How Total Hashrate Is Actually Calculated

It's not possible to see the total amount of bitcoin mining hashrate at any given time.

Miners don't generally disclose this information and you cannot know if you're accounting for all miners even if they did. It can only be calculated from the variables we do know.

Those variables are the network difficulty and average block times.

Network difficulty: The measure of how difficult it is to mine a new bitcoin block.

It determines how hard it is for miners to find a valid hash below a given target when creating a new block. It is adjusted regularly to keep the average time between new blocks at 10 minutes. If blocks are coming in faster than 10 minutes (on average), it's adjusted higher to make it more difficult to find new blocks (thus slowing the speed at which they're found back down). And vice versa if the blocks are being found slower than 10 minutes.

Average Block time: The amount of time it has been taking on average for bitcoin miners to find a block over a given timeframe.

The target block time is set to 10 minutes. The average block time is how long it's actually been taking over a given time. The difficulty adjusts based on the difference.

We calculate what the total hashrate must be by how quickly the blocks are being found at the current difficulty.

The equation looks like this:

Bitcoin Hashrate = [(blocks solved over last 24 hrs/144) difficulty 2^32 ]/600 (https://hashrateindex.com/blog/what-is-bitcoins-hashrate/)

The point is that we're determining what the hashrate must be, not recording what it is.

For this reason, viewing the total hashrate number over too short a timeframe is inherently prone to the luck of the miners.

If you want a useful number to measure, look at either the 7-day or 30-day moving average instead.