I understand that in the espionage tab of the game, there are stars next to the your cities and that they refer to the potential that each city has. However, I have more cities with 5-star potential than the number of spies I have (I have 4 spies by the way). Thus, I would like to know how to calculate the chances of your cities' information to be stolen and eliminate the enemy spies that are camped in my cities.

Is there a way to calculate the chance of your cities information? For example, maybe via your cities population, the science, or maybe the difficulty of the game.

In case this matters, I am also using the Gods and Kings expansion pack and not the Brave New World expansion pack.

  • Hi, and welcome to Arqade! I've just edited a couple of your questions, so I figured I'd mention that putting "thanks" or the like on the end of your questions is generally frowned upon. Aug 20, 2013 at 16:00

2 Answers 2


As already mentioned, tech stealing has a 100% success rate regardless of detection, unless the spy dies from the presence of a counterspy in it. The chance of the latter is ~33/37/42% for levels 1/2/3 of the counterspy.

As for how long it takes for a spy to steal a tech - the raw science output of the city at the start of the spy's mission is stored and accumulated each turn. Once it accumulates more than 1.25x the cost of the highest tech the target civilization can research (again, determined at the start of the spy's mission), you trigger a steal.

The raw science output is modified by the presence of anti-spy buildings and the level of the spy (0/25/50% bonus for levels 1/2/3).

Source code from Steam -> Tools -> Sid Meir's Civilization 5 SDK:
Functions CvPlayerEspionage::ProcessSpy, CvPlayerEspionage::CalcPerTurn, CvPlayerEspionage::CalcRequired

GlobalDefines.xml from the installation directory

  • So that's why my game where I tried to maximize the population of a one-city build (200+ megalopolis) had all the AIs set to easiest suddenly match my tech level from being 3 eras behind in 20 turns. The way that bonus works makes them effectively exceed my science output due to the spies.
    – aphid
    Jul 24, 2020 at 11:19

You can see the exact potential value by mousing over the stars.

Potential is not about probability. Spies steal technologies more quickly (fewer turns) from a city with high potential.

If you have a spy defending your city, your spy has a chance of killing the enemy spy, but otherwise spies always successfully steal a technology.

  • Can you tell me what affects the amount of potential a city has?
    – David Toh
    Aug 20, 2013 at 13:34
  • I believe that potential is based on science output, but I do not know of the formula nor have I tested it. Aug 20, 2013 at 13:43
  • 1
    I've found that when I specialize a city for science (all the science buildings, all the specialists, a couple of Jungles being worked for science), its potential is incredibly higher than the rest of my cities (5 stars vs. 1-3 stars). I think potential is roughly-proportional to science output, but since I neglected to write down the numbers, I can't prove it. Aug 20, 2013 at 17:58

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