Numbers & Units You Must Know Cold
Round to powers of 10 — never 1024
In a real design discussion you never use 1024. The unit ladder is just “add three zeros”:
| Unit | ≈ bytes | Think |
|---|---|---|
| 1 KB | 10³ | a thousand |
| 1 MB | 10⁶ | a million |
| 1 GB | 10⁹ | a billion |
| 1 TB | 10¹² | a trillion |
| 1 PB | 10¹⁵ | a quadrillion |
“Bytes per thing” — memorize ~8 anchors
You can’t size storage without knowing how big one record is. Memorize these and interpolate the rest:
| Thing | Size |
|---|---|
| char (ASCII) / int / bool | 1 byte |
| int64 / timestamp · UUID | 8 bytes · 16 bytes |
| a short text post (~280 chars) | ~300 bytes |
| one DB metadata row | ~1 KB |
| a web page (HTML) | ~100 KB |
| a compressed photo | ~1 MB |
| 1 min of audio (mp3) | ~1 MB |
| 1 min of HD video | ~10–50 MB |
The time trick that unlocks “per second”
Seconds in a day ≈ 86,400 ≈ 10⁵. Memorize this one number.
- 1 request/sec ≈ 100K/day ≈ 2.5M/month ≈ 30M/year.
- Going back: 1 million events/day ÷ 10⁵ ≈ ~10/sec.
- 1 month ≈ 2.5 × 10⁶ s · 1 year ≈ 3 × 10⁷ s.
Latency numbers (order of magnitude)
| Operation | Time |
|---|---|
| L1 cache reference | ~1 ns |
| Main memory (RAM) | ~100 ns |
| SSD random read (4 KB) | ~100 µs |
| Round trip, same datacenter | ~0.5 ms |
| Read 1 MB sequentially from SSD | ~1 ms |
| Disk seek (HDD) | ~10 ms |
| Round trip, cross-continent | ~150 ms |
Takeaway: RAM is ~1,000× faster than SSD and ~100,000× faster than a disk seek; a cross-continent round trip dwarfs everything — minimize them.
Throughput anchors (for server-count math)
| Resource | Rough anchor |
|---|---|
| One app server | ~1K–10K simple QPS |
| Redis / in-memory store | ~100K ops/s |
| Postgres/MySQL (simple reads) | ~a few K – 50K QPS |
| SSD | ~500 MB/s sequential |
| 1 Gbps NIC | = 125 MB/s |
| Kafka (per partition) | ~MBs/s |
Availability — the “nines”
| Availability | Downtime / year |
|---|---|
| 99% (“two nines”) | ~3.65 days |
| 99.9% | ~8.8 hours |
| 99.99% | ~52 minutes |
| 99.999% (“five nines”) | ~5 minutes |
These anchors are exactly what the Capacity Estimation playbooks plug into. Memorize the bold ones first.
Formulas are standard/public-domain engineering math. Approach and reference-table format adapted from the System Design Primer (CC BY 4.0), Jeff Dean’s latency numbers, the DesignGurus capacity-estimation guide, and Little’s Law.
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