Lol, we are both a little off. It's been a while since I used the calculation. So the correct formula is HxLxW, that number is multiply by the number of times you want to move the air per minute. 1 is the minimum recommendation, 3 is the common. This new number you divide by 60 (seconds) to get the cfm. So for a 3x3x7 the correct calculation would be 3x3x7(63).. 63 x 3 =189.. Divide 189 by 60 to get cfm needed. So for my 3x3 it would be 3.1 cfm, not 21. Lol. Either way. Most inline fans will be overkill for smaller grow tents without a speed controller.