If the room is rectangular, you measure two connecting sides in feet and multiply those numbers together to figure out the size of the room in square feet. The area of a rectangle is length times width.
When you are figuring out room size, you are solving a geometry problem by figuring out the area. If your room is a square or a rectangle, the calculation is fairly simple; you only need two sides to calculate the area. However, it becomes more complicated if your room is not a standard square or rectangle.
If that's the case, you need to divide the room into shapes. Maybe your room is one large rectangle, one smaller rectangle and one triangle. In that case, you need to measure two of the sides of each of the rectangles. Calculate the area for each rectangle by multiplying the sides together. Add the area of the rectangles together.
Next, you need to figure out the area of the triangle. The area of a rectangle is half of the base times the height. Pick a side for the base. Measure from the base up to the angle above it for the height. If it's a right triangle, the base and the height are the two sides adjacent to the right angle. Multiply the base times the height, and then divide by two to reach the area. Add the area of the triangle to the areas of the rectangles to reach the area of the room in square feet.