What halving is, and who sets the rule

In brief

Halving is the moment the reward Bitcoin pays a miner for producing a new block is cut in half.

No committee or company decides this. The rule sits inside the software that runs the network as a piece of arithmetic, and the comment in Bitcoin Core's source puts it this way:

"Subsidy is cut in half every 210,000 blocks which will occur approximately every 4 years."

What the code does

The function that computes the reward fits in three lines. It divides the block height by 210,000 and calls the resulting integer the number of halvings so far. The starting reward is 50 BTC, and that number is halved once per period.

So the reward follows this sequence: 50, then 25, then 12.5, then 6.25, then 3.125.

The code also carries a ceiling: once the number of halvings reaches 64, the function returns zero outright. The reward does not shrink forever; it ends.

Why "approximately four years"

The rule counts blocks, not calendar days. At an average of one block every ten minutes, 210,000 blocks work out to roughly four years. When the network's total mining power rises, blocks arrive faster and the date moves earlier; when it falls, the date slips later.

That is why every prediction naming an exact halving day is a prediction. The block height is the certain part.

What it measures and what it does not

A halving changes only the rate of new supply. It says nothing about the supply already in circulation, about demand, or about price.

Slower supply growth does not mean a higher price: price forms where supply and demand meet, and a halving touches only one side of that.