The Calculations I Make

One of the most routine parts of my daily life is pulling out my calculator and determining someone’s energy and protein needs.

The process I use most frequently looks like this:

1.) Determine person’s BMI
To do this, I need to know their weight in pounds and height in inches. I divide their weight in pounds by their height in inches, then divide that number by their height in inches again. Finally, I multiply this number by 705.

To use myself as an example (which is why I’m doing this anyway), I would take my weight (142#) and divide it by my height (70″).

142/70=2.02857 (don’t round at this point)

This number needs to be divided by my height again:

2.02857/70=.028979 (still no rounding)

Now I multiply this by 705:

.028979*705=20.4 (now is where I round to one decimal place).

2.) Use BMI to determine whether a person is underweight, normal weight, overweight, or obese

This can be easily done using the following schema:
–BMI <18.5 is underweight --BMI 18.5-24.9 is normal weight --BMI 25.0-29.9 is overweight --BMI >30.0 is obese

By this categorization, we can see that with a BMI of 20.4, I am in the “normal weight” category.

3.) If the person is obese, determine their ideal body weight and develop an “adjusted body weight”

To determine an ideal body weight, you need to know whether your person is a woman or a man.

Women get 100 lbs to start with and then add on 5 lbs for every inch in height over 5 feet (60 inches)–or take away 5 lbs for every inch in height less than 5 feet.

Since I’m 70 inches tall, I get 100# for the first 60″ and add on 10*5=50 for the final 10 inches. So my ideal body weight is 150#.

Men start with 106 lbs and get 6 lbs for every inch in height over 6 feet.

Which means that if I was a man, I’d get 106# for my first 60″ and add on 10*6=60 for my final 10 inches, making my ideal body weight 166#.

Now, I can make an “adjusted body weight” for my obese residents. I take their actual body weight and subtract their ideal body weight to get the approximate pounds of fat on their bodies. I divide this by four and then add that number to their ideal body weight. This is their adjusted body weight (I do this because fat requires fewer Calories to keep it going–and I don’t want to overfeed the fat!)

Because I’m not obese, I can’t do this for me for real, but I can pretend that I’m only 60″ tall, with an ideal body weight of 100#.

Then I’d take my actual body weight (142#) and subtract my ideal body weight (100# in this example)

142-100=42

I’d take this answer and divide it by four

42/4=10.5

Then I’d add it to my ideal body weight

10.5+100=110.5

And there we have it–my adjusted body weight (if I were 60″ tall) is 110.5#

4) Convert the person’s body weight (or adjusted body weight, if the person is obese) into kilograms.

This is pretty easy. Just divide by 2.2.

So my weight in kilograms is 142/2.2=64.6

5) Finally, multiply the person’s body weight in kilograms by a defined factor to determine an estimate of daily energy needs

Generally, the factors I use are as follows:
…if the person is underweight, multiply actual body weight in kg by 30 to 35 (30 is lower range, 35 is upper range)
…if the person is in the normal range, multiply actual body weight in kg by 28 to 30
…if the person is in the overweight range, multiply actual body weight in kg by 25 to 28
…if the person is in the obese range, multiply adjusted body weight in kg by 25 to 30

Thus, if I were underweight, my energy needs would be 64.6*30=1938 kcal/day to 64.6*35=2261 kcal/day

Since I’m in the normal weight range, my energy needs are actually closer to 64.6*28=1808 kcal/day to 64.6*30=1938 kcal/day

If I were overweight, I’d calculate my needs at 64.6*25=1612 kcal/day to 64.6*28=1808 kcal/day.

And if I were obese, using the adjusted body weight determined above, I’d need 50.2*25=1255 kcal/day to 50.2*30=1506 kcal/day

Of course, the above factors are simply generalities. I would adjust them further if, for instance, an individual had a disease that increased or decreased energy needs or if an individual had recently lost a lot of weight (or gained a lot of weight). And even once I’ve calculated all this, I still have to monitor other indicators (such as weight) to make sure that what I’m providing is sufficient.

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While it looks complicated when all typed out like this, this process is about as natural to me as breathing. I make these calculations at least a dozen times a day–with hardly any conscious thought.

Just one (tiny) piece of what it means to be a Registered Dietitian.

(Of course, this is just one of many ways to estimate nutrient needs. I could use others, but feel that this is one of the simplest and most effective for adjusting to the needs of a mostly elderly, long term care population–many of whom are overweight or obese.)