1. The square footage of your attic. This is usually the same as the square footage of your house. If you have a two story home, you can find the foot print if the attic on Google Maps and use their scale to get the length and width of your attic. If you have an addition with a flat roof you should not include this in your square footage calculation unless you plan on cutting an access (typically there is insulation present, but a smaller hole can tell the same thing).
2. The current depth and type of insulation you have and the depth you want to be at. If you don't know and don't want to go in your attic to measure the insulation depth, you can assume 4" which is about the point you can see the floor studs. Use Energy Star's R-value map at this link to find the zone you live in and what R-value they recommend for your zone. In Phoenix, R-38 is Energy Star standard.
You can find the current R-value of your insulation from the manufacturers specifications or use BPI R-value table I simplified below.
Cellulose R-3.2 per inch
Loose fill fiberglass R-2.8 per inch
Rolled fiberglass batts R-2.6 per inch
3. The insulation manufacturer's specification chart. You can find this information on thel bags of insulation themselves. Each bag of insulation is stamped with an R-value chart that tell us what depth in inches gives us a desired R-value, how many bags per 1000 sq ft are required to reach that R-value. For your convenience I've included some R-value charts here. Notice that the insulation bags Lowes and Home Depot sell are much smaller and have a high bag count than the bags an insulation contractor will use.