I often find I have too much information to remember (and for some reason people expect me to be able to spout pixel aspect ratio numbers like a multiplication table). Fortunately the fine folks over at Digital Rebellion has solved this (and for free). They offer several video widgets that do important math, these run on a web browser or iPhone (and a couple can run offline as well).
Film Rate Calculator – Determines how much film you need for a particular scenario.
Web – iPhone version – Offline version
Aspect Ratio Calculator – Determines the aspect ratio for different footage formats.
Web – iPhone version
These tools are cool, free, and useful.... three points that make them a must have for my iPhone.