---
title: "Jeff Dean Numbers"
url: "https://laigary.com/interview/system-design/jeff-dean-numbers"
type: "note"
section: "system-design"
date: "2026-08-07"
updated: "2026-08-07"
---

# Jeff Dean Numbers

"Jeff Dean numbers" refers to The Latency Numbers Every Programmer Should Know, a famous reference list of computer hardware operation timings compiled by legendary Google engineer Jeff Dean. Engineers use these metrics for quick back-of-the-envelope estimations to see how system designs will perform.


| Operation                                 | ns             | µs         | ms     | note                        |
| ----------------------------------------- | -------------- | ---------- | ------ | --------------------------- |
| L1 cache reference                        | 0.5 ns         |            |        |                             |
| Branch mispredict                         | 5 ns           |            |        |                             |
| L2 cache reference                        | 7 ns           |            |        | 14x L1 cache                |
| Mutex lock/unlock                         | 25 ns          |            |        |                             |
| Main memory reference                     | 100 ns         |            |        | 20x L2 cache, 200x L1 cache |
| Compress 1K bytes with Zippy              | 3,000 ns       | 3 µs       |        |                             |
| Send 1K bytes over 1 Gbps network         | 10,000 ns      | 10 µs      |        |                             |
| Read 4K randomly from SSD\*               | 150,000 ns     | 150 µs     |        | \~1GB/sec SSD               |
| Read 1 MB sequentially from memory        | 250,000 ns     | 250 µs     |        |                             |
| Round trip within same datacenter         | 500,000 ns     | 500 µs     |        |                             |
| Read 1 MB sequentially from SSD\*         | 1,000,000 ns   | 1,000 µs   | 1 ms   | \~1GB/sec SSD, 4X memory    |
| Disk seek                                 | 10,000,000 ns  | 10,000 µs  | 10 ms  | 20x datacenter roundtrip    |
| Read 1 MB sequentially from disk          | 20,000,000 ns  | 20,000 µs  | 20 ms  | 80x memory, 20X SSD         |
| Send packet CA -> Netherlands -> CA | 150,000,000 ns | 150,000 µs | 150 ms |                             |


## Notes

1 ns = 10^-9 seconds
1 us = 10^-6 seconds = 1,000 ns
1 ms = 10^-3 seconds = 1,000 us = 1,000,000 ns

## Credit

By Jeff Dean:               [http://research.google.com/people/jeff/](http://research.google.com/people/jeff/)
Originally by Peter Norvig: [http://norvig.com/21-days.html#answers](http://norvig.com/21-days.html#answers)

## Contributions

'Humanized' comparison:    [https://gist.github.com/hellerbarde/2843375](https://gist.github.com/hellerbarde/2843375)
Visual comparison chart:   [http://i.imgur.com/k0t1e.png](http://i.imgur.com/k0t1e.png)
Interactive Prezi version: [https://prezi.com/pdkvgys-r0y6/latency-numbers-for-programmers-web-development/latency.txt](https://prezi.com/pdkvgys-r0y6/latency-numbers-for-programmers-web-development/latency.txt)