As tempting as it may be to use the cord as a neat measuring point, you'll end up with sheets that don't fit. Instead, place the end of the tape measure on the top of the box spring, as shown here. Then place your hand on the top of the mattress and measure up to it. As you can see here, the depth of this mattress is 14". Remember to round off your depth to the nearest inch.
Also, bear in mind that the sheets you order will actually about two inches deeper than the mattress measurement you give us. This is to provide enough return under the mattress. Remember our bottom fitted sheets have elastic sewn around the entire bottom, not just the corners.