Many loyal Cruft readers had questions about the size of Tivo "hours" storage in relation to hard drive size. I have stolen this paragraph directly from the Weaknees site, a place you can buy upgrades to your Tivo.

How are the number of hours calculated on a TiVo?

TiVo standalone units (i.e., TiVos without a built-in DirecTV receiver) have four levels of recording quality. At the best recording quality, the rule of thumb is that the number of hours is equal to 0.35 times the number of gigabytes. At the lowest recording quality (basic), the number of TiVo recording hours is 1.2 times the number of gigabytes. Because drive sizes vary slightly (an 80 gigabyte drive may be 81.9 gigabytes or 80.0 gigabytes, for example), the exact number of hours in any given TiVo cannot be calculated precisely in advance. DirecTiVo units generally get about 0.875 hours per gigabyte of storage space. The hours reported on a DirecTiVo is a maximum amount; the actual number of hours yielded will vary based on the type of shows you record (eg. sports take up more space on a hard drive than do, say, soap operas, due to the amount of motion).

So, let's do the math here. We'll do the simple case first.

If Martin has a DirecTivo with a 40 GB drive, he should expect 35 hours (40 GB * 0.875 hr/GB = 35 hours). If Martin was to upgrade to a 120 GB drive, he should expect 105 hours (120 GB * 0.875 hr/GB = 105 hours)

If I have a standalone Tivo with a 30 GB drive that I use in High Quality mode, with a 30 GB hard drive there should be about 18 hours (30 GB * 0.6 hr/GB = 18 hr). If I upgraded to a 120 GB drive I roughly have 72 hours (120 GB * 0.6 hr/GB = 72 hr).

So the rough effect of my upgrade was to take me from 18 hours to 72 hours of recording time. The actual amount of storage will vary depending on the quality I choose to use for each show. Most of the kids shows record at Medium quality and most of our favorite shows record at Best Quality.

Make sense?