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
Is 13.1 Kg a Healthy Weight?
13.1 kilograms converts to 28.9 pounds (or 2 stone 0.9 lbs in UK measurements). Whether this weight is healthy depends entirely on your height — the same weight can be perfectly normal for one person and concerning for another. To understand if 13.1 kg is right for you, we need to calculate your BMI using both your weight and height. Use our free BMI calculator above to check your personal BMI instantly, and discover what the NHS considers a healthy weight range for your specific 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
A weight of 13.1 kg would only be healthy for a very young child — it is far too low for any adult. To understand healthy weights, here are three real-world examples using metric to imperial weight conversion:
- Adult 160 cm tall (5ft 3in): Healthy weight range is 47–63 kg (7st 6lbs – 9st 13lbs). At 13.1 kg, this person would be severely underweight.
- Adult 170 cm tall (5ft 7in): Healthy weight range is 53–71 kg (8st 5lbs – 11st 3lbs). At 13.1 kg, this person would be dangerously underweight.
- Adult 180 cm tall (5ft 11in): Healthy weight range is 60–80 kg (9st 6lbs – 12st 9lbs). At 13.1 kg, this person would be critically underweight.
If you weigh 13.1 kg as an adult, please speak to your GP immediately. If this is a child's weight, use the NHS Healthy Weight Calculator for children to assess growth properly.
Clinically BackedWeight Loss
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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 to Do If You're Underweight
Being significantly underweight carries serious health risks including malnutrition, weakened immunity, bone loss, and hormonal problems. The NHS recommends that adults maintain a BMI between 18.5 and 24.9. If you're underweight, your GP can investigate the cause — it may be medical, nutritional, or related to an eating disorder.
If you're concerned about your weight, contact your GP for personalised advice. They can refer you to a dietitian or specialist if needed. Our free weight converter helps you understand your weight in both metric and imperial units, but your healthcare professional should guide treatment. Never attempt rapid weight gain without medical supervision.
- 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 13.1 kg in pounds exactly?
13.1 kilograms equals 28.9 pounds (or 28 pounds 14.4 ounces). In UK stone measurements, 13.1 kg is 2 stone and 0.9 pounds. Use our free weight converter above to convert between kilograms, stone, and pounds instantly.
Is 13.1 kg a healthy weight for an adult?
No. 13.1 kg is dangerously low for any adult, regardless of height. A healthy adult weight typically ranges from 45 kg (7st) to over 100 kg (15st 10lbs) depending on height. If you weigh 13.1 kg, contact your GP urgently — this indicates severe malnutrition or illness.
Could 13.1 kg be healthy for a child?
It depends on the child's age. 13.1 kg might be appropriate for a child aged 4–6 years, but it varies significantly by age and growth pattern. Use the NHS Healthy Weight Calculator for children, or ask your health visitor for personalised guidance on your child's healthy weight range.
How do I check if my weight is healthy?
Calculate your BMI using our free BMI calculator on this page — it combines your height and weight to show whether you're in the healthy range (18.5–24.9 for most adults, or 18.5–22.9 for South Asian adults). Your GP can also assess your individual healthy weight range.
What should I do if I'm significantly underweight?
Contact your GP for assessment. Significant underweight can indicate malnutrition, illness, or eating disorders — all of which need professional support. Your GP can refer you to a dietitian or specialist. Never attempt rapid weight gain alone; medical guidance is essential for safe, healthy recovery.
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.