Child BMI Calculator

Same sum, completely different rules

The arithmetic here is identical to the adult version. Weight in kilograms divided by height in metres squared. Nothing clever.

Then everything else is different, and the reason is worth understanding because it is the whole point of this tool.

For an adult, that number is compared against fixed lines: under 18.5, under 25, under 30. Those lines never move. They are the same for a 25 year old and a 65 year old, for a woman and a man.

For a child, the number is compared against nothing of the sort. It is compared against other children of the same age and the same sex, and the answer comes back as a percentile rather than a category.

Here is why that is not fussiness. A BMI of 17 in an adult is underweight, and a doctor would want to know why. A BMI of 17 in a six-year-old is entirely unremarkable. Same number, same formula, and the adult lines would tell you a healthy child was underweight. Apply adult categories to children and you will misclassify them constantly, in both directions.

So this tool needs your child's age in years and months, and their sex, and it is not being nosy. Without those it cannot answer the question at all.

Children are supposed to get thinner, then rounder again

The reason the lines cannot be fixed is one of the more charming facts in child development, and almost no parent has heard of it.

A child's BMI is not supposed to hold steady as they grow. It is supposed to follow a curve, and the curve goes down before it goes up.

Think about the babies you have met. A one-year-old is round. That is the correct state of a one-year-old, and their BMI is high. Then the round toddler turns into a lanky four-year-old, all knees and elbows, and their BMI falls, reaching its lowest point somewhere around five or six. Nothing has gone wrong. They are stretching upward faster than they are filling out, exactly as they should.

Then it turns around and begins climbing again through later childhood and adolescence. That turning point has a name, the adiposity rebound, and clinicians pay attention to it, because when it arrives is informative in itself.

Now look at what that does to any fixed line. A BMI of 17 is near the peak of thinness for a six-year-old and utterly normal. The same 17 in a fifteen-year-old, whose BMI should have been rising for years, is low. The number has not changed. The child's place on their own curve has. This is why children get percentiles and adults get categories, and it is not a detail, it is the entire reason this is a separate tool.

What the four categories are, and where the lines sit

The percentile gets sorted into one of four bands, which are the standard CDC categories for children and teenagers:

Two things about that table are worth noticing, because they surprise people.

The first is how wide the healthy band is: from the 5th all the way to the 85th. That is eighty percent of children. Four children in five land in it. If your child is in that band, and most children are, the tool has told you the boringly good news and there is nothing further to do.

The second is that these are percentile lines, which means they are defined by where children sit relative to each other rather than by any biological threshold. The 95th percentile is, by construction, roughly the heaviest one in twenty. That is a comparison, not a diagnosis, and it is why the categories are best read as flags for a conversation rather than as verdicts.

The chart runs from two to twenty. Below two, BMI is not used at all, and you want our Baby Growth Percentile Calculator instead.

Running one through

Take a boy of 10 years and 6 months, 140 cm tall, weighing 34 kg. First the arithmetic, which is the easy half:

BMI = 34 ÷ (1.40 × 1.40)
BMI = 34 ÷ 1.96
BMI ≈ 17.3

Now the half that matters. That 17.3 is looked up against the BMI-for-age curves for boys at 10 years 6 months, and the tool works out where it falls between the plotted percentile lines, reporting something like "approximately the 47th percentile" and a category of healthy weight.

Notice what the adult rules would have done with the same number. A BMI of 17.3 in an adult is under 18.5, which means underweight, which means a doctor asks questions. In this ten-year-old it is almost exactly average. The formula gave the same answer; the comparison changed the meaning completely.

That is the single most useful thing to take away from this page. Never compare a child's BMI number to an adult's. Never compare it to a sibling's at a different age. The only comparison that means anything is against other children of the same age and sex, which is what the percentile already did for you.

The things it still cannot see

BMI-for-age is a considerable improvement on adult BMI applied to a child. It is still BMI, and it inherits the family flaw: it knows mass and height, and nothing about what the mass is made of.

• It cannot tell muscle from fat. A well-built, athletic twelve-year-old can land in the overweight band while carrying very little fat. This is a routine misclassification, not an edge case.
• Puberty scrambles it. Children develop on wildly different timetables, and BMI-for-age compares by chronological age rather than developmental stage. An early developer and a late developer of the same birthday are not really comparable, and for a few years the percentile is partly measuring the timing of puberty.
• Body composition changes at different rates by sex. Girls gain body fat through puberty as a normal and necessary part of development. The girls' chart accounts for this; a parent's instinct often does not.
• It is a reference, not a standard. These charts describe how American children actually grew between 1963 and 1994, not how children ought to grow. The CDC is explicit that the charts do not imply the growth they capture is healthy. Our Child Weight Percentile Calculator goes into what that means.
• One reading is a dot. As with every growth chart, the trend over years is the information. A child tracking the same band is doing the normal thing. A child crossing bands is the observation worth making.

If the answer worried you

Two directions, and the same underlying advice.

If it came back low: check it against the height percentile before doing anything. A small child in proportion is a small child, not a problem. What is worth a doctor's attention is a child whose BMI has been falling across bands, a child who has lost weight, or a child who seems unwell or tired alongside it. Unintended weight loss in a child is always worth asking about.

If it came back high: please be careful about what happens next, and particularly about what your child hears. This is the part we would most want you to take from the page. Weight-focused talk aimed at children does not have a good record: the evidence associates it with worse outcomes rather than better ones, including the outcome it is meant to prevent, and it can start something that outlasts childhood by decades.

What does help is quiet and structural, and none of it requires your child to know a number. Food that is simply what the family eats, at roughly the times the family eats. Movement that is play rather than exercise. Sleep, which affects this more than most parents expect. No forbidden foods, since forbidding is the most reliable way to make something fascinating. And nothing framed as a diet, or as your child's problem.

Above all, take a real concern to a paediatrician rather than acting on a web page. They can see years of growth rather than one dot, they can check whether height and weight are moving together, and they can do the whole thing without a child ever being told that a calculator had an opinion about their body.

Questions people ask

Why can I not just use the adult BMI categories?

Because a child's BMI is supposed to change with age. It falls to a low point around five or six and then rises again, so a fixed line would misclassify healthy children constantly. Children are compared against other children of the same age and sex instead.

What percentile is healthy?

Anywhere from the 5th to below the 85th, which is four children in five. It is a deliberately wide band, and being inside it is the ordinary outcome.

My child is sporty and it says overweight. Is it wrong?

Quite possibly. BMI cannot tell muscle from fat at any age, and a well-built athletic child is one of the most common misclassifications. Worth mentioning to a doctor, who can look at the child rather than the number.

Can I use this for a toddler?

Not under two. BMI is not used below that age. Use our Baby Growth Percentile Calculator, which uses the WHO standards recommended for the early years.

Should I tell my child their number?

We would not. There is little upside and a real, documented downside to making a child's body a subject of measurement and discussion. If something needs addressing, it can be addressed through what the family does rather than through what the child is told.

References

Where the figures come from. The BMI-for-age percentile categories, and the charts behind them, are the CDC growth references for children and teenagers aged 2 to 20, recommended by the CDC and the American Academy of Pediatrics for that age range. The CDC also sets out the distinction between a prescriptive standard and a descriptive reference, and the recommendation to use the WHO growth standards below two years, published formally by Grummer-Strawn and colleagues.

1. Centers for Disease Control and Prevention. About Body Mass Index (BMI). https://www.cdc.gov/bmi/about/
2. Centers for Disease Control and Prevention. Growth charts: recommendations and rationale. https://www.cdc.gov/growth-chart-training/hcp/using-growth-charts/recommendations-and-rationale.html
3. Grummer-Strawn LM, Reinold C, Krebs NF. Use of World Health Organization and CDC growth charts for children aged 0-59 months in the United States. MMWR Recommendations and Reports. 2010;59(RR-9):1-15. https://pubmed.ncbi.nlm.nih.gov/20829749/