I' trying to build a ported box for my 2 Q15 tunes to 30hz but everytime i tried to calculate it always came out with different results.

My box dimension (external) is W33 H17 D26 and I'm planning to use 2 X 6.25" round port with 3mm thickness. This will come to 6.52 cu ft. after speaker displacement. My problem is do we calculate port length based on net volume (after speaker and port displacement) or do we take the gross volume (after port displacement) as point of reference. My guess is the first one.

So anyone could help me calculate the port length for 2 round ports. If the net volume is below recommended (2.8 cu ft per sub) I could raise depth to 27 but that is as far as I can go.

Also any program that allow you to calculate 2 ports? I've tried using winisd but everytime I enter the TS parameter it wont recognize it.