Whether you are interested in putting in a new rug or planning your furniture relocation, you'll want to know the area of a room's floor. This is easy if you have a nice square room, but what happens if you have five sides to your room? It's just as easy, but it takes a little longer. It's all about triangles.
Things You'll Need
 Tape measure
 Paper
 Pencil
 Calculator
Setting Up

Draw on the paper a rough floor plan of the room, following its basic shape. This will be just for reference and notes, so it doesn't have to be to perfect scale. Starting at the top corner and moving clockwise, label each corner with a letter of the alphabetA, B, C, D and E. Locate and mark each corner of the room according to which corner of the drawing represents it.

Measure each side of the room and note that on your paper.

Measure the distance from point A to the opposite wall (the wall defined by points C and D). This measuring line must be perpendicular to the wall. Mark this line on your paper with a dotted line, and mark the end of the line point F.

Make two dividing lines on your map from point A to point D and from point A to point C. These will be called line AC and line AD. Measure these distances and note them on the paper.

Measure the distance from point B to line AC in a perpendicular line, and note that on the paper with a dotted line and the measurement. Mark that intersection point G.

Make a similar line and measurement from point E to line AD and note the measurement. Mark this intersection point H. You are now ready to make your calculations.
Making Your Calculations

Perform the following calculations:
Multiply the measurements of wall DC and line AF.
Multiply the measurements of line AC and line BG.
Multiply the measurements of line AD and line EH. 
Add all three values.

Divide the total in half. This number is the area of your room in square feet (if you measured in feet).
Tips & Warnings
 Work slowly and deliberately without rushing.
 It helps to clear the floor as much as possible for measuring.
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References
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