How many Catholics are in the Austin Diocese?
This is a very difficult question to answer.
This webpage, http://austindiocese.org/about-us, says 450,000. This page, http://austinvocations.com/history-of-the-diocese, says "roughly 500,000". This PDF, Fact Sheet says 571,000. This page, http://www.catholic-hierarchy.org/diocese/daust.html says 519,000 (in 2012).
This diocesan pastoral plan statistical data document prepared in 2014, posits 18% of the local population of 2.9 million (at the time) is Catholic, a number that works out to about 520,000.
I don’t know where these number come from. There is no official membership list of the Catholic Church. The closest thing is an entry in a log when a person is baptized, and that record stays with the parish of baptism, which is often a good distance from where the adult lives Further, some people are more engaged with the Church then others.
The diocese website lists numbers of families at each parish – families, not individuals – but even these numbers are suspect. While it is possible for individuals and families to register as members of a parish, the registration numbers give only a partial look into who is affiliated with a parish. Some people attend mass at a parish for years without ever registering; some people register at more than one parish; some register and later move away while their name stays on the rolls.
A planning document to support the diocesan pastoral plan in 2014 included the number 112,698 families. Dividing that into the number of Catholics in the diocese the document uses, that works out to an average family size of 4.6. This seems an extremely high number for average family size when you consider the number of single (never married) adults, childless couples, empty nester couple, and empty nester widows and widowers. It is not reasonable that an average of 4.6 members of any family attend a given parish.
All of which leads to the conclusion: nobody really knows.
This is not a criticism, just a recognition of how shaky numbers are in this area.
A top down approach to estimating the Catholic population
Pew Research estimates 20.8% of the US population is Catholic. That’s a 2014 estimate. The 2007 estimate was 23.9%. (These numbers trigger some people into righteous rhetoric about how "most of those people aren’t REALLY Catholic", but at least for sociological and demographic purposes these numbers are the best we have.)
A 2009 Dallas Morning News column http://religionblog.dallasnews.com/2009/03/texas-is-getting-more-catholic.html said the Texas population was 32% Catholic. The writer was going from a Pew poll. The percentage has probably dropped since then (as it has nationwide) but it is still reasonable to assume Texas is more Catholic than the rest of the country. The Dallas Morning News writer speculated the reason was largely due to the high Hispanic population in this state, which sounds reasonable to me.
The population in the Austin Diocese is more Hispanic than the US as a whole, but less Hispanic than the rest of Texas. All other things being equal, I would expect the diocese population to be somewhere more than 20% and less than 32% Catholic.
I did a top down calculation from Census Bureau data for county populations for each of the 25 counties and Census Bureau estimates of the demographic groups in each county, for four groups: Hispanic, Anglo White, Asian, and Black (African American). I then applied Pew estimates of the percentage of each of these groups that identify as Catholic: Hispanic 58.9%, Anglo White 21.6%, Asian 19.1%, and Black 5.6%. These percentages may be outdated but they were the most current Pew had.
Doing the calculation this way, I got a diocese population of 3.02 million in the diocese, and a Catholic population of 907,000. The fact sheet on the diocese website estimates the population at 3.09 million (close to mine) but the Catholic population at only 571,000. My estimate works out to 30% of locals are Catholic while theirs is only 18%.