Length times width times depth. Then divide by 27 when using foot increments. That will get you cubic yards. Add 10%. Verify with concrete company when placing order. Good luck.
Sample: 12' by 12' by 4" deep. 12x12=144 x .33=47.52 divided by 27 = 1.76 cu. yd. Order 1.9 to 2 yards. Think of something else small to make with leftover (patio blocks, base under downspout, a step, etc.)