QTUM Capped Supply — The Halving

Here is a question that comes up regularly in the Community: is the supply of QTUM capped, how many will there ever be?

This report answers that question — yes, the supply is capped — and gives the maximum number.

TL;DR: the supply of QTUM is capped at 107,822,406.25, reached in about 31 years.

I am an independent researcher, occasional blogger, social moderator, and appreciate the technical guidance from the Qtum Team and discussions in the Community. If you have any comments or corrections for this article, please reach out on the social medias.

New Explorer

First, let me call your attention to a new explorer at qtum.info.

This powerful explorer has a clean layout, can easily navigate to the first transaction for busy addresses, and has a “Rich List” that covers 98,345 addresses (some not so rich).

Network Weight

For the period February 26 to March 4, 2018, big wallets with known balances of 6.0 million won 712 block rewards of 4,185 total, or 17.01%, giving a network weight of 35.2 million and an annual return of 2.49%

Capped Supply

Two lovers see each other across the playing field, from 50 meters apart. Being mathematics students at Uni, they decide to approach each other by half, to 25 meters, then half again, to 12.5 meters, then half again, half again, etc. As mathematicians, they know that reducing the distance by half each time they will never actually reach each other, but as realists, they know they will get close enough for all practical purposes.

Before researching this, I thought QTUM was uncapped, but it turns out that QTUM is capped similar to how bitcoin is capped. Many people know the maximum number for bitcoin is 21,000,000. What is the equivalent maximum number for QTUM?

The Halving

To figure out the maximum number for QTUM, we need to look at halving (taking something by half). I’ll use bitcoin as the reference because it has the most halvings so far, and everyone knows the 21 million number. Many people know that the block reward for bitcoin drops by half (the “halving”) every 4 years. Bitcoin started with 50 coins for the block reward, halved to 25, and is currently paying 12.5 coins for each block reward.

Qtum will follow a similar halving pattern, controlled by software in the wallets where the halving period is set:

This code says the halving interval is every 985,500 blocks, about 4 1/2 years for current block spacing. The first having for Qtum will be at block 990,501, approximately March 9, 2022, at 07:48:48 UTC (mark your calendars!) [2].

There is the “for all practical purposes” issue we saw with the lovers, but also a hard limit in the source code for calculating the Qtum block reward. The limit for bitcoin is calculated (with some different considerations) in the reference listed below [1]. To get the maximum number for QTUM, the table below shows how the block reward gets successively halved and how the total QTUM accumulate over the next 31 years:

This table takes some explanation.

The genesis blocks 1–5,000 created the original 100,000,000 QTUM, minting 20,000 QTUM with each block (a coinbase block reward like bitcoin, not a coinstake block reward like Qtum). Starting at block 5,001, the block reward was 4.0 QTUM, and continuing for 985,500 blocks to block 990,500 this will add 3,942,000 new QTUM. Starting with block 990,501, the block reward will be halved to 2.0 QTUM, and every 4 ½ years after, halved again.

In 31 years at block 6,903,501, the block reward will be set to zero by the software [3], and we will have reached the final cap of 107,822,406.25 QTUM. By that time transaction fees and gas will have long since replaced the block rewards as the incentive for the stakers. The chart below shows how the QTUM supply grows toward the cap.

There are reports on the dark web that Team Satoshi has the number 21,000,000 tattooed on their left butt cheek. I can neither confirm nor deny any of the Qtum Team have the number 107,822,406.25 proudly inked on their epidermis [4]. I hope everyone can remember 107,822,406.25 now, because that is the cap, and there will never be more QTUM.

Jackson

References

[1] Bitcoinwiki, Controlled supply

[2] Check the calendar math at the Epoch and Unix Timestamp Converter — use 144 seconds per block.

[3] Qtum code in validations.cpp:

Explanation

Line 1375: for the first 5,000 blocks (nLastPOWBlock) the block reward is 20,000 QTUM. These are the genesis blocks.

Line 1378: halvings = the integer value (throw the fractional part away) of (current block — 5,001) / 985,500

Line 1380: If halvings is greater than or equal to 7, set the block reward to 0. Starts at block 6,903,501.

Lines 1382, 1385: Otherwise, the block reward is 4.0 QTUM divided by 2 x number of halvings.

[4] These sentences are a joke, but 107,822,406.25 is for real!

[5] If you are similarly obsessed with the genesis blocks creation story, you might enjoy seeing block 0, which predates the launch of Mainnet, and seems to hold an unspent block reward of 50.0 QTUM. In fact, the address received BOT in the recent airdrop. I have not included these 50 QTUM in the above capped calculations, but you could make a case they should be.

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