Ask HN: Do you know any useful "Back of the Envelope Calculations"?

One I use often is that 1 Mbps of IP packets ~~ 10 GB / day of data. (1 Mbps is 10.8 GB/day, but TCP/IP overhead will typically eat up at least 0.4 GB and usually close to 0.8 GB from that.)<p>Another, more often financial calculation: If you have X% growth, it takes 72/X time periods to double. (Accurate to within 1 time period for X% &#62; 2%, and accurate to within 10% for X% &#60; 29%.)

Memorize powers of 2 up to 9<p><pre><code> 0 1 2 3 4 5 6 7 8 9 1 2 4 8 16 32 64 128 256 512 </code></pre> and<p><pre><code> 2^10 = 1K, 2^20 = 1M, 2^30 = 1G etc. </code></pre> then split anything else<p><pre><code> ex. 2^23 = 2^20 * 2^3 = 8M ex. 2^37 = 2^30 * 2^7 = 128G </code></pre> Edit: Also useful in reverse<p><pre><code> ex. 400K: 256K &#60; 400K &#60; 512K &#60;=&#62; 2^18 &#60; 400K &#60; 2^19 </code></pre> At least memorize 2^4 = 16 (hex) and 2^8 = 256 (byte), then do doubling or halving as necessary.

It's trivial but useful that an hourly rate multiplied by 2000 gives a year's worth (for a standard working year).

Not quite a mental calculation, but still in the spirit: One rough estimation I often find myself using is that a US dollar bill is six inches long. Using either folding or flipping, you can approximate most distances in the wave-your-arms-about scale. Be it thinking about a new physical device or ball-parking blinds at Home Depot, I use this all the time.

I use revenue per employee as a rough guide to the type of company in question (is it human-based or tech/marketing-based? is it a startup or an established business?). It's simple to calculate, and revenue is usually less sensitive information than net income per employee (so easier to get for non-public companies). My feeling is that for the average (non-public, not particularly successful but OK, non-automated) company the average salary is about 1/3 of that (more or less). Actual net income per employee of course depends on the degree of automation and success.<p>Example tables:<p><a href="http://www.jbryanscott.com/2009/02/07/nasdaq-100-revenue-per-employee/" rel="nofollow">http://www.jbryanscott.com/2009/02/07/nasdaq-100-revenue-per...</a><p><a href="http://37signals.com/svn/posts/2283-ranking-tech-companies-by-revenue-per-employee" rel="nofollow">http://37signals.com/svn/posts/2283-ranking-tech-companies-b...</a>

I've always liked this page - <a href="http://www.vendian.org/envelope/" rel="nofollow">http://www.vendian.org/envelope/</a><p>and the dots clock is interesting as well

The rule of 72 for me is a classic that comes to my mind.<p><a href="http://en.wikipedia.org/wiki/Rule_of_72" rel="nofollow">http://en.wikipedia.org/wiki/Rule_of_72</a>

Two:<p>* Golden ratio is also miles to ks. Which means you can use Fibonacci to convert miles to ks.<p>* Divide by square route of two for compound doubling time.