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Since bitcoins are being regularly rewarded to miners, will the number of bitcoins continue to grow indefinitely, or will there be a maximum total number of bitcoins in existence? And if there is some kind of limit, what is it and how is it enforced?

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retagged as "basics" is a meta-tag and not recommended by stackexchange (meta.bitcoin.stackexchange.com/questions/14/…) –  David Perry Aug 31 '11 at 4:40
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5 Answers 5

up vote 16 down vote accepted

A pre-defined schedule limits the total number of bitcoins so that they gradually approach a total of 21 million (ignoring those that have been lost through deleted or misplaced wallet files). The limit of 21 million bitcoins is "hard-wired" in to the protocol, and there will never be more bitcoins than this:

The Schedule on which Bitcoins are released into the economy through being rewarded to miners

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Note that there are some assumptions built into the timing and unless the protocol is changed, they will actually be mined a bit earlier than this chart suggests. –  David Schwartz Aug 31 '11 at 0:00
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Or later--if the value drops precipitously and difficulty takes a while to get low enough again. But the graph is a good rough approximation. –  eMansipater Aug 31 '11 at 0:02
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I think that's very unlikely. Even if there are a few precipitous drops, I think that will be outweighed by the overall trend of increasing hashing power (and they'll be followed be precipitous drops in difficulty). But, yes, that is possible. –  David Schwartz Aug 31 '11 at 0:04
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Every block introduces 50 new coins in the system. This quantity (50) halves every 210,000 blocks. So, getting the limit of coins it is possible to generate is quite easy : it's the sum of a geometric series.

latex formula

Also, note that this is an upper bound ; the actual quantity will probably be a bit lower due to rounding issues (BTC has a finite number of decimals, 8).

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Artefact, yes but that is not the theoretical limit... Basically its (10^8)... 100,000,000 –  intmain Nov 19 '12 at 22:34
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As Artefact2 mentioned, Bitcoin uses fixed-point math to calculate the block subsidies. So, ignoring the unspendable genesis block, the sundry lost coins and unclaimed rewards, the maximum number of bitcoins is 20999999.9769 BTC.

I found that number through the following python program:

COIN = 100 * 1000 * 1000
nSubsidy = 50 * COIN
nHeight = 0
total = 0
while nSubsidy != 0:
    nSubsidy = 50 * COIN
    nSubsidy >>= nHeight / 210000
    nHeight += 1
    total += nSubsidy

print total / float(COIN)

It's intended to mirror this code from the Bitcoin client:

int64 static GetBlockValue(int nHeight, int64 nFees)
{
    int64 nSubsidy = 50 * COIN;

    // Subsidy is cut in half every 210000 blocks, which will occur approximately every 4 years
    nSubsidy >>= (nHeight / 210000);

    return nSubsidy + nFees;
}

Of course, it only differs from 21 million BTC by only 3 bitcents, so the difference isn't significant.

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As per the current rules there will only ever be 21 million coins at most (explained in other answers here). However, I'd like to add that this is by general agreement, which means that it can be changed.

See this question: Could there be hyperinflation in Bitcoin?

Looking at the history of money, I am skeptical that there will only ever be 21 million coins. I don't know of too many instances in history when money could be created out of thin air and wasn't. It would be foolish to ignore history. Whether or not there will be more than 21 million coins depends on whether or not "the people" demand it, and once again history is our guide.

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According to a BIP (Bitcoin Improvement Proposal) I found, this code:

 nSubsidy >>= (nHeight / 210000);

will actually wrap the reward generation back to 50 at some point in 2200 or so. Making the supply of bit coins infinite. The python code above does not have this property. The C++ code does. See BIP https://github.com/bitcoin/bips/blob/master/bip-0042.mediawiki

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