This post is all about weight percentile calculation and BMI percentile

I made this page because I occasionally receive emails from parents of small children, wondering why the BMI calculator classifies their child as overweight or obese and yet the Weight Percentile value didn’t seem overweight.

Back to the BMI and Weight percentile calculator

The bottom line

Weight Percentiles are different from BMI percentiles.

 
Please do not think that the ‘Weight percentile’ value shown below the ‘Body Description’ is a BMI percentile, because these are not exactly the same.

 

 
 

A bit of background about BMI percentiles

 
Before I explain the difference, let me give some background about BMI percentiles. In adults, BMI percentiles are not usually relevant because the classification of people as overweight is based on fixed BMI thresholds of 25 to 30.

Since most of the visitors to my website are adults, especially young adults and since BMI as a classifier of fatness levels has many known flaws, I decided to show weight percentiles.

In adults, using a BMI threshold of 25 as the definition of ‘overweight’ classifies over half of the American population as overweight.

Thus, it is entirely possible to have a Weight percentile at the 50^th percentile and still be considered overweight. This means that you can be average weight and overweight at the same time.

 

 
 

In Children, the Body Description shown on my BMI calculator uses the BMI percentiles 85^th and 95^th percentiles, as thresholds¹.

My calculator differs from the CDC-based calculators, only in the wording: CDC define over 25 as, ‘at risk of… overweight’. In comparison, I just come right out and simply say ‘overweight/obese’.
 

For children:	halls.md terminology	CDC terminology
BMI > 85th percentile:	"Overweight"	"At risk of overweight"
BMI > 95th percentile:	"Obese"	"overweight"

 

Why did I choose to differ from the CDC’s wording?

 
Because most of my website visitors are young adults and I did not want my calculator to behave inconsistently in the transition years between childhood and adult.

A person of age 17.9 years would not be pleased to be ‘at risk of overweight’ and then suddenly be labelled ‘obese’ when they become 18 years old. That does not make sense.

However, for smaller children and their parents, I agree it is both harsh and overly judgemental to label young children ‘obese’. The word ‘obese’ is not an appropriate descriptor of younger children.

Since I can not think of a scientific justification to set a specific age boundary, I instead simply hope parents will use common sense to realize that these body description words are not suitable for young children.

 

 
 

The BMI percentile thresholds for children and the BMI value thresholds for adults¹, are based on Chronological Age. But what if a child is taller than average? Think about it.

 

 
 

Tall children and BMI: The Problem

 
A tall child could easily be mislabelled as obese, even by my own BMI calculator, or on any other calculator or chart based on the CDC thresholds.

However, the Weight percentile calculation takes height into account, and this makes it special and different from the BMI thresholds.

For both children and adults, the Weight percentile calculation considers both chronological age and median-age-for-height. Furthermore, the weight percentile calculation estimates how your weight compares to a population of other people of similar gender, age and height.

 

 
 

Why do Body Mass Index descriptions and Weight Percentile disagree

 
If I haven’t confused you yet, I’m about to… There is another reason why the BMI-derived Body Description and the Weight Percentile appear to disagree.

The body descriptions are based on different data sets from different years. Strange as it may seem, the CDC published new charts⁵ in the year 2000. The charts are based on height, weight and BMI for children.

However, these charts are based on a broad mix of data that spans many decades². Thus, the end result are charts that reflect the way kids were in the 1970’s³.

On the other hand, the Weight Percentile calculation is based exclusively on the NHANES III dataset from the 1990’s⁴.

Hence, you can almost think of the CDC’s BMI thresholds as idealized criteria from the 1970’s – before America started to get really fat. Conversely, the Weight Percentile calculation allows you to compare yourself to the present population.

 

 
 

halls.md BMI Calculator and Children

 
Is the halls.md BMI calculator suitable for evaluating weight status? It is not perfect, but it could be of help. Hardly any other BMI calculators on the internet bother to compare a child’s BMI to the 85^th and 95^th BMI thresholds.

No other BMI calculators on the internet can show Weight Percentiles adjusted for Age AND height. As long as you remember that the Weight Percentile is not a BMI percentile, and you do not apply the labels ‘overweight’ and ‘obese’ to children, then you should be OK.

 

 
 

References

1. Body Mass Index evaluation Criteria
2. Observatons about the CDC child growth charts
3. NHANES I study’s similarity to current CDC’s BMI curves
4. NHANES III. Third National Health and Nutrition Examination Survey. 1988-1994 data from USA.
5. http://www.cdc.gov/nccdphp/dnpa/bmi/bmi-for-age.htm
6. Additional References about Body Mass Index