Trade Route Formula Explained

After digging around in the game data and doing some testing, I'm pretty comfortable with the way that trade routes between cities are calculated. I hope this will be helpful to everyone trying to decide when building a road between cities becomes profitable. The same calculation is used for all difficulty levels, but playing at anything below Prince reduces your route costs. See the modifiers section for more detail about this.

Revenue Calculation:
Trade route income is calculated based on the size of the city which is being connected to your capital's trade network. The size of your capital city is not currently used in the calculation and can be ignored when deciding to build a road or not.

Trade route yield is roughly equal to (Connected Non-capital City Size * 1.25). This appears to be true even when your capital city is smaller than your connected city.

For example, from one of my test games:
Kyoto(4) to Osaka(4):Trade Route Income 5.01
Kyoto(4) to Tokyo(5): Trade Route Income 6.26
Kyoto(4) to Satsuma(2):Trade Route Income 2.51

Roads:
Roads cost 1 gold per turn in upkeep...so if you need to build roads on ten tiles to connect your city to your trade network, you wouldn't break even from that expenditure until your connected city reaches size eight (8 * 1.25 = 10). Keep in mind that a longer road may still be profitable to build if it allows your trade network to be extended to multiple cities in another part of the map.

Harbors:
For remote cities, harbors may be a better investment than roads provided that at least one of your cities already within your capital's trade network also has a harbor to complete the link between cities. Harbors have an upkeep of three gold per turn. goodolarchie posted a cost analysis that compares the investment of buying a harbor outright with the number of turns needed to break even here.

Trade Route Modifiers:
I haven't tested all of the modifiers yet, but I'm listing the following based on the game data and/or documentation. Let me know if I'm missing something and I'll add it to the list.

Great Wonder - Machu Picchu: +20% more Gold from all Trade Routes. Must be built in a City within two tiles of a Mountain. Requires Currency.

Leader - Harun al-Rashid: Trade Caravan trait, +1 Gold from each Trade Route

Policy - Commerce/Trade Unions: Maintenance paid on Roads and Railroads reduced by 20%

Difficulty - Settler/Chieftrain/Warlord difficulty levels reduce road and railroad costs by 66/50/25 percent.

Thanks for the info.

Awesome info Daniel. Much appreciated.

that is helpful

Very helpful. I'm glad people are starting to figure out the mechanics of this game and teach them to the rest of us.

Awesome, very helpful. I was slowly realizing that roads are one of the last improvements I want to build, this gives me exactly when I can start.

It seems like building the road from each side would help slightly as well. If two workers build four road sections between a city, it saves (3 + 6 + 9 - 2*3) = 12 gold of maintenance (assuming no rough terrain) over building it with one worker.

Great info Dan and thanks for sharing it with everyone im sure many people will keep your findings in mind in their games.

I do wonder though (as i have never built any), if railroads contribute any income for train routes. I know they cost 2/turn but perhaps faster travel should mean move can be traded no? Anyway i have no clue

Keep in mind roads aren't only there for economic benefit. I would road up your cities as soon as your worker is available to do so.

Daniel D said:

The size of your capital city is not currently used in the calculation and can be ignored when deciding to build a road or not.

Click to expand...

That's... odd.

StoneColdNuts said:

Awesome, very helpful. I was slowly realizing that roads are one of the last improvements I want to build, this gives me exactly when I can start.

Click to expand...

As long as the cities aren't too far apart, it'll basically be a wash even if you build it a bit too early. I've been using the heuristic that if the number of improved tiles around the city is greater than its population, it's probably time to build a road, which in practice has meant that I usually end up with roads right around the break-even point for trade route income.

Numi said:

I do wonder though (as i have never built any), if railroads contribute any income for train routes. I know they cost 2/turn but perhaps faster travel should mean move can be traded no? Anyway i have no clue

Click to expand...

Nope. No extra income, but they do give the city a huge production bonus if the railroad connects to the capital (+50%, IIRC), so they're definitely worth building.

And that's why the port (I don't remember the name) that connect the city to the capital can be useful since it has only 3 maintenance. So it's better than a 4 tile road (If you are willing to take the time needed to build it in that city of course.).

Thanks for the road and trade route tips. Very helpful.

Wasn't the maintentance of roads 1 gold / 3 roads?

krunsh said:

And that's why the port (I don't remember the name) that connect the city to the capital can be useful since it has only 3 maintenance. So it's better than a 4 tile road (If you are willing to take the time needed to build it in that city of course.).

Click to expand...

But it also uses up precious hammer time, while you should already have a worker or two out already for other improvements. Plus, being on the coast doesn't seem to be as good anymore, other than having a city that can provide you with a navy.

Awesome thread, thank you for the various tips

Even with formulas, I plan on building 1 road between all my cities always. I think the trade route income is ment to pay for a road between each city. That is because the road maintenance is so we don't build roads everywhere, but they still want us to have 1 road to each city.

In my game the difference from what I gain from traderoutes and what I pay for roads has been always very small. I don't think it's ment to be a source of income, considering how much gold we get for other things. I think it's just that, a way to make you connect all cities but not spam roads.

Daniel D said:

After digging around in the game data and doing some testing, I'm pretty comfortable with the way that trade routes between cities are calculated. I hope this will be helpful to everyone trying to decide when building a road between cities becomes profitable. The following should be accurate for difficulty levels Prince and up, assuming no other modifiers from leader trait, etc.

Trade route income is calculated based on the size of the city which is being connected to your capital's trade network. Your capital's size doesn't come into play at all.

Trade route yield is roughly equal to (Connected Non-capital City Size * 1.25). The size of your capital city is not currently used in the calculation and can be ignored when deciding to build a road or not.

Roads cost 1 gold per turn in upkeep...so if you need to build roads on ten tiles to connect your city to your trade network, you wouldn't break even from that expenditure until your connected city reaches size eight (8 * 1.25 = 10).

Click to expand...

Is this true even if your capital is smaller than the other city?

I haven't tried it but I heard elsewhere that it was:
Minimum(city size, capital size) * 1.25

Can you confirm this is NOT the case?

This is great info because one of the things I did very early was road my capital to my cities. Yes, I agree for defense that it may be a good idea, but I would essentially do it for the TR.

The great thing about the cities being able to defend themeselves is that it buys you time to get your warrior our spearman there to defend. Cities defending themeselves is definately an element Civ4 would have benefited from.

Lord Olleus said:

Is this true even if your capital is smaller than the other city?

I haven't tried it but I heard elsewhere that it was:
Minimum(city size, capital size) * 1.25

Can you confirm this is NOT the case?

Click to expand...

Confirmed. I just made one of my other cities outgrow my capital and I still saw the trade bonus for that city's growth in the trade route detail.

Kyoto(4) to Osaka(4): Income 5.01
Kyoto(4) to Tokyo(5): Income 6.26
Kyoto(4) to Satsuma(2): Income 2.51

DalekDavros said:

That's... odd.

Click to expand...

I thought so too. There's actually what looks like a modifier for capital city size in the game data, but it is set to zero by default. I'd expect that this will be modded by someone eventually (along with everything else).

i think your wrong about the roads though, from what ive seen it seems ot be 1gb per 2 road hexes.

Thanks!
Was not sure if it was really profitable to maintain roads between my cities, some are far apart but with this now I know that the whole network is compensating for those exceptions...