Ask HN: Do you know any useful "Back of the Envelope Calculations"? • Hacker News Board

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

#11

Post by porter » Sat, Sep 18, 2010, 6:11 PM UTC

cperciva wrote: post #2

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.) 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% > 2%, and accurate to within 10% for X% < 29%.)

Using 70/X will also work just fine.

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

#12

Post by cperciva » Sat, Sep 18, 2010, 9:14 PM UTC

porter wrote: post #11

cperciva wrote: post #2

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.) 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% > 2%, and accurate to within 10% for X% < 29%.)

Using 70/X will also work just fine.

Yes, and for small values of X, 70/X is more accurate, too. 72 has the advantage of being a multiple of 2, 3, 4, 6, 8, 9, and 12, whereas 70 is a multiple of 2, 5, 7, 10, and 14 -- knowing both of these gives you a whole-number answer for any growth rate under 15% except 11% (correct answer is 6.64) and 13% (correct answer is 5.67).

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

#13

Post by fauxfauxpas » Sun, Sep 19, 2010, 2:16 AM UTC

I've always liked this page - http://www.vendian.org/envelope/

and the dots clock is interesting as well

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

#14

Post by mchadwick » Mon, Sep 20, 2010, 2:25 AM UTC

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.