user78: If all of the world suddenly decided they want to use BTC as their currency, what would the value of 1 BTC be, as measured in today's US dollars?
(Of course dollars would drop to zero in this theoretical case, but we can still measure BTC in today's dollars and get a finite result)

user845: All of those answer are right but they do not take in effect that bitcoins can, are and will be destroyed (I am talking about lost bitcoins not the network crash).
That said the value of one bitcoin will be higher than the expected value we calculated and should continue to rise as more bitcoins are destroyed.
Bitcoins can be destroyed by many ways:
-Losing one's wallet
-Making a payment to an invalid address (like the 0x0 puclic key, an elleptic curve in ECDSA cannot pass by (0,0))

user2606: As of 2009, we have some random guy on the Internet estimating $4.5 trillion as the value of all paper and coin around the world. That's a pretty decent number to use if you're looking to imagine replacing all fiat money with Bitcoin.
One big problem: there would be more demand for "cash" transactions since Bitcoins are more convenient than physical cash.
That is, due to convenience factors, a lot of us like monetary transactions that don't require the existence of any paper or coin. If we all switched to Bitcoin, how many of us would transfer BTC to a bank for the promise to pay back?
Anyway, $4.5 trillion divided by $21 million is $214,285.71. That said, that's in 2009 dollars with the 2009 world economy, and a lot of assumptions. Also, with a deflationary currency that is far more convenient than paper and coin, its value would likely be far greater than that if every single human being wanted to only use Bitcoin and have all financial transactions denominated in BTC.

user323: Basing on this set of data from XKCD, world's GDP is $62,900,000,000,000, If all of it was expressed in 21 million bitcoins, then each bitcoin would be worth $2,995,238.10, so roughly 3 million dollars.

user1528: I think a more reliable way is to compare it to gold, since the exact value of currency in the world is very difficult to calculate. It's estimated that 171,300 tonnes of gold have been mined in human history, which is roughly equivalent to 5.5 billion troy ounces. 
5.5 billion oz/21 million bitcoin = 261.9 oz/bitcoin

The value of one ounce of gold is about $1,668 so the value of bitcoins in current dollars would be about:
261.9 oz/bitcoin * 1668 USD/oz = 436,849.20 USD/bitcoin

If bitcoin is lucky enough (and I hope it is) to be valued at 1% of the total gold supply, then each bitcoin would be around $4,368.49. This would make the entire bitcoin supply worth about $91 billion.

78: <p>If all of the world suddenly decided they want to use BTC as their currency, what would the value of 1 BTC be, as measured in today's US dollars?</p>

<p>(Of course dollars would drop to zero in this theoretical case, but we can still measure BTC in today's dollars and get a finite result)</p>
 845: <p>All of those answer are right but they do not take in effect that bitcoins can, are and will be destroyed (I am talking about lost bitcoins not the network crash).</p>

<p>That said the value of one bitcoin will be higher than the expected value we calculated and should continue to rise as more bitcoins are destroyed.</p>

<p>Bitcoins can be destroyed by many ways:</p>

<p>-Losing one's wallet<br>
-Making a payment to an invalid address (like the 0x0 puclic key, an elleptic curve in ECDSA cannot pass by (0,0))</p>
 2606: <p>As of 2009, we have <a href="http://goldnews.bullionvault.com/all_the_money_in_the_world_102720093" rel="nofollow">some random guy on the Internet</a> estimating $4.5 trillion as the value of all paper and coin around the world. That's a pretty decent number to use if you're looking to imagine replacing all fiat money with Bitcoin.</p>

<p>One big problem: there would be more demand for "cash" transactions since Bitcoins are more convenient than physical cash.</p>

<p>That is, due to convenience factors, a lot of us like monetary transactions that don't require the existence of any paper or coin. If we all switched to Bitcoin, how many of us would transfer BTC to a bank for the promise to pay back?</p>

<p>Anyway, $4.5 trillion divided by $21 million is $214,285.71. That said, that's in 2009 dollars with the 2009 world economy, and a lot of assumptions. Also, with a deflationary currency that is far more convenient than paper and coin, its value would likely be far greater than that if every single human being wanted to only use Bitcoin and have all financial transactions denominated in BTC.</p>
 323: <p>Basing on <a href="http://xkcd.com/980/huge/#x=-11222&amp;y=-7614&amp;z=5">this set of data from XKCD</a>, world's GDP is $62,900,000,000,000, If all of it was expressed in 21 million bitcoins, then each bitcoin would be worth $2,995,238.10, so roughly 3 million dollars.</p>
 1528: <p>I think a more reliable way is to compare it to gold, since the exact value of currency in the world is very difficult to calculate. <a href="http://en.wikipedia.org/wiki/Gold" rel="noreferrer">It's estimated that 171,300 tonnes of gold have been mined in human history, which is roughly equivalent to 5.5 billion troy ounces.</a> </p>

<pre><code>5.5 billion oz/21 million bitcoin = 261.9 oz/bitcoin
</code></pre>

<p>The value of <a href="http://www.monex.com/pmr" rel="noreferrer">one ounce of gold is about $1,668</a> so the value of bitcoins in current dollars would be about:</p>

<pre><code>261.9 oz/bitcoin * 1668 USD/oz = 436,849.20 USD/bitcoin
</code></pre>

<p>If bitcoin is lucky enough (and I hope it is) to be valued at 1% of the total gold supply, then each bitcoin would be around $4,368.49. This would make the entire bitcoin supply worth about $91 billion.</p>
