As I'm in the process of building a couple 18" sonotube subwoofers, with the terrific help from the forum here, I thought I'd mention something that's not obvious about the sonosub.exe program I found while using it.
By default, the program has endcaps of 3/4" x 3 layers, for a total of 1.5" inside and .75" outside each end. If you just plug in the volume, tuning and port size dimensions as shown earlier in the thread, and you plan to have the more common one x 3/4" piece inside each end, your tube will be too long.
To get an accurate tube length using the more common one x 3/4" plate inside, you need to go to the endcap tab and change the height of the endcaps from the default 2.25" to 1.5" each (both E & F), then go to the main screen and make sure your volume and tuning are still set to the desired numbers. If not, change them back and you'll get a new shorter length.
Comparing the screen grab from earlier in the thread to the ones below, this changes the overall height dimension from 5'-8.39" to 5'-7.0". Almost 1.5" shorter.
I went over this detail yesterday with Mike P, who is helping as well, and he concurred on the need to change the endcap dimensions to get the correct length.
I don't know what you plan to do for a port and flares, but on Mike Ps recommendation I bought Precision Sound Products ports with flares from parts-express.com. According to Bill who makes the sonosub.exe program, if you use PSP ports you will want to use the port length calculation that they come with, or plug in the numbers in the ports tab in his sonosub.exe program. In my case they came out essentially the same and Bill said they were so close either would work. Both were about 1" longer than winISD showed without flares.
While it's a small detail, if you also put in the driver volume occupied number, your length calculation will be even more accurate.
Having said all this, I'm on my first Sonotube build, and by no means an expert. But I have been careful with my understanding of how the program works.