Things You'll Need
Two tacks (for inside measurements) or two stakes (for outdoor measurements)
Many products are sold by the square foot and require you to calculate a square area (for example, carpet, wood flooring or turf). If you are calculating the square footage of a room or yard with round corners or other round elements, the equation for an area of a circle (pi * r * r) can help you in your task. However, before you use the equation, you'll need to measure the length of the radius (the distance from the center of the circle to the edge of the circle). With two of the most basic tools, you can create a larger version of a school compass (a tool that traces circles).
Tie a knot in one end of the string and attach it to the stake or tack.
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Imagine that the rounded corner is the edge of a circle. Place the stake or tack in the approximate center of the circle (the tack represents the pointed end of a real compass).
Lay out the string so that it is approximately the length of the radius, placing the other end at the edge of the "circle." Cut the string with a foot to spare (you'll need this leeway in case you guessed wrong for the radius).
Move the end of the string (the end opposite the tack, which should still be in place) back and forth along the edge of the rounded corner. If your end of the string traces the edge of the circle, then you have selected the correct point for the radius. If not, move the tack or stake a few inches and try again.
When you have found the correct position, trim the string at the edge of the circle.
Measure the length of string with the tape measure.
If you are having trouble "eyeballing" the best place to put the tack, draw a sketch of the room on paper and try to visualize it that way.