RAM ~100 ns, SSD ~100 µs, spinning disk ~10 ms — each rung roughly 100× slower.
Five orders of magnitude between memory and disk is why caching dominates systems design — one disk seek costs ~100,000 RAM reads. The rungs are not a constant factor, though: memory to SSD is nearer 1000× than 100×, and the mnemonic understates the gap that matters most.
The rungs have moved. NVMe is nearer 20 µs than 100, and Optane closed the gap to RAM far further than the ladder allows. Queue depth matters more than the medium in many workloads: a spinning disk streaming sequentially can beat an SSD serving random 4 K reads.
Estimate with the rule, then check it against the calculator that models it properly.
Open Latency Ladder →