# How to calculate calories needed for designed creatures?

How does one calculate the caloric needs of various creatures considering their different behaviours, heartbeats... hormones, blood types, abilities, body temperature, external temperature, gravity, air density and stances like bipedalism, quadrupedalism or multipedalism.

I'm looking for a single formula that accounts for all of these factors at once.

• I think "abilities" is going to be a major sticking point. The number of possible "abilities" an animal might have are countless, and I don't expect there to be any single formula that can account for each and every one. May 22, 2021 at 11:14
• Multipedal exists, so multipedal-ism can always be formed using the regular English word derivation process. Multipede also exists, but it's a noun, not an adjective. May 22, 2021 at 16:11

Thankfully most of that does not really matter, Its fairly simple you only need two things for a rough estimate.

## Body mass and metabolism work with kleiber's law for a quick estimate.

Body mass is fairly straight forward, base metabolism depends on how accurate you need the estimate, you can break it down by species, clade, or type of metabolism. for a quick an dirty approach clade or -thermy is fine. Based on the clade is usually, mammal, bird, amphibian, archosaurs, ect. . If you don't want to get into that fine of detail you can also do it by whether they are endothermic or exothermic, it won't be quite as accurate as breaking it down by clade but it is good enough for a quick estimate.

So your calculations area as follows.

for large animals

Kcal/h = X(mass^3/4)

for very small animals like insects and micro-organisms is is instead Kcal/h = R (mass^2/3)

Body mass is in kilograms

X is 3.52 for mammals and birds and 2.66 in ectotherms

you can also search individual animal groups, passerine birds of instance can have an X as high as 5.

Is this perfect, absolutely not, but it is good enough for a quick a dirty estimate.

Also don't forget this gives you the caloric consumption per hour, be sure to multiply by 24 for daily consumption and 8769 for yearly.

• This Stack constantly amazes me. I couldn't find squat on a rapid search this morning. I'd delighted to be proven wrong. +1!
– JBH
May 22, 2021 at 15:03
• @JBH this in particular is a pain because google will keep sending you to all the stuff for human weight lifters and diet. I took me a while to dig up the constants because of this.
– John
May 22, 2021 at 15:05
• Just curious, look at dogs - something like a whippet and sleigh dog, with extremely different degree of insulation from the fur, but quite similar in size and weigh - are these estimates still good approximations?
– d-b
May 22, 2021 at 21:33
• @d-b I think breeding species out of their normal environment breaks this. Whippets would likely die out quickly without human intervention!
– pipe
May 22, 2021 at 22:07
• @d-b yes they are still good estimates, animals in colder environment burn more calories but are also often larger, Again you can get very detailed if you want but this equation is fine for rough estimates and for a fictional creature. It does not work if you move an animal into a drastically different environment than the one it adapted for, but fine for its native environment.
– John
May 22, 2021 at 23:26