Measure the distance from the center of your circle to its perimeter using your tape measure. This distance is called the radius.
Line your protractor up with the center of the circle, making sure that the placement hole is as close to the center as possible. The protractor allows you to find the angle in degrees of any given section of a circle
Move the arm of the protractor until you get to the portion of your circle you are calculating the curvature of. If you want to know the angle for the entire circle, that is always 360 degrees.
Look at where the moving arm meets the protractor and take note of how many degrees your angle is for your desired section of curvature.
Divide your angle in degrees by 180 and then multiply that number by pi (3.14159...) using your calculator. This will convert your angle from degrees into radians.
Multiply your angle (in radians) by the radius you found in Step 1 using your calculator. The answer you get here is the curvature of the circle.