Quick Answer
How to say back of the envelope estimation in English: state assumptions first, round the math out loud, label every unit, then turn the number into a design choice.
Diagrams and what to say while drawing
Narrate each figure out loud. Point to the box you are talking about.
What to say on figure 1
Powers of two keep unit jumps clear.
What to say on figure 2
Memory is fast; disk is slower; cross-region is expensive.
What to say on figure 3
Use the visual scale to justify cache or CDN.
What to say on figure 4
Translate nines into downtime, then into redundancy.
Key vocabulary
| Term | Plain meaning | Say it in an interview |
|---|---|---|
| QPS | Queries per second | “Average write QPS is about two thousand.” |
| Peak QPS | Highest short-window traffic | “I’ll size for two times peak.” |
| Retention | How long you keep data | “I’m assuming five years of retention.” |
| Order of magnitude | Rough 10x scale | “We only need order-of-magnitude accuracy.” |
ESL English phrases and transitions
Clarify
- “I’ll estimate QPS and storage with round numbers, then use them to size the system.”
- “I’ll write assumptions on the board so we can change them later.”
Propose and check in
- “The next bottleneck is X, so I would add Y.”
- “Does this level of detail work, or should I go deeper?”
Tradeoffs
- “The advantage is simplicity; the downside is…”
- “I’d choose A over B because…”
Key English language pitfalls
| Pitfall | Sounds like | Say instead |
|---|---|---|
| Silent arithmetic | (writing with no speech) | Speak every multiply and divide. |
| Mixing average and peak | Sizing for average only | “Average is X; I’ll size for peak at about 2X.” |
| Summary | Every new box needs one spoken reason. | |
Common English mistakes
| Mistake | What the interviewer concludes | Better English |
|---|---|---|
| Missing units | Confused scale | “30 terabytes per day.” |
| Fake precision | Sounds rehearsed | “Roughly three and a half thousand QPS.” |
What the interviewer is testing
This is a communication test as much as a design test. Can you explain back-of-the-envelope estimates as a calm collaboration?
Interviewers listen for structure, tradeoffs, and clear English under pressure.
Key takeaway
Your job is to narrate decisions the interviewer can follow and challenge — not to dump tools.
How to open the problem in English
Opening script you can practise aloud
- “I’ll estimate QPS and storage with round numbers, then use them to size the system.”
- “I’ll write assumptions on the board so we can change them later.”
Watch Out
Do not open with a finished architecture. Show the path.
Back-of-the-envelope estimation system design
Use this sequence for back-of-the-envelope estimation system design. For each stage, say what changes, why, and what can still go wrong.
Assumptions
Write DAU, actions/day, media %, retention — then invite correction.
Average and peak QPS
Users → actions → ÷ seconds/day → × peak factor.
Storage
Object size × count/day × retention days, with units every line.
Design impact
Turn the number into servers, object storage, or retention policy.
Weak vs strong answers
Weak
“QPS is about 5,000, so we need Redis.”
Strong
“Assuming 100M DAU and two writes each… about 2,000 average write QPS, maybe 4,000 peak — so multiple app servers.”
Metrics and how to say numbers
| Write | Say |
|---|---|
| 86,400 seconds/day | about one hundred thousand seconds a day |
| 99.9% uptime | three nines |
Follow-up questions you will get
Likely follow-ups
- “What about read QPS?” → “Reads can be ten times writes; I’d estimate them separately and size caches.”
- “Why double for peak?” → “Simple spike model — I can use 3× if you prefer.”
Practice drill
Checklist
- Assumptions
- QPS path
- Storage path
- Two design impacts
“If you can explain back-of-the-envelope estimates without notes, you are ready to draw it in an interview.”
Answer frameworks you can reuse
Use these spoken templates under pressure. Fill the blanks — do not invent a new structure mid-interview.
Framework 1 — Whole estimate (A-C-R-D)
Assumptions → Calculation → Result → Design impact.
Framework 2 — QPS path
Users → Actions → Seconds → Peak.
Framework 3 — Storage path
Item size × Count × Retention.
Framework 4 — Write → Say → Why
Narrate every number you write on the board.
Interview tip
Pick one framework and stay inside it. Switching structures mid-talk makes English sound messy.
System Design English knowledge base
← System Design English knowledge base — start at How to Explain Scaling from Zero to Millions in English for the full chapter index and reading order.
Explore every Mockly article on the blog hub.
All chapters in this series
Jump between System Design English posts below. Each chapter builds spoken English for the same book — use the KB overview when you want the big picture.
| # | Chapter | Slug |
|---|---|---|
| 1 | How to Explain Scaling from Zero to Millions in English | how-to-explain-scaling-from-zero-to-millions-in-english |
| 2 | How to Say Back-of-the-Envelope Estimates in English (you are here) | how-to-say-back-of-the-envelope-estimates-in-english |
| 3 | Spoken System Design Framework in English (ESL) | spoken-system-design-interview-framework-in-english |
| 4 | How to Explain a Rate Limiter in English | how-to-explain-a-rate-limiter-in-english |
| 5 | Consistent Hashing Interview Vocabulary in English | consistent-hashing-interview-vocabulary-in-english |
| 6 | How to Explain a Key-Value Store in English | how-to-explain-a-key-value-store-in-english |
| 7 | How to Explain a Unique ID Generator in English | how-to-explain-a-unique-id-generator-in-english |
| 8 | URL Shortener Interview Phrases in English (ESL) | url-shortener-interview-phrases-in-english |
| 9 | How to Narrate a Web Crawler in English | how-to-narrate-a-web-crawler-in-english |
| 10 | How to Explain a Notification System in English | how-to-explain-a-notification-system-in-english |
| 11 | How to Explain a News Feed in English | how-to-explain-a-news-feed-in-english |
| 12 | Chat System Interview Phrases in English (ESL) | chat-system-interview-phrases-in-english |
| 13 | Think Out Loud: Search Autocomplete in English | think-out-loud-search-autocomplete-in-english |
| 14 | How to Explain YouTube System Design in English | how-to-explain-youtube-system-design-in-english |
| 15 | How to Explain Google Drive System Design in English | how-to-explain-google-drive-system-design-in-english |
- How to Explain Scaling from Zero to Millions in English
- How to Say Back-of-the-Envelope Estimates in English — you are here
- Spoken System Design Framework in English (ESL)
- How to Explain a Rate Limiter in English
- Consistent Hashing Interview Vocabulary in English
- How to Explain a Key-Value Store in English
- How to Explain a Unique ID Generator in English
- URL Shortener Interview Phrases in English (ESL)
- How to Narrate a Web Crawler in English
- How to Explain a Notification System in English
- How to Explain a News Feed in English
- Chat System Interview Phrases in English (ESL)
- Think Out Loud: Search Autocomplete in English
- How to Explain YouTube System Design in English
- How to Explain Google Drive System Design in English
Frequently Asked Questions
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