Hash rate is a measure of how well a miner is performing, or another way of putting it is how quickly a miner solves the Bitcoin code to confirm the existence of a new coin. Back in 2014, mining performance was expressed in billions of hashes/s.
The higher the hash rate the greater the amount of cryptocurrency that can be mined and the more block rewards the miner will receive. Hash/s is an expression of mining efficiency, as is W/Ghash/s. It’s also possible to express efficiency in terms of J/Ghash, which means joules per billion hashes.
Hash per second is used with SHA256 algorithms. Hash rate is commonly written as h/s.
Bitcoin Hash Rate
On the Bitcoin network, the hash rate is expressed as hash/s. Mining pools or individual miners work on solving blocks, and the whole network’s hash rate is calculated according to the time it takes to complete each of those blocks. For an accurate measurement though, snapshots aren’t considered a reliable measure, so an average is taken over time.
In January 2015, the hash rate for the network was something like 300 quadrillion hashes/s or 300 Phash/s.
How do we measure the Hash Rate?
Hash power or Hash rate is expressed in hashes per second or h/s. Here are some other measures of hash rate that are in common use:

1 KH/s = 1,000 (one thousand) hashes per second

1 MH/s = 1,000,000 (one million) hashes per second

1 GH/s = 1,000,000,000 (one billion) hashes per second

1 TH/s = 1,000,000,000,000 (one trillion) hashes per second

1 PH/s = 1,000,000,000,000,000 (one quadrillion) hashes per second

1 EH/s = 1,000,000,000,000,000,000 (one quintillion) hashes per second
Some Typical Hash Rate Conversions

1 MH/s = 1,000 kH/s

1 GH/s = 1,000 MH/s = 1,000,000 kH/s

1 TH/s = 1,000 GH/s = 1,000,000 MH/s = 1,000,000,000 kH/s
How Rewards for Mining Correlate with Hash Rate and Difficulty
12.5 BTC is paid for every solved block, and that’s fixed. Hash rate, difficulty, and miner’s reward are changeable though and are all related to each other because the system is set up to be balanced. It’s all designed to even out the time taken to compute each block, so if more miners go on the Bitcoin network then the difficulty level increases to compensate, the hash rate or hash power also increases (more computational power will be required to successfully solve a cryptographic problem).
This correlation between these variable factors is baked into Bitcoin to keep the average Bitcoin block time at 10 minutes.