BMI Calculator

BMI Basics

Human bodies are oddly shaped and it’s not easy applying any generic mathematical formula to it. But for the sake of simplicity, a Belgian mathematician and statistician Adolphe Quetelet came up with this calculation, because we needed a way to study the population as a whole, rather  than any single individual. His calculation proved to be so successful, it stuck all the way to modern days.

So what is this formula? - It goes like this BMI = kg/m2; in other words, you divide your weight by the square of your height. One important caveat about BMI is that it’s not an end all be all indicator of one’s “body fatness” or their weight. It’s a number that quantifies the tissue mass and puts you into a certain category, based on the number of the entire population. You can be:

• Underweight
• Normal weight
• Overweight or
• Obese

depending on the BMI range your results fall under. These categories are based on the general population and each one is associated with its own level of health risks (or the probability of developing them). 

But, as we already said, this number is just an indication researchers/doctors use to point them into a certain direction - whether further tests are needed. Though BMI proved relatively accurate and reliable when used for general population, it becomes less accurate when used in extreme cases such as:

• Bodybuilders
• Endurance/long-distance athletes
• Pregnant women
• People with certain disabilities etc

Why? Because BMI calculation “doesn’t” know if the number is high because of excessive fat or muscle. In other words, if you are a big buff dude, your BMI might still put you in the “overweight” category, simply because your number is just too high.

BMI Health Risk Classification

Based on your BMI, your result will put you into one of the following groups:

BMI Categorization

When talking about BMI calculations, we saw that this is not, by any means, an exact science. Similar can be said for the BMI ranges and percentages. For simplicity’s sake, we just talk about  and use one scale, but there are many different ones. The most commonly used are:

• WHO/CDC Standard BMI scale
• Gallagher et al.
• American Council on Exercise (ACE) and
• Deurenberg formula

WHO/CDC BMI Index Table

Gallagher et al.

American Council on Exercise (ACE)

This table is mostly used in sports and general fitness:

BMI for Children

Above-listed charts mostly apply on adults, but if we’re talking about children (starting at the age of 2) and teens, the numbers are slightly modified:

Risks and Dangers

We all know about the risks associated with excess weight and obesity, but not many people talk about the dangers of being underweight and malnourished. To help you fully comprehend the issue, we’re going to be talking about the issues from both ends of the spectrum.

Risks Associated with Being Underweight

Though not as severe as being obese, being underweight comes with its own set of issues. Some of the most common are:

• Vitamin deficiencies and anemia (not having enough red blood cells or hemoglobin to carry oxygen)
• Osteoporosis (disease leading to weak, brittle bones)
• Development issues (especially in children and teenagers)
• Hormone imbalances which could lead to reproductive issues (misscarriage and menstrual cycle problems)
• Post-operative recovery problems
• Reduced immune system capabilities

Though being underweight is not a disease in itself, it might be an indication of an underlying cause (illness) and it almost always requires further investigation - both physical and psychological, as a person might also be suffering from some form of anxiety.

Risks Associated With Being Overweight or Obese

Being overweight (obese) increases the risk of a number of serious health problems and diseases, some of which are:

• High blood pressure
• Increased levels of LDL (so-called, bad) cholesterol, reduced levels of HDL (“good”) cholesterol, increased levels of triglycerides
• Increased risk of type II diabetes
• Heart (attack) and blood vessel problems
• Increased risk of stroke
• Osteoarthritis (joint cartilage breakdown)
• Breathing problems (accompanied by sleep apnea)
• Various body pains and restricted movement
• Increased likelihood of some types of cancers
• Mental problems (depression, anxiety etc.)
• Increased risk of mortality and reduced quality of life

As you can see, the list is quite a serious one with a number of severe health problems, some of which might be directly linked to a fatal outcome. This is why being overweight is by no means a joke, but something to be taken quite seriously and dealt with utmost care. As a general rule of thumb, you should keep your BMI levels below 25 kg/m2, to stay on the safe side. Should you cross it, consider drastic changes in your lifestyle before it becomes too late.

Measuring BMI

The BMI calculator you see at the beginning of the page uses the “simplest” way of assessing your BMI based only on a few measurements (mainly height, weight, and gender), but if you want a calculation that’s a bit more accurate, but requires some work on your part, you can employ US Navy’s Body Fat Circumference method. What you’ll need to do is measure your:

• Neck - just below your larynx (Adam’s apple in men)
• Waist (men) - at the navel
• Waist (women) - at the narrowest point (commonly above the navel) Waist (women) - at the narrowest point (commonly above the navel)
• Hips (women only) - at the widest measure (across the buttocks) Hips (women only) - at the widest measure (across the buttocks)
• Height - standing straight

The BMI Measurement for men is as follows:

Body Fat Percentage = 86.01 x log10(waist - neck) - 70.041 x log10(height) + 36.76

Example calculation for a male who is 70in (178cm tall), whose neck is 16in in circumference, and waist is 34in.

This is what the calculation would look like: 

BFP = 86.01 x log10(34-16) - 70.041 x log10(70) + 36.76
= 86.01 x 1.255 - 70.041 x 1.845 + 36.76
= 107 - 129.2 + 36.8 = 14.6%

So, the estimated body fat percentage for this person would be 14.6%.

The BMI Measurement for women is as follows:

Body Fat Percentage = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387

Example calculation for a woman who is 65in (165) tall, whose neck is 13in in circumference, waist is 30in, and hips are 38 inches.