I'm creating a TTRPG game where players can play as vertebrates from any time period on earth. I'm trying to make my game as realistic as possible and one thing I have to do to make it more realistic is to find out how much water a player's character needs to drink each day to survive. The only things related to my problem on the internet are formulas for how much a human needs to drink or how much livestock needs to drink. Livestock as a rule of thumb need 1 gallon of water per 100 pounds of body mass, but I'm not sure if this would work for all vertebrates. I don't need a formula that includes animals that have specially adapted to need more or less than most other creatures, such as camels. Those can be covered in the species traits. But I need a formula that can cover the different vertebrate groups, such as reptiles, fish, mammals, birds, and amphibians in general.

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    $\begingroup$ -1 because you're straining at a gnat. You're being too realistic. There isn't and won't be a single formula that identifies the water needed per-day for all vertebrates. This is because there's a lot of variables and such a formula would be (IMO) a waste of time outside of your game. Examples: Is the animal male or female? lactating or not? working hard or not? Living in a hot or cold area? Favor high muscle mass or low? Favor high fat content or low? Etc. Consequently, you're looking for a generalization in an effort to be "more realistic." It's really not what you want. $\endgroup$
    – JBH
    Commented Mar 6, 2022 at 20:58
  • $\begingroup$ Depends on your animals.. even in the same group there are huge differences. First populate your world, I would say... decide your species and their counts. Then, to estimate what each animal group needs in total. To find the amounts, try Google, I get a lot of results with this google.com/… $\endgroup$
    – Goodies
    Commented Mar 6, 2022 at 21:05
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    $\begingroup$ This is going to fail the book test: the answer to this is going to be as long as a significant portion of a book, which is against site rules. Any shorter answer will be missing details that appear to be required. $\endgroup$
    – Monty Wild
    Commented Mar 6, 2022 at 21:14
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    $\begingroup$ Amphibians need very very very much more water than reptiles or birs. Mammals need very much more water than reptiles or birds, but also very much less water than amphibians. In fact, most amphibians required permanent moisture; their skins cannot support a dry environment. Among mammals, some need much more water than others. There is no general formula. You just need to allocate the time to read in depth about the water requirements of each and every species which is of interest to you. $\endgroup$
    – AlexP
    Commented Mar 6, 2022 at 21:45
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    $\begingroup$ Velociraptors were closely related to birds, and about the size of a turkey. A large (25 kg = 55 pounds) turkey in typical conditions drinks about 1 liter (0.3 U.S. gallons) of water per day. You are welcome. $\endgroup$
    – AlexP
    Commented Mar 7, 2022 at 10:29

1 Answer 1


Ectotherms need half as much water as endotherms.

Endotherms: that is us and llamas and chicken, with various guest stars each shot. 1 gallon / 100 lbs. That is pretty generous but OK. It is close to what humans need. A chicken needs a pint a day (pints! sorry metric folks) and assuming a 10 lb chicken that is close also; 8 pints to a gallon so 100 lb chicken would need 10 pints or 1 gallon 2 pints. BEWARE ITS PECK!

Yes. Now ectotherms: iguanas and toads. Iguanas are about 8 lbs (4 kg) and need 20 ml/kg so 160 ml. 8 x 12.5 = 100 lb iguana; 160 ml x 12.5 = 2000 ml which is 1/2 gallon. I found somewhere estimates from frogs which were also about half of the gallon / 100 lbs. So ectotherms need 1/2 as much water as endotherms.

That makes intuitive sense. Ectotherms are colder so they don't evaporate as much. That can be a good simple rule for your game and also a reason your player might opt to be a toad if water was scarce.

  • $\begingroup$ Reptiles needs much less water than half of what a regular warm blooded mammal would need. And humans do not need anything even close to 3 L / 50 kg. (I certainly don't drink 6 liters of water per day.) Maybe half of that. Of course it depends on the environment; a human in St. Petersburg needs very much less water than the same human in Timbuktu. $\endgroup$
    – AlexP
    Commented Mar 7, 2022 at 10:23
  • $\begingroup$ What do reptiles need? $\endgroup$
    – Jayleth
    Commented Mar 7, 2022 at 22:01
  • $\begingroup$ I found that iguanas and bearded dragons need about 1/5 of a gallon per 100 pounds of mass, although that might just be those two specific reptiles and the sources might not be credible. $\endgroup$
    – Jayleth
    Commented Mar 8, 2022 at 1:04
  • $\begingroup$ @Jayleth That is a very big iguana! $\endgroup$
    – Daron
    Commented May 14, 2023 at 14:45
  • $\begingroup$ Toads need loads of water, Willk! $\endgroup$
    – Daron
    Commented May 14, 2023 at 14:46

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