My ISP, Tiscali, are offering a 2mb BB service with a 15Gb monthly usage cap.
In their members area it's possible to view your upload/download figures for about a 45 day/6 week period.
Between 14 Feb and 31 March (45 days) my upload was 269.78mb and download 1,783.32mb. So the total of that was 2,053.10mb. Divide that by 45 and I've averaged 45.62mb per day. Multiply that by 31 and I've averaged 1,414.36mb per month. Divide 1,414.36 by 1,024 and that gives 1.38Gb per month.
Is that correct? Do usage caps take into account upload AND download figures?