Here’s how to make your estimates:
How many drywall panels?The drywall itself will be your biggest expense. Professionals estimate jobs based on the square footage of the surfaces they will be covering, but for a relatively small DIY job, I think it makes more sense to simply figure out how many panels will be needed.
Calculate the number of panels you will need for each wall separately (I find it easiest to make a sketch of each wall). If you have a typical 22-foot-square garage and you plan to install the drywall horizontally, you will need three 8-foot-long panels or two 12-foot-long panels for each row. In both cases, you will have a 2-foot piece left over.
If the walls are 8-feet high, just double these totals. If you are drywalling the ceiling, calculate what you'll need for that surface as well. Once you find out what each panel costs, multiply the unit cost by the number of panels.
How many screws?I usually use 1 1/4-inch drywall screws, which are sold by the pound. Figuring about 300 screws per pound, plan to need one pound of screws for every 500 square feet of surface you will be covering. Screws are cheap, and you will likely find many other uses for them, so don’t be afraid to buy more than you think you might need.
How much joint compound?
You have to make a rough guess at this, as the quantity of “mud” you use will depend on your skill and on the number of joints that need finishing. Here’s a formula that should get you pretty close: Figure out the total number of square feet you will be covering with drywall, then divide that figure by 7.25. The result will be the approximate number of pounds of all-purpose joint compound needed.
Calculate square footage by multiplying length times height (or width). A single 22-foot-long wall that is 8-feet tall covers 176 square feet.