMI (Body Mass Index) as a screening tool to assess if your weight is healthy for your height. This guide shows you how to interpret your weight in health terms and what the NHS recommends. Use our free BMI calculator above to check your personal BMI instantly based on your exact weight and height.
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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.
Healthy Weight Ranges by Height
Whether 5.58 kg (12.3 lbs) is healthy depends on your height. The NHS defines a healthy BMI range as 18.5 to 24.9 for most UK adults. Here are three concrete examples showing how the same weight falls into different health categories:
- At 150 cm (4ft 11in): 5.58 kg gives a BMI of 2.5 (severely underweight) — significantly below the healthy range
- At 165 cm (5ft 5in): 5.58 kg gives a BMI of 2.0 (severely underweight) — well below healthy weight
- At 180 cm (5ft 11in): 5.58 kg gives a BMI of 1.7 (severely underweight) — dangerously low
At any height, 5.58 kg (12.3 lbs) falls far below a healthy adult weight. A healthy weight range for an adult typically starts at around 40–50 kg depending on height. To convert your own weight between kg, stone, and lbs, use our 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.
What the NHS Recommends If You're Underweight
Being significantly underweight can pose serious health risks, including malnutrition, weak immunity, bone density loss, and hormonal imbalances. The NHS recommends seeing your GP if you're underweight or concerned about your weight. Your GP can assess your individual circumstances, check for underlying health conditions, and refer you to appropriate support such as a dietitian or nutritionist.
Waist circumference also matters for health — the NHS recommends keeping waist measurements under 94 cm (37 inches) for men and under 80 cm (31.5 inches) for women to reduce cardiovascular risk. If weight management is a concern, discuss a personalised approach with your GP rather than attempting rapid changes alone.
- 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 12.3 lbs in kilograms exactly?
12.3 lbs equals 5.58 kilograms. To convert pounds to kilograms, multiply the pounds value by 0.453592. This conversion applies to weight in any context — food, body weight, or other measurements.
Is 12.3 lbs a healthy weight for an adult?
No. 12.3 lbs (5.58 kg) is far below a healthy adult weight at any height, indicating severe underweight status. A healthy weight for an adult typically ranges from 40 kg to 120 kg depending on height and build. See your GP if you have concerns about being underweight.
How do I know my healthy weight range?
Use your height and the NHS BMI guidelines. A healthy BMI is 18.5 to 24.9. Calculate your BMI using our free calculator above, or multiply your height in metres by itself, then divide your weight in kg by that number. Your GP can also advise on your personal healthy weight range.
Why does healthy weight depend on height?
BMI accounts for both weight and height together because taller people naturally weigh more. A weight that's healthy for a 6ft person would be overweight for a 5ft person. That's why the NHS uses BMI rather than a single weight target for all adults.
What should I do if I'm significantly underweight?
Contact your GP for a health assessment. Being severely underweight can affect energy levels, bone health, and immune function. Your GP can identify any underlying causes and refer you to a dietitian or specialist support if 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.