Measure the length of the room with the tape measure. A retractable builders tape measure is the most effective way of measuring. These are available in several sized lengths. Put the end of the tape measure against the wall and roll out to the opposite wall. Be as accurate as possible.
Write down on your piece of paper the length of the room. Ensure you mark the word "length" on the paper. A set of figures with no headings will become confusing.
Measure the width of the room from wall to wall.
Jot down the width of the room on the piece of paper. Ensure you write "width" next to the figure.
Use the calculator to multiply the length by the width. The final figure will be the square footage of the room.
Measure the longest side of the triangle shape. This side will be the base so write "base" next to your figure.
Measure the height of the triangle. If you are calculating room size, measure a straight line from the point of the triangle to the wall of the base side.
Write down the figure under the heading of "height."
Multiply the figures of base and height.
Divide the figure by two to calculate the square footage.