[cperciva]

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%.)

[porter]

Using 70/X will also work just fine.

[cperciva]

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).