Little's law
The average number of "things in the system" = arrival rate × average time they stay. L = λW.
Proven by John Little in 1961, independent of distributions. Despite its simplicity it's one of the most-used tools in capacity planning. Example: if your API receives 1000 req/s and each request takes 200 ms ⇒ you have on average 200 concurrent in-flight requests. If the concurrency pool is 100, you have backpressure. The law applies at every level: threads, queues, customers in a shop, packets in a network.