What percentage of US marriages are religious ceremonies?

A commenter on this post pointed me towards a USA today article claiming that rates of civil marriage in the US have increased significantly over the years. According to the article, there are no nation-wide statistics on this, but in the 18 states that keep track of these things, 40% of marriages are civil marriages.

This surprised me. Previously, the statistic I had heard was that 80% of marriages in the US are religious. Googling, I found this page which made that claim, but did not cite a source. Or rather, it said that 80% of weddings are performed in churches and synagogues, which isn’t strictly incompatible with the 40% civil marriages statistic, if there’s a sizable chunk of people getting civil marriages but also having a big church ceremony for the relatives. Though that seems unlikely.

My personal impressions are largely based on the weddings my parents dragged me to as a little kid, and the weddings I’ve seen in TV shows and movies. I’m honestly not sure I ever saw a non-religious wedding ceremony, live or on a screen, until my roommate’s wedding in grad school.

But the cultural stereotype I grew up with may no longer hold true. If 40% (or perhaps now, since it’s been a decade since when that statistic came from) of marriages are civil marriages, that’s actually a really encouraging sign. It shows that religion is losing its grip on US society, particularly the segment that’s of marrying age.

But I’m not totally confident about the 40% figure. Does anyone know where I can get better statistics on this?

What arguments are popular among liberal Christians?
Peter van Inwagen's argument for Christianity
Did Chris Mooney have a point?
From the archives: Gary Gutting on Mackie, Plantinga, and the problem of evil