network hash rate
Network hash rate is the total number of guesses per second that all the miners on a proof-of-work chain are collectively making as they race to find a valid block. Mining is essentially repeated guessing: a miner tweaks a nonce, hashes the block, and checks whether the result is below the target; each such attempt is one 'hash.' Adding up every miner's attempts gives the network hash rate, the single best measure of how much computational muscle is currently securing the chain. Bitcoin's is measured in exahashes per second (EH/s) — quintillions of attempts every second.
Hash rate, difficulty, and block time are locked together by a feedback loop. The protocol targets a fixed average block interval (about 10 minutes for Bitcoin) and periodically adjusts the difficulty so that the current hash rate produces blocks at that pace — Bitcoin retargets every 2016 blocks, roughly every two weeks. If miners join and hash rate rises, blocks initially come faster, then difficulty ratchets up to slow them back to 10 minutes; if miners leave, the reverse happens. So you cannot read hash rate directly off the chain — it is estimated by observing how fast blocks arrive relative to the known difficulty.
Hash rate is the dollar-denominated wall protecting the chain's history. To rewrite recent blocks, an attacker must out-hash the honest network long enough to build a longer competing chain — a 51% attack — and the more hash rate honest miners contribute, the more hardware and electricity an attacker would need to match and exceed it. A high, growing hash rate therefore signals strong security; a sudden drop (after a price crash or a regional mining ban) signals that the chain has temporarily become cheaper to attack.
Two honest caveats. First, hash rate measures security only against rewriting history; it says nothing about software bugs, governance capture, or which transactions get censored. Second, because hash rate is inferred statistically from block timing, short-term figures are noisy — a string of lucky fast blocks can make hash rate look higher than it really is — so it is best read as a smoothed, longer-term trend rather than a precise instantaneous number.
If a network mines blocks 20% faster than its 10-minute target over a retarget period, the next difficulty adjustment raises difficulty by roughly 20%, pushing block times back toward 10 minutes — the protocol's automatic response to hash rate having grown.
You cannot observe hash rate directly; it is estimated from how fast blocks arrive given the current difficulty, so short-term readings are statistically noisy.