Bitcoin Forum

Why 21 Million? Why that number? This was an arbitrary decision made by Satoshi or there is a reason behind the 21?
Thank you!

There isn't an obvious answer about that, I don't think you'll get a proper one unless you ask him/her. I think he/she had answered that 21 million is just a nice number. The first 209,999 blocks rewarded 50 bitcoins and every 210,000 blocks they were cutting in half. So the number 21,000,000 comes from the halving, not directly from an integer.

What Satoshi decided is the halving frequency. If instead of 210,000 blocks, they were 10,000 and every other consensus rule remained the same, then only 1 million coins would be in circulation.

It's probably just arbitrary, why not 21 million? 21 is an often used example in educating people on cryptography as its devisible by two simple prime numbers (3 and 7).

21 million may be the population of a few states, countries or areas too (only one I can think of is scandinavia).

It's likely that he designed the mining rewards algorithm and that just generated the number from that (just below the 21 million).

I have read somewhere that this was a decision made so that eventually Bitcoin unit prices align with FIAT. Now, that's where the easy part of this ends, then we get to the math XD And here is a math explanation from bitcoin stack exchange:

Calculate the number of blocks per 4 year cycle:

6 blocks per hour
* 24 hours per day
* 365 days per year
* 4 years per cycle
= 210,240
~= 210,000

Sum all the block reward sizes:

50 + 25 + 12.5 + 6.25 + 3.125 + ... = 100
Multiply the two:

210,000 * 100 = 21 million.

Seems like voodoo to me, but it works that way. The bottom line is that Satoshi could not predict that correctly so he probably used a number that was the closest and realistic to his own calculations.

That number was not just taken, it was taken in accordance to some other certain reasons that will make bitcoin deflationary.

If Satoshi likes, he can make the number 21 billion instead of 21 million, in which mining rewards before the first halvings will be 50000BTC instead of 50, while the mining block before next halvings will be 210,000,000 instead of 210,000.

If Satoshi likes, he can make the number 2.1 million instead of 21 million, in which mining rewards before the first halvings will be 5BTC instead of 50, while the mining block before next halvings will be 210,00 instead of 210,000.

This depends on Satoshi Nakamoto, but it has to be done in a way bitcoin will remain deflationary in design, and he chose 21 million after other certain reasons have been considered.

Block hieght before next halving, mining reward and the time set to solve proof-of-work algorithm by miners are all very important before he choose bitcoin total supply.

Bro you won't get 210,000 if you don't have 21 Million Bitcoin so what you posted didn't help at all IMO. You just basically explained how halving works which happens every 210,000 mined blocks so it only means mining isn't really the answer to the question why 21 Million Bitcoins exists.  For all we know it is just a random number thrown in before Bitcoin is created and there might be no science involved on why they have picked that number up.

Thank you for the answers!
Incredible that there is a chance that this was just a random number. For a system like this, so well thought out, defining things just arbitrarily sounds a little off. Maybe someday we will have confirmation of this.

The one theory that makes most sense to me is the M1 money supply. It is said that the total supply of money during that time is estimated to be around $21 trillion. Satoshi wanted a currency that will be worth more per unit so he probably took that figure and reduced it to 21 Million.

I think it's an arbitrary decision by Satoshi, there are no specific reasons.

This is the most reasonable explanation at the moment.
https://twitter.com/ArcaChemist/status/1167543733245472768
Sasha Fleyshman(C)
https://i.ibb.co/sC4K44W/EDPy11-Xo-AEX3-Iq.jpg (https://ibb.co/7QDnDDk)
"This is offset by what is called a "Difficulty Adjustment", where the 'hash'(random generated number) that is required to solve the block is adjusted - this occurs every 2016 blocks (14 days). So, for all related purposes, 6 blocks/hour is a safe metric to use.

By multiplying that number to expand it to blocks/year, then blocks/cycle (a block cycle is the time between block halvings, where the reward for mining a block is cut in half). That is where the ~210,000 number comes from.

Next, they ran a summation notation of the block rewards approaching zero, as shown by the second image in the above graphic. That number comes out to be 100.

By multiplying 210,000 by 100, you arrive at 21,000,000 BTC - so that hard-capped supply number is just a math based output of the amount of blocks mined multiplied by the rewards for mining each block over time as rewards approach zero.

Now, pulling on this string opens up a new line of questioning, of which I have no answer (as of yet); just theories.

I think it is very important to question what you do not understand with #Bitcoin - the answer is not as important as the thought process."

Given the explanation aboved. Someone with caliber to introduced a unique case study for Bitcoin is 100% a genius and knowing the 21million decision supply is not by choice but also a consideration to the flow of the plan. If its just a random picking, the reasons here wouldn't be derived based on mathematical approach isn't it?

 my guess could have the ground.

One additional, more romantic reason...in addition to the block award math listed above.


SOURCE: Why are there only 21 million Bitcoin? (https://www.luno.com/learn/en/article/why-are-there-only-million-bitcoin#:~:text=%E2%80%9CI%20wanted%20to%20pick%20something,per%20unit%20than%20existing%20currencies.)