Art of Codeartofcode
Lesson 1 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

The picture to keep in your head

3 min read+10 XP

Every question below is answered twice: first at the wedding, so you can picture it, then in the system, so you know exactly how to do it. Each answer ends with a memory code to lock it in.

The picture to keep in your head

Your cousin is getting married and you're in charge of the food. The first thing you ask isn't "biryani or pulao?". It's "how many guests?" Fifty guests and you cook at home. Five thousand and you need caterers, a big kitchen, delivery vans and a plan for the moment everyone walks into the dining hall at 1 pm.

You don't need the exact number. "About 500" is enough to decide how big a kitchen, how much rice, how many cooks. Getting it wrong by 10 or 20 guests doesn't matter. Getting it wrong by 10 times does.

Back-of-envelope estimation is the same skill for systems: quick, rough numbers that tell you how big to build.

Our running example: a photo-sharing app (call it PicShare). People open it, scroll through photos, and sometimes upload one. All numbers about PicShare are assumptions you'd agree with the interviewer, not real figures for any company.

At the weddingIn the system
Number of guestsDaily active users (DAU)
Plates each guest eatsRequests each user makes
Everyone arriving at 1 pmPeak traffic
Rice and stock in the store roomStorage
Vans bringing food in and taking it outBandwidth (network)
Popular dishes kept ready at the front counterCache memory
Cooks in the kitchenServers

the wedding plan, mapped step by step to the system plan.

Figure: the wedding plan, mapped step by step to the system plan. Everything starts from one number: how many people are coming.