There was a time when figuring out how much space your export would need was easy. Now, with so many codecs, I have difficulty remembering what the total per minute is for most of them. There are some calculators you can install on your computer, but I have found that using
VideoSpace Online is the better way to go for me. They do a good job of keeping it updated as new formats are introduced and it is clean, simple and free. While you are on their site,
Digital Heaven has some other nifty free tools you might check out including some very comprehensive grid and guides overlays and a plug-in to keep your drives from going to sleep.
