If you want to calculate the area of a space in square feet, it's best to express your measurements in feet. If you made the measurements in inches, one option is to convert the measurements to feet before calculating area. Another option is to make the calculation using inches, and then convert the result to square feet. Neither option is more difficult than the other, but converting to feet before calculating the area usually helps avoid having to deal with large, unwieldy numbers.
TL;DR (Too Long; Didn't Read)
Convert measurements from inches to feet by dividing by each measurement by 12. Then calculate the area using feet to get square feet. Alternatively, calculate the area in square inches and divide the result by 144.
Option 1: Convert to Feet Before Calculating Area
When you're looking for the area of a small item, it sometimes makes sense to make the measurements you need in inches. For example, suppose you want to cover a bathroom floor with small porcelain tiles, and you want to know the area of one tile to figure out how many you need. If you know the area of the bathroom floor, you'll probably have that figure in square feet, so it makes sense to calculate the area of the tile in square feet so you can divide its area into the area of the floor to obtain the number of tiles that will fit. However, the tiles are small, so you measured in inches.
In this case, divide each measurement you make in inches by 12 to convert it to feet. Once you have expressed the measurements in feet, you can calculate area in square feet.
Example: Suppose the tiles are square, and you measure the length and width of one of them to be 2 inches by 2 inches.
Divide each measurement by 12 to convert to feet and you find the tiles are 1/6 ft by 1/6 ft = 0.17 ft. x 0.17 ft.
Multiply these together to find the area of a single tile to be 1/36 sq ft = 0.0289 sq ft.
Option 2: Calculate Area in Square Inches and Convert
If you go ahead and calculate area in square inches, you can convert your result directly to square feet by dividing by 144. Why this number? Picture a square with an area of 1 square foot. Each side of the square is 12 inches long, and multiplying the lengths of the sides together produces an area of 144 square inches. So 1 square foot = 144 square inches.
Example: If you calculate the area in square inches of one of the 2-inch-square bathroom tiles in the example above, you'll find that each tile has an area of 4 sq in. Divide this number by 144 to arrive at the area in square feet: 4 sq in/144 = 0.0278 sq ft.