There are multiple BMI calculators out there, including our own online Body Mass Index calculator. Some of them make a distinction between the sexes and produce different results for men and women. Others don’t make such a distinction, including those put forth by the most respected health organizations such as the U.S. CDC, NIH, and the U.K. NHS*. In this article we set out to explore whether it makes sense to have a separate BMI for women, and therefore whether having a BMI calculator for women makes any sense.
Here is how this article is structured:
- What is the Body Mass Index (BMI)? – a brief refresher on its definition and utility
- Does BMI for women make sense?
- Should we calculate women’s BMI differently?
- Should we use a separate BMI scale for women?
- A BMI calculator for women?
What is the Body Mass Index (BMI)?
BMI, an acronym of Body Mass Index, is a composite measure for body mass relative to body size. The BMI formula is simply:
BMI (kg/m2) = weight (kg) / height2 (m)
It is therefore obvious that BMI only takes into account two of your body measurements – your body mass (weight) and your height. These are measurements anyone can take in mere seconds and this ease of use is by design.
To be clear, your body mass index doesn’t incorporate information like body fat percentage (doesn’t differentiate between lean body mass and body fat), age, or gender. Put simply, a 46 year old man who is 180 cm (5 ft. 11 in.) high and has a body mass of 82 kg (181 lbs) has exactly the same BMI as a 25 year old woman whose stature is 180 cm and whose body mass is 82 kg (181 lbs).
Why is BMI useful?
Given its simplicity, the body mass index is an incredibly useful statistic. Different BMI ranges have been associated with an increased relative risk of suffering from health conditions such as gallstones, high blood pressure, type II diabetes, cancer, heart disease, as well as all-cause mortality. Being a measure for obesity it is also related to overall well-being.
The body mass index has its limitations, especially when it comes to athletes involved in particular sports which require the development of unusual percentages of muscle tissue such as weightlifting, american football, and others. Furthermore, as a statistical measure, its application to individual cases of both men and women is subject to significant caveats.
Does BMI for women make sense?
That said, we can turn to the major question: does BMI for women actually make sense? Under what conditions would it be useful to calculate the index differently depending on gender?
Is average Body Mass Index different for men and women?
The first scenario in which we might want to calculate BMI differently, or apply adjustments to the calculated number, is if men and women show markedly different average values or percentile distributions. Say if the average for men is much higher than that of women, or if the 90-th percentile for men is at BMI > 30 while the 90-th percentile for women is at BMI > 28.
Several large sample studies show different levels of difference in average BMI between men and women:
| Source | Sample Size | Mean BMI (Men) | Mean BMI (Women) | Difference (Men-Women) |
|---|---|---|---|---|
| Dorn et al. [1] | 1,308 | 25.5 | 24.7 | 0.8 kg/m2 |
| Johansson et al. [2] (1986 data) | 126,546** | 25.5 | 24.8 | 0.7 kg/m2 |
| Johansson et al. [2] (2010 data) | 126,546** | 27.1 | 25.9 | 1.2 kg/m2 |
| Rho et al. [3] | 172,395 | 27.5 | 27.7 | -0.2 kg/m2 |
It should also be noted that Dorn et al. [1] found the 85-the percentile for both men and women in their sample to be just over BMI 27 (27.5 for men, 27.1 for women).
The data seems contradictory with the largest study (Rho et al.) showing a result opposite to the others. Even if we accept a difference of 1 kg/m2 it would mean that only borderline cases would be potentially classified incorrectly. Adhering to and acting based on strict classification bounds is not something physicians and nutritionists should do anyways. Therefore, even if we accept that a difference in average BMI is grounds for a separate scale for women the impact of such a difference should be negligible in practice due to how small it is.
Now we come to the more important question:
Do BMI-related health outcomes differ between men and women?
Even if we establish a marked difference in average BMI or certain BMI quantiles, it is not yet enough to justify using a different BMI scale for women as opposed to men. Remember that the purpose of the body mass index is to allow quick assessment of certain risks, as well to serve as a possible predictor for particular health outcomes.
So, for a women-specific BMI to make any sense, there has to be marked and uniform difference in risks and health outcomes for the two genders.
To work with a concrete example, let us take Logue et al. [4] wherein in a study of more than 95,000 people it was found that men diagnosed with type II diabetes had mean BMI of 31.83 whereas as women of the same category had a mean BMI of 33.69. If such a relationship held for gallstones, high blood pressure, cancer, heart disease, all-cause mortality and other diseases, then maybe having separate BMI scales (underweight, normal, overweight, obese, etc.) for mean and women would make sense.
An alternative would be to have a separate BMI calculation formula for women and for men.
However that is simply not the case. BMI has the same predictive value for men and women in some diseases or it might exhibit opposing relationships for others. Having separate scales or body mass indices would only confuse physicians and the general public who are worried about their health or are looking to lose weight.
Should we calculate women’s BMI differently?
As argued above, it doesn’t seem that having a different BMI formula for women makes sense. There is no justification for an adjustment of the standard formula in any direction as the health outcomes and risks associated with BMI do not change uniformly for adult women versus men.
Using a separate BMI equation for women would result in biasing the index up or down for certain diseases more so than it might currently be thus adding unnecessary confusion regarding its interpretation.
Should we use a separate BMI scale for women?
The standards for the U.S. Center for Disease Control, the U.S. National Institute for Health, the U.K. National Health Institute, and the World Health Organization all agree that men and women should not be judged on a different scale. Based on the arguments presented above I also don’t think a generalized gender-based differential scaling would have any value.
However, disease-specific scales might be developed separately so that a man with a BMI of 31 is considered as having the same risk of type two diabetes as a woman with a BMI of 33, for example (following Logue et al.). Following the same logic, age-specific scales can be developed as well. Based on Fig 1. in Logue et al. we can say that a BMI of 30 for men and 31 for women aged 60 is equivalent to a BMI 35 for men and 37-38 for women in terms of associated risk of type II diabetes.
However, if we go this way, we might as well add other known predictors such as body fat percentage and so on. This, however, will kind of defeat the purpose of the body mass index as being a simple and easy to compute metric that can be used in a quick heuristic.
A BMI calculator for women?
In light of the above, we can answer the second question posed in the title – does a body mass index calculator specific to women make sense? We can answer in the negative, as far as the output is BMI in kg/m2. The answer is also in the negative if the calculator outputs general categories like overweight, obese, etc.
However, it might be interesting to develop a calculator with disease-specific output which includes gender-specific risk group or risk level corresponding to the resulting BMI. This also requires that a number of high-quality large population studies are present for each disease, which would limit the number of supported diseases significantly.
* While the NHS BMI calculator has an input field for gender, it states in its documentation that: “The adult BMI does not take into account age, gender or muscle mass.”
** The sample size consists of the entire dataset over the period from 1986 to 2010
References
[1] Dorn et al. (1997) “Body mass index and mortality in a general population sample of men and women. The Buffalo Health Study.” American Journal of Epidemiology, 146(11) p.919-31, DOI: 10.1093/oxfordjournals.aje.a009218
[2] Johansson et al. (2012) “Associations among 25-year trends in diet, cholesterol and BMI from 140,000 observations in men and women in Northern Sweden”, Nutrition Journal, (11),40, DOI: 10.1186/1475-2891-11-40
[3] Rho et al. (2014) “Independent impact of gout on the risk of diabetes mellitus among women and men: a population-based, BMI-matched cohort study.” Annals of the Rheumatic Diseases 75(1) p.91-5, DOI: 10.1136/annrheumdis-2014-205827
[4] Logue et al. (2011) “Do men develop type 2 diabetes at lower body mass indices than women?” Diabetologia 54(12) p.3003-6, DOI: 10.1007/s00125-011-2313-3
An applied statistician, data analyst, and optimizer by calling, Georgi has expertise in web analytics, statistics, design of experiments, and business risk management. He covers a variety of topics where mathematical models and statistics are useful. Georgi is also the author of “Statistical Methods in Online A/B Testing”.