What exactly do granaries do?

[stebbinsd]

The in-game explanation of a granary is that it stores 50% of food after a population increase.

But ... what exactly does that mean?

First of all, 50% of what? The food on all the tiles I'm working at the time?

And what does it mean to "store" that food? Does it provide instant progress towards my next population increase? Or does it provide more food for the population to consume each turn?

For example: Suppose I have a city with 9 population and 20 food per turn. That's a surplus of 2 food. Thus, I am slowly but surely making progress to my next population increase. Suppose my #10 guy starts working another 2-food tile, so I'm still working on a 2-food surplus.

So, when the city increases to 10 population, what exactly happens, depending on whether or not I have a granary?

 

[Macksideshow]

Every city has a food bin. Any food produced which is not consumed (2 food per population is consumed) goes into the food bin. Once the food bin is full the city grows 1 population and the food bin is emptied. The food bin gets bigger with population so, for example, it takes more food to grow from size 4 to 5 than it does to grow from size 3 to 4.

What the granary does when the food bin is full and the city grows 1 population, the food bin is only halved, not emptied. This effectively halves the amount of food the city needs to accumulate in the bin to grow to the next population size.

It also allows a city to go into starvation mode (food deficit) without losing population immediately after growing in population, since the food bin is still half full. But that is a fairly niche situation.

Additionally the granary provides one extra health to the city for each of the grain resources (rice, corn and wheat) that the city has access too.

This makes the granary a very important building as a city will grow population twice as fast once it has a granary.

 

[Jivilov]

What happens is that the food bin is still half-full, instead of being empty as it would be without one. The granary doesn't actually "store" anything, or increase the present (or future) food surplus. Yet its effect is to double pop growth by the aforementioned effect on the food bin--assuming that after the next pop growth there's still a food surplus.

Having granaries is practically a prerequisite for whipping, since otherwise it would take twice as long to replace the "whippees."

Edit: And it appears that Macksideshow beat me to it, along with adding more info.

 

[Neinet]

You may be interested in this war academy article.
http://civfanatics.com/civ4/strategy/food_growth.php

Here's a basic version of it, using the example of growing 9 to 10.

Normally, it takes 38 food to grow from population 9 to population 10. In basically all cases** your city will store 19 food (Because this is 1/2 of 38)upon growing to size 10, starting with 19/40 food instead of 0/40 food towards population 11 whenever the city grows. This would continue, with the city starting at 20/42 food towards population 12.


**(Other cases are outlined in the war academy article, but they are rare)

 

[stebbinsd]

You say that the food bin is half full. But you also say that the food bin gets bigger as population gets bigger.

So, wouldn't it be slightly less than half full?

 

[Lennier]

See the post above yours; the granary keeps half the food just before growth, so it will be just under half full just after growth.

 

[Jivilov]

You say that the food bin is half full. But you also say that the food bin gets bigger as population gets bigger.

So, wouldn't it be slightly less than half full?

Yes, with respect to the next pop threshold.