Calculating area
To calculate area, you simply use this formula:area = length × width a = lw
From there, you can get the other information you need:

Figuring how much seed you need: Use the area formula to calculate the area you want to cover and then calculate the number of pounds of seed you need. Seed companies indicate how much coverage a pound of seed gives you. Simply divide the area you need to cover by the amount of area that 1 pound of seed covers. For example, if you want to cover an area of 1,625 square feet, and 1 pound of seed covers 600 square feet, you need about 3 pounds of seed (1,625 divided by 600 is 2.7)

Calculating area to be painted: You can calculate the area of each wall and then add the values together, or you can determine the perimeter of the room (add the length of each wall) and multiply that value by the room's height. A room that has a 42foot perimeter and 8foot ceilings has an area of 336 feet.

Calculating how many square yards of carpeting you need: First calculate the area of the room and then convert that number from square feet to square yards by dividing the area by 9 (a square yard is 3 feet wide and 3 feet long).
Calculating volume
When the area you want to cover has depth as well as height and width, you need to determine volume rather than area. Mulching a flowerbed is one such project: To protect your flowers, you need to spread a few inches of mulch over the whole bed.To calculate volume, you use this formula:
volume = length × width × height v = lwh
With that info, you can then do additional calculations to figure out how many bags of mulch to buy.
Say, for example, that you want to cover 6.68 cubic feet with mulch (that's the volume you need for a small bed, mulching to a depth of 3 inches). Simply divide the volume you need by the number of cubic feet in a bag of mulch. If each bag holds 3 cubic feet, you need 2.23 bags. So buy 3 to be safe.