Well im in my garage right now trying to decide if i should make a 5" slot port tuned to 32hz, a 4" slot port tuned to 31hz, or a 3" tuned to 27hz... Im using the RE calculator to build a box and when im looking at the 5" port it says the Volume internal, and volume displaced is 2000...on the others its 1500 and below..i have no idea what this means, is it how loud the box will be or what..Which port u think i should design???Also the larger the port the less box volume i have..its been between 3.6-4ft3..i cant make a bigger box so let me kno which port is gonna be the best!! PS. the box i have been running is below 1000 volume internal and displaced, so im guessing the higher the number the greater the sound???i could be wrong..

80-100 inches of port area is enough, but 3.6-4 ft does not sound good for box volume on an 18. . .

well what should i do then i cant build a bigger box, should i build the small port or a larger port????????

well what should i do then i cant build a bigger box, should i build the small port or a larger port????????

A smaller port will take up less volume. Really no need for a 5" slot port on one 18" BL. You need at least 5cuft. to build a proper enclosure for that BL. The RE box builder also doesnt account for driver displacement either which is .26cuft. This will change your internal volume as well. Have you thought about extending the port outside the enclosure for maximum volume?

80 inches of port area for this driver would be nice, you could get away with 60.  Did you already buy it??  Sounds like you need the 15" version. . .