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Old 03-07-2006, 12:29 PM   #1
Gadrin
 
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Default Calculating Weight in Relation to Height

having a brain cramp and I've seen this posted on various RPG forums over the years.

I'm trying to get a rough formula to calculate a persons weight based on their height.

so a 6 foot, 200 lb man growing 1 foot will be 7 feet tall and approx ?

same with shrinking by the same amount.

thanks.
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Old 03-07-2006, 01:29 PM   #2
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Default Re: Calculating Weight in Relation to Height

Assuming proportionate change in all three dimensions, growing 1 foot will result in a weight of (7/6)^3 * 200 lbs., or roughly 318 lbs; shrinking 1 foot will result in a weight of (5/6)^3 * 200 lbs., or roughly 116 lbs.
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Old 03-07-2006, 02:33 PM   #3
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Default Re: Calculating Weight in Relation to Height

Note that typically creatures do not grow in all dimensions equally. For a human, it's closer to the 2.5 power of height
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Old 03-07-2006, 03:44 PM   #4
Ze'Manel Cunha
 
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Default Re: Calculating Weight in Relation to Height

And of course, you could go by BMI (Body Mass Index).
BMI is calculated by dividing a person's weight (in kilograms) by his or her height (in meters, squared). BMI can also be calculated by multiplying weight (in pounds) by 705, then dividing by height (in inches) twice.

So a 6 foot, 200 lb man has a BMI of (200*705/72^2) = 27.2
(clinically overweight)

A 7 foot man with a BMI of 27.2 would weigh (27.2*84^2)/705 = 272 lbs

A 5 foot man with a BMI of 27.2 would weigh (27.2*60^2)/705 = 139 lbs
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Old 03-07-2006, 04:05 PM   #5
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Default Re: Calculating Weight in Relation to Height

Quote:
Originally Posted by Ze'Manel Cunha
So a 6 foot, 200 lb man has a BMI of (200*705/72^2) = 27.2
(clinically overweight)
clinical observations are irrelevant, and BMI would force a table to be onhand.

Quote:
Originally Posted by thastygliax
Assuming proportionate change in all three dimensions, growing 1 foot will result in a weight of (7/6)^3 * 200 lbs., or roughly 318 lbs; shrinking 1 foot will result in a weight of (5/6)^3 * 200 lbs., or roughly 116 lbs.
thanks, I'll use this formula.
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Old 03-07-2006, 04:18 PM   #6
Ze'Manel Cunha
 
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Default Re: Calculating Weight in Relation to Height

Quote:
Originally Posted by Gadrin
clinical observations are irrelevant, and BMI would force a table to be onhand.
*shrug*
Just offering a formula which is tailor made for human body masses at different heights.
You wouldn't need a table, you'd just have to calculate BMI for base form, then derive final weight as I showed above, just two very basic calculations.


Then again, you should just go with whatever you find easier to use, especially if pseudo-realism isn't a big issue.
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Old 03-08-2006, 07:22 AM   #7
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Default Re: Calculating Weight in Relation to Height

Quote:
Originally Posted by Gadrin
having a brain cramp and I've seen this posted on various RPG forums over the years.

I'm trying to get a rough formula to calculate a persons weight based on their height.

so a 6 foot, 200 lb man growing 1 foot will be 7 feet tall and approx ?

same with shrinking by the same amount.

thanks.
There are several different ways to approach this.

If you're assuming height and weight change within a single species, Homo sapiens, you might want to use the BMI. You convert height to meters (or take height in meters, if you're not an American); square it; and multiply by the BMI, where 25 is a typical value for humans. Weight increases as the square of height. So you have a height increase by a factor of 7/6; you take weight increase as the square of that: 200 x (7/6) x (7/6) = 272.

If you simply scale up geometrically, so that all body proportions are unchanged, then you take weight increase as the cube of height increase: 200 x (7/6)^3 = 317.

If you assume interspecies size changes, the actual scaling law for land animals has body weight going nearly as the fourth power of height (perhaps because the formula for buckling of a column gives this result): 200 x (7/6)^4 = 370. By using this you are reflecting the observable fact that bigger animals tend to have thicker legs.
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Old 03-08-2006, 10:47 AM   #8
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Default Re: Calculating Weight in Relation to Height

Quote:
Originally Posted by whswhs
If you're assuming height and weight change within a single species, Homo sapiens, you might want to use the BMI. You convert height to meters (or take height in meters, if you're not an American); square it; and multiply by the BMI, where 25 is a typical value for humans. Weight increases as the square of height. So you have a height increase by a factor of 7/6; you take weight increase as the square of that: 200 x (7/6) x (7/6) = 272.
That is both better written and clearer than what I said.
(I definitely need to work on my clarity, possibly my tact too, though I don't know why a clinical observation would be taken the wrong way.)
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Old 03-09-2006, 12:03 AM   #9
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Default Re: Calculating Weight in Relation to Height

Quote:
Originally Posted by Ze'Manel Cunha
(I definitely need to work on my clarity, possibly my tact too, though I don't know why a clinical observation would be taken the wrong way.)
Writing clearly is a skill that can be learned. I'm a professional developmental editor; it's my job to write clearly—or help other people do so! There are formal grammatical rules and such, but the real key is to read what you've written and think of all the ways a reader who isn't you could misunderstand it.
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Old 03-09-2006, 12:14 AM   #10
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Default Re: Calculating Weight in Relation to Height

Quote:
Originally Posted by whswhs
If you're assuming height and weight change within a single species, Homo sapiens, you might want to use the BMI. You convert height to meters (or take height in meters, if you're not an American); square it; and multiply by the BMI, where 25 is a typical value for humans.
25 is a modern value. 15-20 is closer to the people of yore.
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