This is done by dividing total tariff revenues by the total consumption billed across all usage categories and all levels of consumption. Another way is to determine a typical level of consumption and calculate the residential tariff that corresponds to this consumption. Depending on which of these two methods is used the resulting tariff can vary significantly for the same utility. The comparison of water and wastewater tariffs across countries is further complicated by the choice of the appropriate exchange rate (nominal exchange rates for a given year or over the average of several years, or purchasing power parity exchange rates). Furthermore, providing a global overview of water tariff levels is complicated by the large number of service providers (utilities). In urban areas in the United States alone, there are more than 4,000 water utilities. In Germany there are more than 1,200 utilities. Few countries in the world maintain national databases of water and wastewater tariffs charged by utilities. There are few countries that maintain national tariff databases, usually those with a specialized regulatory agency for the water sector such as England (OFWAT), Chile (SISS), Colombia (CRA) or Peru (SUNASS). Among the 310 cities in the GWI 2012 tariff survey the average combined water and wastewater tariff was US$1.98/m3 for the 15 m3/month "benchmark" customer used by the survey. Utilities in four of the surveyed cities provide residential water and wastewater services for free: Dublin and Cork (see Ireland), as well as Belfast and Ashgabat in Turkmenistan. The lowest residential water and wastewater tariffs were found in Saudi Arabia (equivalent to US$0.03/m3) and in Havana, Cuba as well as Damsacus, Syria (equivalent to US$0.04/m3). Rates in the United States in Clovis, CA are $0.42/m3. and $1.60/m3 in Seattle  The highest water and wastewater tariffs were found in Aarhus, Denmark (US$9.21/m3), Essen, Germany (US$7.35/m3; not included in the OECD survey) Copenhagen, Denmark (US$7.09), and four Australian cities (Perth, Brisbane, Adelaide and Sydney) where the tariff for the benchmark user ranged from US$6.38/m3 - US$6.47/m3. Concerning wastewater tariffs, in some countries such as [Nigeria] there is no wastewater tariff at all. In other countries - such as Mexico, Turkey, Belgium, Portugal and Korea - wastewater tariffs are low relative to water tariffs.