The in-game help has this to say about the culture rating:
The Culture Rating measures citywide household access to education, religion, entertainment, and healthcare services; each service being weighed equally for the purpose of this rating. The service with the worst citywide household access caps the scoring of the others to a level only slightly better than its own.
How do those caps work in practice?
Your culture rating depends on four types of buildings:
- Health & hygiene: barber shops, bathhouses, clinics and hospitals
- Religion: shrines and temples
- Education: schools and libraries
- Entertainment: odeums, theaters, arenas, coliseums and the circus
To get a high culture rating, you need to provide your people enough cultural diversity, as well as making sure that most people have access to venues.
All buildings that contribute to the culture rating have a certain service range, which can be looked up in our buildings table. A house has access to a building if it is within its service range. Service ranges work by road distances, not by absolute distances: a house can be two tiles away from a shrine, but if the walking distance by road is over 48 tiles, the house doesn't have access to it. The service range is smaller if the building is understaffed.
Now we know the factors involved in the culture rating, let's take a look at how the actual rating is calculated.
The culture formula
Each of the buildings above is assigned a number of points. For each of the four categories, the points per category add up to 100.
Health and hygiene: Each facility is worth 25 points
Religion: Each god is worth 20 points, it doesn't matter whether the coverage is using shrines or temples
Education: The library is worth 30 points, the school is 70 points
- Odeum: 15 points
- Theater: 20 points
- Arena: 25 points
- Coliseum with gladiators only: 15 points
- Coliseum with animals: 30 points
- Circus: 10 points
To illustrate the culture calculation, we assume the following (small) city in which each building type has perfect coverage. The culture buildings built are: a clinic, a bathhouse, shrines to Jupiter and Mercury, a school, an odeum and a theater.
To calculate the culture rating, we need to know the coverage for each building that contributes to the rating; this is the percentage of houses that have access to that particular building. These coverages are available in the various advisor screens, though they aren't reported as numbers there.
For each of the buildings, the number of points for that building are calculated as follows: coverage * building points.
The coverage is a number between 0 and 1, and the building points can be seen above. For example, if 90% of your houses have access to Jupiter shrines or temples, the total points for Jupiter will be 0.9 * 20 = 18.
For each of the four categories, add up the points, and you will get a number between 0 and 100.
For our example city, we have the following (remember, we have perfect coverage for everything):
- Health: 25 (clinic) + 25 (bathhouse) = 50
- Religion: 20 (Jupiter) + 20 (Mercury) = 40
- Education: 70 (school)
- Entertainment: 15 (odeum) + 20 (theater) = 35
The culture rating consists of a "base" rating plus a maximum of 15 points bonus.
The "base" culture rating is now the minimum of those four values, which is in our example city the entertainment points: 35.
On top of the base rating, you can get a maximum of five points bonus per category. The bonus is calculated as follows: take the difference between the category's rating and the base rating. If it's more than 20, you get the full five bonus points. If it's less than 20, divide it by four to get the bonus.
The total culture rating is the base rating plus the bonus. Let's see how it works for our example city:
- Health: 50 - 35 = 15. This is less than 20, so the bonus is 15 / 4 = 3.75
- Religion: 40 - 35 = 5. Bonus is 5 / 4 = 1.25
- Education: 70 - 35 = 35. This is more than 20, so we get the full bonus: 5
- Entertainment: this decides the base rating, so you get no bonus here
The final culture rating for our example city is thus: 35 + 3.75 + 1.25 + 5 = 45.
To help you with the calculations, we've created a Culture calculator, which will calculate the maximum achievable culture rating for any combination of buildings.
Improving your culture rating
The two key points to improving your culture rating is increasing diversity and increasing coverage.
As you can see from the formula above, if you don't have enough diversity, your culture rating will be limited. If you don't have enough coverage, your rating won't rise either.
Your first step in increasing your rating should be consulting the Ratings advisor: he'll say in which category you need more coverage. Next after that, visit the appropriate Advisor, for example the Entertainment advisor, to find out which buildings have low coverage. Once you've decided which buildings should have increased coverage, return to your city.
To decide where to place your new culture building, use the overlays: they will show you which parts of the city have coverage and which don't. Also keep an eye on your equite employment levels: if your buildings are understaffed, their service range won't be optimal.
Getting a culture rating of 100 using this method might not be possible since you're going to be short on space and might have to demolish part of your city to place those big entertainment venues. If the scenario requires a culture rating of 100 or close to it, it's best to sit down for a few minutes and design your city with culture in mind.