Cut the blue jeans apart, discarding the seams, pockets and waistband. Cut the remaining pieces of denim fabric into 3/4-inch strips. Cut lengthwise up the legs for the longest strips.
Cut a slit on each end of every strip. Place the slit 2 inches from the end, and make it 1 inch long. Hold one strip in your left hand. Thread a second strip through one of the end slits in the first strip. When the second strip is halfway through the first, stop pulling it. Push the end you threaded into the slit at the opposite end of itself and pull it through. This will tighten into a loop, connecting the two strips of denim. Connect all the denim strips in this same manner.
Make a loop at the end of the denims strips and put a size Q crochet hook through. Chain stitch until your chain is the length of your desired rug width.
Turn the work and single crochet in the second chain from the hook. Single crochet in every chain until the end of the row. Chain one and turn the work.
Single crochet across the next row and every row afterward until the rug is the height you want for your bath mat.
Cut off the strip 4 inches from the hook and pull the strip through the last stitch. Weave the end of the strip through the crochet stitches to hide the loose end.