Controlled supply

Block reward halving

Bitcoins are created each time a user discovers a new block. The rate of block creation is adjusted every 2016 blocks to aim for a constant two week adjustment period (equivalent to 6 per hour.) The number of bitcoins generated per block is set to decrease geometrically, with a 50% reduction every 210,000 blocks, or approximately four years. The result is that the number of bitcoins in existence will not exceed slightly less than 21 million.[2] Speculated justifications for the unintuitive value "21 million" are that it matches a 4-year reward halving schedule; or the ultimate total number of Satoshis that will be mined is close to the maximum capacity of a 64-bit floating point number. Satoshi has never really justified or explained many of these constants.

File:SupplyFormula.gif

This decreasing-supply algorithm was chosen because it approximates the rate at which commodities like gold are mined. Users who use their computers to perform calculations to try and discover a block are thus called Miners.

See also: http://bashco.github.io/Bitcoin_Monetary_Inflation/

This chart shows the number of bitcoins that will exist in the near future. The Year is a forecast and may be slightly off.

* In Block 124724, user midnightmagic mined a solo block to himself which underpaid the reward by a single Satoshi and simultaneously destroyed the block's fees. This is one of two only known reductions in the total mined supply of Bitcoin. Therefore, from block 124724 onwards, all total supply estimates must technically be reduced by 1 Satoshi.

Because the number of bitcoins created each time a user discovers a new block - the block reward - is halved based on a fixed interval of blocks, and the time it takes on average to discover a block can vary based on mining power and the network difficulty, the exact time when the block reward is halved can vary as well. Consequently, the time the last Bitcoin will be created will also vary, and is subject to speculation based on assumptions.

If the mining power had remained constant since the first Bitcoin was mined, the last Bitcoin would have been mined somewhere near October 8th, 2140. Due to the mining power having increased overall over time, as of block 367,500 - assuming mining power remained constant from that block forward - the last Bitcoin will be mined on May 7th, 2140.

As it is very difficult to predict how mining power will evolve into the future - i.e. whether technological progress will continue to make hardware faster or whether mining will hit a a technological wall; or whether or not faster methods of SHA2 calculation will be discovered - putting an exact date or even year on this event is difficult.

The total number of bitcoins, as mentioned earlier, has an asymptote at 21 million, due to a side-effect of the data structure of the blockchain - specifically the integer storage type of the transaction output, this exact value would have been 20,999,999.9769 bitcoin. Should this technical limitation be adjusted by increasing the size of the field, the total number will still only approach a maximum of 21 million.

Note: The number of bitcoins are presented in a floating point format. However, these values are based on the number of satoshi per block originally in integer format to prevent compounding error.

* In block 124724, user midnightmagic solo mined a block which caused one less Satoshi to be created than would otherwise have come into existence. Therefore, all calculations from this block onwards must now, to be accurate, include this underpay in total Bitcoins in existence. Then, in an act of sheer stupidity, a more recent miner who failed to implement RSK properly destroyed an entire block reward of 12.5 XBT in block 501726.

The bitcoin inflation rate steadily trends downwards. At the time of writing, more than 3 out of every 4 bitcoins that will ever exist have already been mined, and the annual inflation rate is just 4%. The block reward given to miners is made up of newly-created bitcoins plus transaction fees. As inflation goes to zero miners will obtain an income only from transaction fees which will provide an incentive to keep mining to make transactions irreversible.

Due to deep technical reasons, block space is a scarce commodity, getting a transaction mined can be seen as purchasing a portion of it. By analogy, on average every 10 minutes a fixed amount of land is created and no more, people wanting to make transactions bid for parcels of this land. The sale of this land is what supports the miners even in a zero-inflation regime. The price of this land is set by demand for transactions (because the supply is fixed and known) and the mining difficulty readjusts around this to keep the average interval at 10 minutes.

The theoretical total number of bitcoins, slightly less than 21 million, should not be confused with the total spendable supply. The total spendable supply is always lower than the theoretical total supply, and is subject to accidental loss, willful destruction, and technical peculiarities.

One way to see a part of the destruction of coin is by collecting a sum of all unspent transaction outputs, using a Bitcoin RPC command gettxoutsetinfo. The total_amount value returned is the sum of all outputs that the client deems technically spendable but not currently spent. Note however that this does not take into account outputs that are exceedingly unlikely to be spent as is the case in loss and destruction via constructed addresses, for example.

While the number of bitcoins in existence will never exceed slightly less than 21 million, the money supply of bitcoins can exceed 21 million due to Fractional-reserve banking.

Because the monetary base of bitcoins cannot be expanded, the currency would be subject to severe deflation if it becomes widely used. Keynesian economists argue that deflation is bad for an economy because it incentivises individuals and businesses to save money rather than invest in businesses and create jobs. The Austrian school of thought counters this criticism, claiming that as deflation occurs in all stages of production, entrepreneurs who invest benefit from it. As a result, profit ratios tend to stay the same and only their magnitudes change. In other words, in a deflationary environment, goods and services decrease in price, but at the same time the cost for the production of these goods and services tend to decrease proportionally, effectively not affecting profits. Price deflation encourages an increase in hoarding — hence savings — which in turn tends to lower interest rates and increase the incentive for entrepreneurs to invest in projects of longer term.

• Milton Friedman interview, where he proposed to replace the central bank with a computer, and to fix the money supply growth at 4% annually
• Deflationary spiral
• Chart of total bitcoins in circulation
• Inflation
• Prohibited changes