Ported box for a MMATS Dread 12

"25 square inch by 16 inch long" refers to the volume of the port.  If the port is inside the enclosure, as it usually is, the port volume infringes on the box volume just as much as does the drivers themselves and the added bracing.  The displacement of all these objects must be added up, added together, and subtracted from the gross box volume.  This will give you the new net box volume.  It is this final net volume that you use to determine tuning frequency.

Multiply three dimensions to find the cubic answer.  In inches, feet, meters or whatever you are working with.  The link in the answer above has calculators that will change from one unit to another unit. 

Now, look at the 25" sq X 16" problem:  that is akin to a slotted port design, or it could likely refer to a roughly 6" round port.  This site has some good, friendly calculators as well, and you can figure this one out using one of those.  Go to https://www.the12volt.com/caraudio/boxcalcs.asp

Scroll down to Volume calculator - Rectangular or square.  Now you have to find two numbers that when multiplied equal 25", because the problem gives you the number as "squared".  Do this by dividing 25 by 2, which equals 12.5.  Now you have the three dimensions you need:  12.5, 2, and 16.  Input those numbers in the calculator.  Enter "0" in thickness, as we will assume these dimensions take the port wall thickness into account already (but you will need to research that).  You will get your answer in cubic inches.

There are 1728 cubic inches in one cubic foot (12 x 12 x 12).  After you have found all the displacements in cu inches, divide by 1728 to convert the sum to cubic feet.  (The ".25cf" you mentioned above refers to a rough, but close enough, port displacement in cubic feet.)

.25 is a rounded-off number in the spec sheet.  Use your figures, and be sure to include the entire port structure including its walls.

When you are deriving box airspace, use only the airspace that is dedicated to the woofer, not the entire amount that includes ports and baffles.  Saying that it is 2.25 overall can cause confusion...if there is exactly 2 cu ft dedicated to the woofer then that is what you would say.  It would be a 2 cu ft box.

...and don't ever be sorry about asking questions.  That's what makes this forum go 'round. 

Gross is the whole thing.  Net is the airspace that's left for the woofer after displacements have been deducted.  You can be reminded of this every time you pick up a jar of something from the grocery store...it will say on the label "nt wt" or "net weight", which will indicate the weight of the product in the jar...but not the jar itself.

otto34, the drawing doesn't indicate how to measure.  But you are definitely on the right track, and this type of knowledge doesn't come quickly.  The drawing indicates, to me, that you are a pretty good drawer but not yet a box designer.  Arrange the port so that it fires out the same side as the woofer, even if you have to put an elbow in it.  If you want to invert the woofer as the pic illustrates, allow an additional approx. .03 cu ft for the area within the cone, which will be a part of the box volume.

I make much cruder illustrations, and here is an old one to prove it.  This shows that an inverted cone driver will add to the airspace in the box:

There is always a little bit of guesswork involved in coming up with net airspace, and the ".25" in the spec sheet that we talked about proves the point that it is alright to be close.

For your port displacement, measure the port structure from the inside of the baffle to the end of the port, in length.  A 16" port that is mounted through a 3/4" baffle will actually measure 15 1/4" long.  Find the outer diameter, not the inner, for port structure measurement.  A 4" inside diameter might measure 4.25" on the outside.  This total volume is displacing air from the enclosure.

Oh, I just double-checked your question and, yes, round or slotted doesn't matter as long as the square inch opening is 25".  That's inside opening area.