Back-of-the-envelope estimate
Also called: Capacity estimate
A quick calculation with round numbers that shows the size of a problem: requests per second, storage, bandwidth.
Definition
Instead of exact figures, you use rough facts such as "a day has about 100,000 seconds" to turn daily numbers into per-second rates, add a factor for peaks, and multiply sizes to get storage. The goal is the order of magnitude and what it implies: whether data fits in memory, whether one database is enough, whether files belong in object storage.
In practice
100 million feed loads a day is about 1,000 per second on average, and maybe 3,000 at peak.