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In Civ 5 connecting cities to your capital earns you income, but each road costs 1G to maintain, so you don't want to connect cities too early. What is the formula for calculating how much gold you will get by linking cities?

From my experience it seems to be related to the population of the cities, but there also seems to be a benefit to having lots of trade routes. So connecting a second city of roughly the same size seemed to give me more then double the income from a single city. I now have 6 cities in the "network" and despite them being farther away, my trade income is more than paying for the roads.

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I note the TRADE_ROUTE_CAPITAL_POP_GOLD_MULTIPLIER seems to want the game to divide zero. Perhaps this is where the bug that Oak is referring to comes into it? –  user19322 Feb 2 '12 at 5:25
1  
@Kate Not divide, multiply. And with the latest patch, it isn't a 0 anymore. –  bwarner Feb 2 '12 at 14:21
    
Given the answers below, is the explanation to the OP's original question (essentially, "Why did it seem that the second city I added with the same population increase my income by more than double that of the original city?") simply that the OP's capital population size increased by more than double during the intervening turns? –  Kirk Woll Sep 4 '13 at 2:12
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3 Answers

up vote 25 down vote accepted

UPDATE The formula has changed as of the March 2011 patch, it is now

(city population * 1.1) + (capital population * .15) - 1 for each city connected to the capital, not including the capital.

  • The Machu Pichu wonder increases the modifier by 20%, to (city population * 1.3) + (capital population * .15) - 1.
  • Arabia increases the constant by 1, to (city population * 1.1) + (capital population * .15) (thanks WillfulWizard).

The economic overview also gives a far better explanation of how it is calculated - in other words, this question is now trivially solved by simply looking at that economic overview :)

Original answer below.


Okay, I did a bit more testing, and bwarner's answer is almost accurate:

Each city, excluding the capital, provides (city's population * 1.25) + 0.01 gold per turn. Owning the Machu Pichu wonder increases the modifier by 20%, to (population * 1.5) + 0.01, for all the cities.

The capital does not provide any gold.

The 1.25 and 0.01 can be seen in the Assets\Gameplay\XML\GlobalDefines.xml file:

<Row Name="TRADE_ROUTE_BASE_GOLD">
    <Value>1</Value>
</Row>
<Row Name="TRADE_ROUTE_CAPITAL_POP_GOLD_MULTIPLIER">
    <Value>0</Value>
</Row>
<Row Name="TRADE_ROUTE_CITY_POP_GOLD_MULTIPLIER">
    <Value>125</Value>
</Row>

The economic overview also demonstrates these values.

I couldn't find any other factor which affects these values, and I checked different difficulty levels, different distances between cities, different city route type (road vs railroad, road/railroad vs harbor), different city health and whether it is occupied. Looks like it's solely the population.

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Does the dangling .01 get multiplied in by the population? I'm not understanding why everything would be kicked up by a hundredth of a gold piece. –  Raven Dreamer Sep 29 '10 at 16:28
    
Nope, it doesn't, it's a fixed number, and that means its effect is negligible, even if you have dozens of cities. Actually, my developer instincts tell me someone implemented that 0.01 as a workaround to some bug, but it's just a guess :) anyway I've edited my answer to clarify that, thanks. –  Oak Sep 29 '10 at 16:33
    
Fair enough. It'd make more sense to me if cities could have 0 population, as insurance that the number would always be non-zero and non-negative. –  Raven Dreamer Sep 29 '10 at 16:37
2  
There is one more thing that DOES affect trade income: Arabia. Arabia gets +1 income/trade route. I checked to be sure, and this is applied after the population multiplier, just like the + 0.01. So a normal city gets 1.26,2.51,3.76,.. and Arabia gets 2.26,3.51,4.76,... –  WillfulWizard Sep 30 '10 at 3:03
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Looking at the economy details screen shows how much you are making from trade routes to each city. Currently I seem to be making population * 1.25 from each city (where population is the target city's population). Not sure where the 1.25 comes from or what might modify it.

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3  
The increase with the population size does seem linear, but that 1.25 you mention is definitely not fixed. –  Oak Sep 23 '10 at 16:19
1  
I take it back - it is fixed after all, with the exception of Machu Pichu. –  Oak Sep 29 '10 at 16:08
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Sadly the Manual entry for trade routes is not very useful, but confirms population is involved:

Each trade route is worth a certain amount of gold each turn, the amount determined by the population of the connected city.

EDIT: The Civilpedia is only slightly more forthcoming, saying that the population of both cities matters, not just the connected city.

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Maybe it literally is just the population of the city? I guess I can check and see if that matches what I'm seeing. Maybe I imagined the part where it was bigger for the second city of the same size. –  bwarner Sep 22 '10 at 20:27
    
Maybe its the two cities trading with each other... so if you have a capital at 3, trading with cities at 2 and 1, its 3+2 and 3+1, for 9? This is complete speculation, but its something we can test. –  WillfulWizard Sep 22 '10 at 20:30
    
I've tested it a bit, @WillfulWizard, and while it does seem to depend on population, it (1) is not the sum population pairs, and (2) it does not depend solely on population. –  Oak Sep 23 '10 at 16:43
    
@Oak sigh yeah, we're definitely missing something important. –  WillfulWizard Sep 23 '10 at 16:53
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