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 Should Be The Weight Of A 13-Year-Old Boy In Kg?
There is no single "ideal weight" for a 13-year-old boy — healthy weight depends entirely on his height, growth stage, and body composition. Unlike adults, children and teenagers should not use adult BMI categories. The NHS uses age and sex-specific growth charts to assess whether a young person's weight is healthy. If you're concerned about your son's weight or development, speak to your GP, who can plot his measurements on the UK90 growth reference charts and provide personalised guidance based on where he sits relative to his peers.
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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 For 13-Year-Old Boys
The table below shows approximate healthy weight ranges for 13-year-old boys at common heights, based on UK growth standards. These are guidelines only — individual healthy weight varies depending on muscle mass, bone density, and frame size.
- 4ft 8in (142cm): 35–48 kg (77–106 lbs)
- 4ft 10in (147cm): 38–52 kg (84–115 lbs)
- 5ft 0in (152cm): 42–58 kg (93–128 lbs)
- 5ft 2in (157cm): 46–63 kg (101–139 lbs)
- 5ft 4in (163cm): 50–69 kg (110–152 lbs)
- 5ft 6in (168cm): 55–76 kg (121–168 lbs)
- 5ft 8in (173cm): 60–82 kg (132–181 lbs)
- 5ft 10in (178cm): 65–89 kg (143–196 lbs)
- 6ft 0in (183cm): 71–97 kg (156–214 lbs)
- 6ft 2in (188cm): 77–105 kg (170–231 lbs)
These ranges assume healthy body composition. Boys at 13 grow at different rates — some may not reach their adult height until their late teens. Your GP can advise whether your son's current weight is appropriate for his stage of development.
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.
Why Healthy Weight Matters For Teenagers — And What Affects It
A healthy weight range for a 13-year-old supports physical development, energy levels, mood, and long-term health. However, many factors influence what's healthy for an individual boy: muscle mass (boys who play sport or do strength training may weigh more but be very lean), frame size (some boys naturally have larger or smaller bones), ethnicity (growth patterns vary between populations), and growth timing (some 13-year-olds haven't started their growth spurt yet, while others are finishing it). Rather than focusing on a specific number, the best approach is regular check-ups with your GP or school nurse, who can assess growth trends and identify any concerns early.
- 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.
How do I know if my 13-year-old son's weight is healthy?
The best way is to see your GP or school nurse, who will plot his height and weight on the UK90 growth reference charts. These show where he sits compared to other boys his age. A single measurement doesn't tell the whole story — what matters is the trend over time and his overall health, energy, and fitness.
Can I use BMI for a 13-year-old boy?
Adult BMI categories do not apply to children and teenagers. Children use age and sex-specific BMI centile charts instead. The NHS Healthy Weight Calculator specifically warns against using adult BMI for young people. Always consult your GP for personalised assessment.
What if my 13-year-old boy is heavier than the ranges shown?
Weight alone doesn't indicate poor health. Muscle weighs more than fat, and many athletic teenagers are heavier than sedentary peers. Your GP can assess overall health, fitness, diet, and development. If there are genuine concerns, your GP can refer your son to a specialist or dietitian.
Does a 13-year-old need to lose weight?
Weight loss in teenagers should only ever be managed under professional medical supervision — never at home. If your GP is concerned about your son's health, they will discuss appropriate support, which typically focuses on building healthy habits rather than restriction or dieting.
What's the best way to support a healthy weight in a 13-year-old?
Focus on sustainable habits: regular physical activity (sports, walking, cycling), balanced meals with plenty of vegetables, limiting sugary drinks, and good sleep. Avoid labelling foods as "good" or "bad" or discussing weight negatively — this can harm body image and relationship with food during this sensitive developmental period.
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.