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^{1} 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 thresholds^{1}.

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 adults^{1}, 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.

no pun intended.

### 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 charts^{5} 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 decades^{2}. Thus, the end result are charts that reflect the way kids were in the **1970’s**^{3}.

On the other hand, the Weight Percentile calculation is based exclusively on the NHANES III dataset from the 1990’s^{4}.

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.

Discover your BMI

**and**Weight Percentile here.

### References

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