Body Mass Index (BMI)
- The Body Mass Index (BMI) equation consists of three experimental terms.
- Given all but one of the experimental terms, this tool solves for the missing one.
- To use the tool, one of its form fields must be left empty.
- To empty a field, just double click it.
- If no field is left empty, the tool will randomly empty one and recompute its value based on the values of the other fields.
- This tool cross-maps all three experimental terms that define the body mass index (BMI) of a person, also known as Quetelet's Index (QI).
BMI is similar to the Corpulence Index (CI), but is defined as the body mass divided by the second power of the body height. The index is frequently expressed in SI units of kg/m2, resulting from mass in kilograms and height in meters. So if when using this tool a user input the mass in pounds (lb) and height in inches (in), these will be converted to SI units.
BMI and CI are special cases of a large class of ratios based on scaling of weight for height where the ratio of these two variables is
M/Hx
where x = 2 for BMI and x = 3 for CI.
Notice that dividing a BMI range by height gives a CI range.
- Interpretation of results
BMI is an estimate of a person's thickness. This index suitable for classifying average sedentary populations with an average body composition, and for recognizing trends within sedentary or overweight individuals. The World Health Organization (WHO) uses a BMI classification table similar to the one provided below, but with additional cut-off points (World Health Organization, 2006).
BMI Classification by Categories Category BMI (kg/m2) BMI Prime from to from to Very severely underweight 15 0.60 Severely underweight 15 16 0.60 0.64 Underweight 16 18.5 0.64 0.74 Normal (healthy weight) 18.5 25 0.74 1.0 Overweight 25 30 1.0 1.2 Obese Class I (Moderately obese) 30 35 1.2 1.4 Obese Class II (Severely obese) 35 40 1.4 1.6 Obese Class III (Very severely obese) 40 1.6 where
BMI Prime = BMI/BMIoptimal
where usually
BMIoptimal = 25 kg/m2
because this is the upper limit for a normal, healthy weight.
Notice from the table that individuals with a BMI Prime less than 0.74 are underweight; those with between 0.74 and 1.00 have optimal weight; and those at 1.00 or greater are overweight.
BMI Prime is useful clinically because it shows by what ratio a person deviates from the maximum optimal BMI. For instance, a person with a BMI of 34 kg/m2 has a BMI Prime of 34/25 = 1.36, and is 36% over their upper mass limit.
In general, BMI classification tables vary across countries. In addition, for children (aged 2 - 20), BMI is compared to typical values for other children of same age instead of against fixed classification thresholds like the above.
- Limitations
BMI ignores variations in frames and physical characteristics and does not differentiates between muscle mass and fat mass. The index can be particularly misleading for highly active people like athletes with a high muscle-to-fat ratio.Due to all these limitations, the following BMI alternatives have been proposed:
- Surface-based Body Shape Index (SBSI)
- As Body Shape Index (ABSI)
- Corpulence Index (CI)
- Modified Body Mass Index (mBMI)
- Lab techs as well as physiology or allied health teachers and their students.
- What is the minimum/maximum allowed weight of a 5'10"-tall person with a healthy weight?
- Compute an estimated weight range for severely obese, five-feet tall, people.
- Compute an estimated height range for underweight, 160-pounds heavy, people.
- A six-feet tall man is 10% over his upper mass limit. Estimate his weight.
- 703 is a BMI conversion factor for changing lb/in2 to kg/m2. Derive this factor.
- Wikipedia (2017). Body Mass Index.
- World Health Organization (2006). BMI Classification.
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