Now on your LCD, you have a display of number of tape core rotations underwent while winding this ball of yarn.  Now let’s turn that into a measurement of length!  Tape cores are generally a standard 1” (25.4 mm) in diameter in the US.  We need the circumference, to figure out how much yarn goes around with each rotation.  Easy-peasy – circumference is just diameter x Pi.  So 1 inch x 3.14 = 3.14 inches (7.98 cm) of yarn per rotation.  There are 36 inches per yard (or 100 cm per meter).  So, multiply your number of rotations by 3.14 then divide by 36 to get yards.  Or multiple rotations by 7.98 then divide by 100 to get meters.

But wait!  What if your tape dispenser is a newfangled one with a triangular core?  No problem!  You can take a piece of yarn, wrap it around the core, mark the beginning and end of the wrap, then unwrap it and measure the distance between beginning and end.  This is your equivalent circumference.  Multiply it by the number of rotations to get total length.  If you measured in inches, then divide by 36 to get yards, or if you measured in cm, divide by 100 to get meters.  Our triangular core’s equivalent circumference was about 3 inches.