BMI Calculator

1. What is BMI?
2. BMI formula
3. How to calculate your Body Mass Index?
4. BMI classes / categories
5. What is BMI Prime?
6. How reliable are BMI calculations?
7. BMI calculator for women

    What is BMI?

Body Mass Index (BMI) is an index that attempt to quantify the amount of muscle, fat, and bone mass in an individual, and categorize them as underweight, normal weight, overweight, or obese, based on the index value. The simplicity of the body mass index calculation is one of its advantages: to calculate it one only needs to know his or her weight and height. Our BMI calculator also requires just these two measurements as input and the output is in the units BMI is measured: kg per square meter (kg/m²), which is often left out in both writing and speech.

The concept of the BMI is due to one Adolphe Quetelet - a Belgian astronomer, mathematician, statistician and sociologist and his work on "social physics" between 1830 to 1850. The modern term "body mass index" (BMI) was coined in a paper published in the July 1972 edition of the Journal of Chronic Diseases by Ancel Keys et al.., in which it was argued that what was defined as BMI was "...if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity" ^[3]. In general, BMI is useful when applied to adults, but may be ill-suited when used for young children or even teenagers where the waist to hip ratio and body fat estimates may be more informative.

The importance and usefulness of the index and hence any Body Mass Index calculator is in its curvilinear relationship to all-cause mortality. The bigger your calculated BMI is, the greater the risk for cardiovascular complications like hypertension, type II diabetes, some types of cancer, renal disease, degenerative arthritis, gallstones, all of which have links to excessive weight relative to one's height ^[1][2]. See below for a list of BMI categories (classifications).

    BMI formula

The Body Mass Index formula is simple:

BMI (kg/m²) = weight (kg) / height² (m)

Of course, in our online BMI calculator you can enter either SI units or imperial metrics for height (feet and inches) as well as weight pounds for weight. The tool does the conversions for you. Weight is referred to as "mass" in medical literature, while height is often called "stature".

    How to calculate your Body Mass Index?

So, how to calculate BMI knowling the formula above? It is a straightforward task, especially if you already know your height and weight in meters and kg. Then just multiply your height by itself, and then divide your weight by the result. If you do not, some conversions might be necessary.

For example, if you are 5 feet 10 inches tall, that is 5 x 12 + 10 = 70 inches. 1 inch is 2.54 cm so you are 70 x 2.54 = 177.8 cm tall. 1 cm = 0.01 m, so you are 177.8 x 0.01 = 1.778 meters tall. 1.778 x 1.778 = 3.16

If you also weigh 170 pounds, that is 170 * 0.453 = 77 kg. 77 / 3.16 = 24.36, so your BMI is ~24, putting you just in the healthy weight range for your given height.

    BMI classes / categories

Individuals are classified in several BMI categories (classes) according to their mortality risk. There are different approaches to categorization, but below is the widely accepted WHO categorization.

BMI categories and corresponding BMI prime and risk levels

Category	BMI range (kg/m2)	BMI Prime	Risk level
Very severely underweight	less than 15	less than 0.60	Moderate
Severely underweight	15 - 15.9	0.60 - 0.64	Moderate
Underweight	16 - 18.4	0.64 - 0.74	Moderate
Normal (healthy)	18.5 - 24.9	0.74 - 1.0	Very low
Overweight	25 - 29.9	1.0 - 1.2	Low
Moderately obese	30 - 34.9	1.2 - 1.4	Moderate
Severely obese	35 - 39.9	1.4 - 1.6	High
Very severely obese	40+	1.6+	Very high

As you can see, there is a moderate risk in being underweight, but obesity is certainly a primary risk factor for different sorts of causes for mortality. Sticking to a healthy BMI of between 18.5 and 24.9 (normal weight / healthy weight range) is therefore recommended by health authorities such as the US CDC (Center for Disease Control and Prevention) in order to minimize the risk of health problems. With our calculator you will also get a percentage over/under the normal limit as an output.

Graph - BMI health risks

The graph shows relationship between being overweight or obese according to BMI measures and relative risk of different diseases.

* some health risks are calculated differently than others. Risk of death is versus BMI < 19, risk of type II diabetes is versus BMI < 22-23, high blood pressure vs BMI < 23, arthritis and gallstones are vs. BMI < 24, neural birth defects vs 19-27. This is because the data comes from different studies.

The calculator outputs a chart with 35-year absolute risk of all-cause death for men and women separately ^[4], and plots your BMI score so you can estimate your risk level. Note that the risk of death is not the only negative outcome associated with being underweight or overweight. Conditions like diabetes can severely reduce your quality of life and the risk of developing it increases sharply with increased body mass index values as shown in the next section.

Graph - BMI and type II diabetes

Type II diabetes and its relationship to body mass index needed to be plotted separately due to the extremely high increases of relative risk per point - reaching 50 times increased relative risk at BMI 33-35.

The data for both graphs is from reference 2 from the NCHS. Both support the idea that a healthy weight for one's height - a good proxy for which can be the presence of a slim waist - is good for health outcomes.

    What is BMI Prime?

BMI Prime is a mathematical transformation of the BMI index where you calculate the ratio between the actual BMI and the upper limit optimal BMI of 25. It is a dimensionless number and is unit-independent. Its main advantage is ease of use, e.g. if you are above 1.0 you know you are overweight and having a BMI Prime of 1.30 means you are 30% (0.3 * 100) above the optimum.

Since optimal BMI may vary between populations, the BMI Prime number is a useful comparison tool as it can be calculated differently for the different populations but result in the same scale. Our tool also serves as a BMI Prime calculator - just check the output.

    How reliable are BMI calculations?

BMI, like any other statistical measure applied to an individual, has its limitations. The especially noteworthy limitation concerns athletes in whom a high BMI can lead to incorrectly interpreting it as excess body fat. This happens with individuals with excessive muscle mass due to genetic makeup or training.

Large-size, field-event athletes, body-builders, weight lifters, heavy weight wrestlers and boxers, and professional football players are all at risk of being misclassified. To take a U.S. example: the average BMI of the defensive linemen from a former NFL Super Bowl team averaged almost 32, placing these athletes in the "obese" category with moderate mortality risk - a clear illustration of the limitations in certain cases. The limitations apply to all BMI charts and modern body mass index online calculators.

    BMI calculator for women

We have entertained the idea of developing a calculator specific to women but having a separate BMI score for women doesn't seem to be supported by medical science. At best, it seems that the output can be made disease-specific wherein the associated risk of a certain disease is classified differently for men and women, depending on the available scientific studies. However, this goes beyond what a simple adult BMI calculator is supposed to do. Full discussion on the topic: BMI for Women: Does a BMI Calculator for Women Make Sense?.

* the hazard ratio is relative to the reference population which is the WHO BMI group with a score between 18.5 and 25 and for which the value is 1.00. A hazard ratio of 2.00 would mean there is twice the chance that the event would occur compared to the probability for the reference group.