Free tools
System Design Interview Tools
Capacity estimation and latency intuition are the two number skills interviewers at Google, Meta, Amazon, Anthropic, and OpenAI probe in almost every system design round. These free tools train both.
Back-of-the-Envelope Calculator
Turn DAU, actions per user, and payload size into QPS, storage, bandwidth, and cache estimates — with every formula shown, the way interviewers expect you to derive them.
Open tool→ReferenceLatency Numbers Every Engineer Should Know
The classic latency table on a log-scale chart, with a humanized column that makes the gaps visceral — and how to actually use the numbers in an interview.
Open tool→Interview tools FAQ
Are these system design interview tools free?
Yes. Both tools are completely free, with no account required. They pair with TierOnePrep's interactive system design questions, which include capacity estimation steps in their Design phases.
When do I use back-of-the-envelope estimation in an interview?
Typically in the first 10-15 minutes of a system design interview, right after clarifying requirements. Interviewers at companies like Google, Meta, and Amazon expect you to estimate QPS, storage, and bandwidth before proposing an architecture, because those numbers decide whether you need sharding, caching, or a CDN at all.
Do I need to memorize the latency numbers?
You need the orders of magnitude, not the exact values: memory is roughly 100x faster than SSD for random access, a same-datacenter round trip costs about half a millisecond, and a cross-continent round trip costs about 150 milliseconds. Interviewers check whether your design decisions respect these ratios.
Put the numbers to work
Every TierOnePrep system design question includes a capacity estimation step — practice deriving these numbers inside a real interview walkthrough.