Having embarrassing trouble with a unit conversion dealing with BMI (body mass index)

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cow

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Hello all,
This is my first time on the forums and apparently Friday is just not my day for arithmetic.

My problem is quite simple and I have just hit a mental roadblock after hours of coding.

Basically all I have to do is come up with a formula that will convert bmi (kg/meters^2) into an Americanized version (pounds/feet^2).

I took a shot at it and just multiplied the two conversion rates (for this problem 2.2 lbs per kg and 3.28 feet per meter) into the numerator.

I feel this is incorrect, but I can't come up with a way to check it and would really appreciate some advice.

Thanks for looking and have a great day!
 
Trying writing it all out with units so that they cancel out, like so:

\(\displaystyle \huge\frac{kg}{m^2}*\frac{2.2lbs}{kg}*{(\frac{m}{3.28ft})}^2\)

You're taking the original value in kg/m^2 and multiplying by two factors that equal 1, so you're not changing the overall value. But, now you can start cancelling out units and you reduce to:

\(\displaystyle \huge\frac{2.2lbs}{3.28ft^2}\)

Your conversion factor is \(\displaystyle \huge\frac{2.2}{3.28^2}\). I suspect your mistake was forgetting to square 3.28, because while there are 3.28 feet in a meter, there are 10.76 sqft in a square meter.

EDIT -- Yeah, obviously I messed this up the first time. It's been corrected. Thanks Dr. Phil :)
 
PS - since you're converting mass (kg) to weight (lbs), this calculation is only valid for 1.0 Earth gravity :)
 
cow said:
Hello all,
This is my first time on the forums and apparently Friday is just not my day for arithmetic.

My problem is quite simple and I have just hit a mental roadblock after hours of coding.

Basically all I have to do is come up with a formula that will convert bmi (kg/meters^2) into an Americanized version (pounds/feet^2).

I took a shot at it and just multiplied the two conversion rates (for this problem 2.2 lbs per kg and 3.28 feet per meter) into the numerator.

I feel this is incorrect, but I can't come up with a way to check it and would really appreciate some advice.

Thanks for looking and have a great day!
Click to expand...
I also hit a mental roadblock whenever I have to do conversions with the schizophrenic unit "pound." The problem is keeping "mass" and "weight" separate.

Mass is a property of matter, measured in kg.
Weight is the force of gravity acting on a mass, measured in kg-m/s^2 or N (Newtons).

So what is a pound? I take it to be a unit of Force, or weight, as in the statement "I weigh less because I live at high altitude where the acceleration of gravity is lower." Then it is better to say "A mass of 1 kg has a weight of 2.2 pounds at sea level," instead of equating kg and lb.

So what units are used for Body Mass Index? In real units, it is mass/height^2 = kg/m^2.
To convert from Weight(lb)/(ft)^2, multiply by (3.28^2)/2.2 = 4.89
 
Ted said:
Trying writing it all out with units so that they cancel out, like so:

\(\displaystyle \huge\frac{kg}{m^2}*\frac{2.2lbs}{kg}*{(\frac{3.28m}{ft})}^2\)

You're taking the original value in kg/m^2 and multiplying by two factors that equal 1, so you're not changing the overall value. But, now you can start cancelling out units and you reduce to:

\(\displaystyle \huge\frac{2.2lbs*3.28^2}{ft^2}\)

Your conversion factor is 2.2*3.28^2. I suspect your mistake was forgetting to square 3.28, because while there are 3.28 feet in a meter, there are 10.76 sqft in a square meter.
Click to expand...
OOPS .. 3.28 ft/m
 
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