You can use a ruler or a measuring tape to determine the length of a line. Either one will work. Place one end of the ruler or tape at one end of the line, then stretch it out to the other end while ensuring that it remains straight and does not bend or curve in any way. If you use inches as your unit of measurement, the ruler or tape measure will indicate the length in inches, and if you use centimeters, it will show the distance in centimeters.