How do I calculate combined percentages?
May 10, 2014 1:40 PM   Subscribe

Mathfail: I'm considering supplementing my cat's food with plain meat. I'd like a rough idea of how much it would bump percentage of calories from protein and fat.

Here are the numbers I'm working from:

Total calories: 215

Canned food calories: 180
% calories from protein: 32%
% calories from fat: 59%

Meat calories: 35
% calories from protein: 77%
% calories from fat: 8.5%

If I combine these together, what are the combined calories from fat and protein?

I'm looking at a couple of canned foods, and would be grateful if you could walk me through the calculation. I feel dumb asking this - I'm sure I learned how to do this at some point in my life, but my math skills are rusty.

(For the curious: this is to compare numbers of cheaper food + meat to more expensive food.)
posted by moira to Science & Nature (6 answers total) 1 user marked this as a favorite
Response by poster: Adding to my confusion:

Canned food:
% calories from carbs: 9%

% calories from carbs: 0%
% calories left over from who-knows-what: 14.5%
posted by moira at 1:46 PM on May 10, 2014

Best answer: Start by converting the % calories to actual calories:

32% of 180 = 180*.32 = 57.6 calories from protein (can)
59% of 180 = 106.2 calories from fat (can)

77% of 35 = 27 calories from protein (meat)
8.5% of 35 = 3 calories from fat (meat)

Now, add the calories together:

57.6 + 27 = 84.6 calories total from protein
106.2 + 3 = 109.2 calories total from fat

total calories is 215, so % of calories from protein = 100*84.6/215 = 39.3% approximately

I'll leave it to you to figure out the % of fat :-)
posted by mr vino at 1:50 PM on May 10, 2014

The above is assuming you are combining equal portions. Equal in weight? Equal in volume? Well, it depends how the original percentages were calculated.
posted by Obscure Reference at 2:10 PM on May 10, 2014

Response by poster: Thanks, that's perfect. (I was using ounces for both.)
posted by moira at 2:15 PM on May 10, 2014

Response by poster: Sorry, to correct myself: the calculations for percentages in canned were originally done using grams. I had total number of calories per 5.5 oz. can, and total number of calories per 1 oz. meat.
posted by moira at 2:23 PM on May 10, 2014

Response by poster: I had some numbers wrong. I put the formulas into Excel, crunched the correct numbers, and came to a similar answer: supplementing a decent but cheaper cat food with ~20% calories from chicken results in an awesome, affordable diet. Thanks again!
posted by moira at 3:33 PM on May 10, 2014

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