Worrying about your child's weight is one of the most common concerns parents bring to their child's doctor. You might have noticed clothes fitting differently, or a comment from a relative, or simply a nagging feeling that something has changed. Whatever brought you here, the most important thing to know upfront is this: you cannot reliably assess a child's weight by eye, and adult BMI charts do not apply to children.
This guide walks through how weight is actually assessed in children, what the categories mean, what tends to drive excess weight gain, and — most importantly — what to do next in a way that supports your child without causing harm.
Why adult BMI doesn't apply to children
Body Mass Index (BMI) is a ratio of weight to height squared. In adults, a fixed set of cut-offs applies to everyone — below 18.5 is underweight, 25–29.9 is overweight, and so on. Those thresholds are the same whether you are 25 or 65.
Children are different. Their bodies are constantly changing. The amount of body fat a child carries naturally shifts as they grow, and it differs between boys and girls — particularly around puberty. A BMI that would be "overweight" on an adult chart could be perfectly normal for a nine-year-old at a certain stage of development.
This is why paediatricians and pediatricians use BMI-for-age percentiles rather than fixed BMI thresholds. Instead of asking "what is the BMI number?", the question becomes "how does this child's BMI compare to other children of the same age and sex?" That comparison is made using population-based growth charts — the CDC charts in the United States and the RCPCH (Royal College of Paediatrics and Child Health) charts in the United Kingdom.
A child's BMI percentile tells you where they sit in the distribution of BMIs for children their age. A child at the 60th percentile has a higher BMI than 60% of children their age but a lower BMI than 40% — which is entirely within the normal range.
What do the weight categories mean?
The following categories are used for children and teenagers aged 2–19, based on CDC guidelines. UK guidance from the RCPCH uses the same percentile structure, though the underlying reference population differs slightly.
| Category | BMI-for-age percentile |
|---|---|
| Underweight | Below the 5th percentile |
| Healthy weight | 5th percentile to below the 85th percentile |
| Overweight | 85th percentile to below the 95th percentile |
| Obese | 95th percentile and above |
Source: Centers for Disease Control and Prevention (CDC), 2000 Growth Charts.
A few things worth noting. First, a child at the 84th percentile is still within the healthy weight range — even if they seem larger than many of their peers. Second, these categories are screening tools, not diagnoses. A child with a very muscular build may land at a high percentile without carrying excess body fat. This is why a number on a chart always needs to be interpreted by a clinician alongside the child's full health picture.
How to check your child's weight at home
You can get a rough idea of where your child sits using our child growth calculator, which calculates BMI-for-age percentile when you enter your child's age, sex, height and weight.
For accurate measurements, technique matters more than you might expect. Our guide on how to measure your child's height and weight covers the right way to take both measurements at home so the numbers you enter are reliable.
If you want to understand what percentiles mean in more depth before drawing any conclusions, BMI-for-age in children explained is worth reading first.
Home calculations are a starting point only. If the result places your child at or above the 85th percentile, the appropriate next step is a conversation with your GP or paediatrician — not a change in diet based on a calculator result.
Common causes of excess weight gain in children
Weight in children is shaped by a combination of factors, most of which are environmental and behavioural rather than moral failures on anyone's part.
Sedentary time has increased sharply over the past two decades. Screen time — television, tablets, gaming — displaces physical activity and is also associated with more frequent snacking. The NHS recommends children aged 5–18 get at least 60 minutes of moderate-to-vigorous physical activity every day; many children fall well short of this.
Ultra-processed foods are engineered to be hyper-palatable and are very easy to overconsume. They tend to be calorie-dense, low in fibre and protein, and digest quickly — leaving children hungry again sooner. They are also heavily marketed to children. Reducing ultra-processed food intake is one of the most consistently supported dietary changes in paediatric weight research.
Sleep deprivation is an underappreciated driver of weight gain. Children who sleep fewer hours than recommended for their age have higher levels of the hunger hormone ghrelin and lower levels of leptin, the hormone that signals fullness. This makes it harder to regulate appetite. A child who is consistently tired will also be less active during the day.
Genetic and medical causes do exist but are much less common than lifestyle factors. Conditions such as hypothyroidism (an underactive thyroid) or Cushing's syndrome can cause weight gain, and these are worth ruling out if your child's weight gain has been rapid or is accompanied by other symptoms such as fatigue, slow growth or other changes. Your child's doctor can order straightforward blood tests to check for these.
What to do if you're concerned
The most important first step is to speak to your child's GP or paediatrician before making any significant changes to your child's eating or exercise routine. They can confirm whether your child's weight is a clinical concern, rule out medical causes, and refer you to a registered dietitian or specialist paediatric weight management service if needed.
If your doctor does confirm your child would benefit from some changes, the evidence strongly supports a family-based approach. This means the whole family shifts toward healthier habits together — not singling out the child with a different plate of food or a separate set of rules. Changes made this way tend to stick better and avoid the psychological harm that comes with a child feeling they are the problem.
For most children who are overweight but still growing, the goal is often weight maintenance rather than weight loss — meaning that as the child grows taller, their BMI naturally falls. This is less stressful for the child and medically appropriate for many ages and stages.
Practical family-based changes that the evidence supports: eating more whole foods and fewer ultra-processed snacks, cooking at home more often, eating together at a table without screens, building daily physical activity into the routine (walks, bike rides, swimming — whatever the child enjoys), and protecting adequate sleep.
No single food needs to be banned. Restriction of specific foods often backfires, increasing a child's preoccupation with those foods. A more effective approach is to make nutritious foods the easy, default choice at home.
What NOT to do
Do not put your child on an adult diet plan. Calorie-restricted diets designed for adults are not appropriate for children who are still growing and developing. They can interfere with bone density, growth, and — particularly in adolescents — can contribute to disordered eating.
Avoid commenting on your child's body at the table or in front of others. Children are highly sensitive to how their bodies are discussed. Remarks about weight — even well-intentioned ones — can cause lasting damage to a child's relationship with food and their own body image. If you need to have a conversation with your child, keep it private, focus on health and energy rather than appearance, and keep your tone calm and supportive.
Do not skip meals as a strategy. Restricting meals tends to increase hunger, reduce energy for activity, and lead to larger portions when food is available. Regular, balanced meals and structured snack times are more effective for weight regulation in children.
Do not delay speaking to a doctor out of embarrassment. Paediatricians and GPs discuss childhood weight every day. Raising the topic early — before a child reaches a higher percentile or develops related health issues — gives more options and better outcomes.
For a deeper understanding of how weight is assessed in children, read our guides on BMI-for-age in children explained and understanding growth charts for parents.