Art of Codeartofcode
Lesson 14 of 19

2. Back-of-Envelope Estimation

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The picture to keep in your head
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2.1 Why estimate at all?
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2.2 Which units and sizes should you know?
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2.3 What time shortcuts make the maths easy?
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2.4 Which latency numbers should every developer know?
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2.5 How do you turn daily users into requests per second?
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2.6 Why does the read/write ratio matter?
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2.7 Why size for the peak, not the average?
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2.8 How do you estimate storage over the years?
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2.9 How do you estimate bandwidth?
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2.10 How much memory does the cache need?
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2.11 How many servers do you need?
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2.12 How do you round, and how do you know you're not way off?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
2. Back-of-Envelope Estimation

Memory card and where the analogy breaks

3 min read+10 XP

The memory card (tear this page out)

#QuestionMemory code
2.1Why estimate?Size decides shape
2.2Units?King Midas Gave Tea Parties: KB, MB, GB, TB, PB, each ×1,000
2.3Time shortcuts?A day is 100k seconds. A million a day is 12 a second.
2.4Latency numbers?Nano for memory, micro for the building, milli for the disk and the world
2.5Users to requests per second?Times, divide, times
2.6Read/write ratio?Read-heavy? Copy it. Write-heavy? Spread it.
2.7Peak vs average?Plan for 1 pm, not the daily average
2.8Storage?Writes × size × days × copies
2.9Bandwidth?Network speaks bits, storage speaks bytes. Times eight.
2.10Cache memory?Cache the hot 20%
2.11Servers?Peak ÷ per-server, leave headroom, add a spare
2.12Rounding and checks?Right zeros, not right decimals

The whole chapter in one line: Count the guests, plan for the lunch rush, round hard, and never forget the times eight.

Where the analogy breaks

  • Guests don't multiply overnight. Users can. A viral moment can bring 10 times the traffic in an hour, so systems need room to grow quickly (autoscaling, Chapter 6).
  • Food is eaten once. Data is kept, copied and backed up. Storage keeps growing every day, which is why you multiply by years and copies.
  • A caterer sees the guests arrive. A system has to predict them. That's why you state your assumptions and design for the peak.

Quick check

Five questions on this chapter. Pick an answer, then check it.

quiz