Height and Weight Math Matters

Many years ago, my husband, being a tall and thin student, told his PhD adviser that he came about a trick that allows one to skip actually trying on pants at the store. Your waist size is double your neck size, so what you need to do is measure folded pant's waist around your neck. If it fits on the top, it should fit the bottom. His PhD adviser, being a happy overweight man with a thick waist smiled and said: "Oh if it only were true".

There are a few other weight and height tricks that are believed to be true but you can prove them wrong even with the members of your family. Well, they are OK as approximations.

  • Distance between your wrist and elbow is equal to your shoe size (no need to try on shoes anymore!)
  • Your wingspan is equal to your height

For a large part of our lives we are obsessed with height and weight. When we are born all everyone cares about is how much we weigh. From then till our teens we obsessively mark our height. From teens till golden years we are fighting our weight. Maturing at last we stop caring about either of them, but when we die all that's important is again our height.

Our family doctor recently scared me that our 6-months old baby fell to the 6th percentile by weight, from 60% at age 4-months. I am pretty sure that the doctor made a mistake because the baby looks perfectly normal to me because she is a bit chubby for her height and her height perfectly fits the 6-month old clothes. Still, I started feeding her extra baby formula. The math formula for the amount of baby formula and milk that my doctor suggested to me is:
daily amount of milk and baby formula in ounces = baby weight in pounds  x 2.5

As our babies grow, doctors are starting to be more concerned about their BMI - Body Mass Index (approximate estimation of their body fat). It is defined as person's weight (in kg) divided by the square of his height (in meters).   Transferred to inches and pounds, the formula is:

BMI = (mass in pounds) x 703 / (height in inches)^2

For child of certain gender and age, BMI is compared to the BMI distribution among kids of this age and gender. BMI that is less than 5% is underweight and above 95% is overweight. For adults BMI doesn't depend on age and the guidelines are simple: BMI value of 18.5 to 25 indicates optimal weight, lower than 18.5 suggests underweight, above 25 indicates overweight, and above 30 obesity.

Another measure of person's health is a waist-to-hip size ratio. Beer belly can get you above 1, but in general it is below and the lower the better.  Historically, it also had been considered a measure of female attractiveness, hence the corsets, hip and buttock padding and Spanx nowadays. Marilyn Monroe and Sophia Loren measured 0.63 and Salma Hayek 0.67. Are you starting to hold your breath in? No worries! The sexy Cameron Diaz measures around 0.833, so breath out.

Switching to the vertical dimension. Can you predict how tall your child is going to be? The "Go Figure!" book by Clint Brookhart claims that the following formulas can estimate how tall your kids will grow based on their current age (A) and height (h, in inches):

for girls:
H(woman) = h / (0.00028 x A^3 - 0.0071 x A^2 + 0.0926 x A + 0.3524)

for boys:
H(man) = h  / (0.00011 x A^3 - 0.0032 x A^2 + 0.0604 x A + 0.3796)

My son came about 6'6", taller that I and my husband...
Try it on your kids. They will likely be taller than you, so it is about time to start teaching them that size doesn't rule.

Image by pyxopotamus, distributed under CCL.


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