I haven't recorded enough shows to fill my TiVo DVR, but when I try to schedule more shows, the TiVo DVR says there isn't room.

The TiVo Service includes smart scheduling features that track not only how much space you currently have on your TiVo DVR, but also how much space will be needed in the near future to record all the shows you have scheduled. If your TiVo DVR will be full in the near future, the TiVo Service informs you and suggests options—such as allowing some shows to be deleted early—that will allow you to schedule additional recordings. With the TiVo Service, you always have the final say over what gets recorded and how long it’s kept.

The amount of recording space the TiVo DVR needs varies from channel to channel and from show to show. High-definition shows take up much more space than standard-definition shows.