RE Audio's box calculator
 

I've used this many times for all kind of subs, it works great.

http://reaudio.com/LPort_Box_Calc.php

It's off by like 5-10 Hz, it'll usually tell you it's lower than it really is. That calculator is greeeasy, use The 12 Volt Port Calculator instead, calculating everything manually. BTW mostly everyone knows about that calculator. It works well for a cut-sheet for a slot port, that's about as good as it gets.

Oh look, they recently updated it by coloring the inner port wall blue, what geniuses!

lol,i'm not knowledgable enough to know that kind of stuff but i made 3 boxes that where all pretty good, but i am looking for a way to build a perfect box for my RD's, the new RD site is up but the box calculator is not

that's handy

The 12 volt makes it easy, you just calculate NET air space seperately from the total space and the NET space with the port space ETC, just individual measuring, and then use an end correction.

End correction - with a slot port that uses an outside wall as an outer port wall than the port length should actually be measured from +half of the port width. For example if a port is 4" wide than the port should be measured at 2" after the inside port wall. Nobody seems to know exactly where the port ends and the NET space begins, but an end correction helps to accurify the tune. Accurify? Wtf. It's late.

Atleast with the 12 volt calculator you get a more accurate tune, and you can make different shapes of boxes as well as calculate for round ports.