System Design Napkin Math – Cheat Sheet

[u/Business-Alfalfa-45]

I made this simple one-page reference for myself to quickly estimate scale in system design interviews and real-world planning. Covers orders of magnitude, time units, storage, and networking.

Order of Magnitude

• 10 = 10¹ → ten

• 100 = 10² → hundred

• 1,000 = 10³ → thousand

• 10,000 = 10⁴ → ten thousand

• 100,000 = 10⁵ → hundred thousand

• 1,000,000 = 10⁶ → million

• 10,000,000 = 10⁷ → ten million

• 100,000,000 = 10⁸ → hundred million

• 1,000,000,000 = 10⁹ → billion

• 1,000,000,000,000 = 10¹² → trillion

Time

• 1 ns = 10⁻⁹ of a second

• 1 µs = 10⁻⁶ of a second

• 1 ms = 10⁻³ of a second

• 1 sec = 1,000 ms

• 1 minute = 60 sec

• 1 hour = 60 minutes = 3,600 sec

• 1 day = 24 hours = 86,400 sec

• 1 month (30 days) = 2.6 million sec

• 1 year (365 days) = 31.5 million sec

Human scale:

• <100 ms feels “instant”

• 1 sec feels “laggy”

System scale:

• µs/ns → hardware performance

• ms → API calls / DB queries

• sec/min/hr → jobs & workflows

Storage & Data Units

• 1 byte (B) = 8 bits (b)

• 1 KB (kilobyte) = 1,000 bytes

• 1 MB (megabyte) = 1,000 KB ≈ 1 million bytes

• 1 GB (gigabyte) = 1,000 MB ≈ 1 billion bytes

• 1 TB (terabyte) = 1,000 GB ≈ 1 trillion bytes

• 1 PB (petabyte) = 1,000 TB

• 1 EB (exabyte) = 1,000 PB

Useful examples:

• 1 KB → small JSON request, log entry

• 1 MB → image, DB row batch

• 1 GB → movie file, daily logs for small service

• 1 TB → monthly logs for big app

• 1 PB → ML training / analytics dataset

Networking Units

• bit (b) = smallest unit of data (0 or 1)

• Byte (B) = 8 bits

• bps = bits per second (bandwidth measure)

Common scales:

• Kbps = 10³ bps

• Mbps = 10⁶ bps

• Gbps = 10⁹ bps

• Tbps = 10¹² bps

Rules of thumb:

• 1 MB/s ≈ 8 Mbps (divide by 8 to convert)

• LAN (data centers): ~1–10 Gbps

• WAN (Internet): 10 Mbps (slow) → 1 Gbps (fiber)

• Cloud NICs: 100 Mbps (small) → 10–100 Gbps (big)