Reading the article from Forbes yesterday (
http://www.forbes.com/sites/timworstall/2013/03/31/links-31-march-is-bitcoin-doing-well-or-is-it-just-a-bubble/), the quote is that the US has a GDP of ~$15 trillion, with a monetary base of $600 billion as an example of how the value of the currency supports the economy (and granted the article states that transaction value should be many times higher than GDP). But, for some back-of-the napkin calculations, let use this.
Bitcoinwatch shows roughly 1.2m coins sent yesterday. Generally speaking, the four main areas where bitcoins are sent around seem to be:
1) buying coins as an investment (speculators)
2) trading for goods and services (economy)
3) mining
4) moving between user's own wallets.
1) is pretty easy, as it is basically all done on the major exchanges. (I assume no major trades are down without some service, especially as the value goes higher, as I don't want to just agree to send some guy $10K for coins). This number then is the total of all the exchanges, which on busy days approaches 200K coins.
2) is what we are interested in.
3) is pretty small - 3600 coins per day (although lets say 4000 since the blocks solved per day is a bit higher with increasing hashing)
4) is anybody's guess.
So, using 1) and 3), we get 1,200,000 - 200,000 - 4000 = ~1M BTC.
Let's be generous and say half of those remaining coins are going between a user's own wallets - that leave 500,000 BTC, or about $50M per day for goods and services. Annually that amounts to $18.25 billion. Now, that's proportionally a bit smaller than the US GDP to the monetary base, and certainly much smaller than an expected transaction value), but it is really that far out? I also think the actual number could be much higher - potentially twice that.
And for a rapidly growing economy I would argue its easy to see we are in line with proper numbers...
Just my
BTC.02 and I might be way off base, but is seems interesting... Feel free to correct any glaring errors you see!