Session 2 · Fri 5 June 2026 · 90 min

Capacity Estimation & Requirements.

Turn vague requirements into the few numbers that bound a design before you draw a single box.

Ehsan Gazar
From Senior to Staff · session 2 of 12
Where we're going · 90 minutes

The run sheet.

0–10Requirements → numbersthe inputs you must extract
10–35The estimation toolkitQPS · storage · bandwidth · memory
35–50Numbers you should knowRAM vs SSD vs network vs region
50–75Worked examplesize a real system live
75–90Project 1 worksheethow it gets graded
By the end of tonight

You'll be able to…

1Split functional from non-functional requirements and turn the latter into numbers.
2Estimate traffic, storage, bandwidth and memory from a handful of inputs.
3Carry the latency numbers every engineer should know in their head.
4Use one bounding number to pick an architecture, not a spreadsheet.
The reframe

One number that bounds the design.

You are not building a model. You are finding the single estimate that decides whether this fits on one node or needs a fleet. Peak, not average, and order-of-magnitude is enough.

💡 In plain English

Like estimating how many guests are coming before you decide how much food to cook. You don't need the exact headcount, just "about 10" vs "about 500", because that changes everything about the plan.

Average lies
design for peak: ×2–10 over the mean
rule
Order of magnitude
is 10s, 1000s or millions? that picks the shape
rule
State assumptions
a stated wrong number beats an unstated right one
tell
The toolkit

What to have in your head.

1
QPS from DAU
daily actives × actions/day ÷ 86,400, then × peak factor
2
Storage
items × bytes/item × retention × replication
3
Bandwidth
QPS × payload size, both directions
4
Memory / cache
hot set size → does the working set fit in RAM?
5
Latency budget
p99 target split across each hop
6
The known numbers
RAM ~100ns · SSD ~100µs · same-DC ~0.5ms · cross-region ~70ms+
Recap · then what's next

A few numbers, decided out loud.

Requirements become QPS, storage, bandwidth, memory and a latency budget. Find the one that bounds the design and state your assumptions as you go.

→
Project 1 · Capacity Estimation Worksheet
Everything tonight applies directly. Due Jun 12.
this is the deliverable
Take-home
Estimate peak QPS and storage for one service you own.
bring it next week