Unroll the area rug, and assess it to determine if it will sit flat. A new, stiff area rug may not sit flat, which will result in an inaccurate measurement.
Ask one friend to stand on the edge of the area rug. Then pull the rug taught (Pull it in the same direction that it was unrolled.) and flat; ask a second friend to stand on the opposite end. You can also use heavy objects like a chair or a few phonebooks.
Measure the rug length by extending the tape measure along the long edge of the rug (in the case of a square or rectangular area rug). If the rug is oval, fold it in half (width-wise), and measure along the fold to determine the length.
Measure the rug width along the edge of the rug in the case of a square or rectangular area rug. If the rug is oval or circular, fold the rug in half (lengthwise for an oval carpet) and measure along the fold to determine width (or diameter, in the case of a circular carpet).
Measure the height of the rug pile if the rug will extend beneath a door or if a door will pass over the rug when opened. The space between the bottom of the door and the floor surface must be equal to or larger than the height of the rug pile.
Write down all of the measurements--length, width and height, or just diameter and height in the case of a circular rug--on a piece of paper, and use this for reference.