A yard of concrete will cover about 68 square feet at 4 inches thick,so in saying that you are looking ruffly at 2 yards to do the job.

Do the math, I think its 1.7 yards,not 3.6. Because I just poured one 12 x 24 and used 3.7yards

About 3.6 cubic yards but the concrete company will do the math and tell you for certain