# Back-of-the-Envelope Estimation — System Design

Source: https://www.geekswithgeeks.com/en/system-design/sd-estimation

> Quick math for QPS, storage, and bandwidth.

## From DAU to QPS

A day has ~86,400 seconds (~100k). So 1M requests/day ≈ ~12 QPS average. Multiply by a peak factor of 2–10x for the real target.

## Storage estimate

Estimate per-record size, multiply by write rate and retention.

```text
500 bytes/tweet x 100M tweets/day
= 50 GB/day
x 365 x 5 years
~= 90 TB (before replication)
```

Output:

```
Plan for 3x with replicas ~= 270 TB
```

## Round aggressively

Use powers of 10 and 2. The goal is the right order of magnitude — is it GB or PB? — not a precise figure.

**Quiz:** A service gets 8.6M requests/day evenly. Roughly what is the average QPS?

- [ ] ~1 QPS
- [x] ~100 QPS
- [ ] ~1,000 QPS

*Answer:* ~100 QPS. 8.6M / 86,400s ≈ 100 QPS average; peak would be higher.