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Im being really retarded right now, and Im sure that this is easy simple math, actually I know it is...but **** I dont know...lol

plot any numbers you want for example's sake, I just wanna know how to calculate the air space being that its not a typical square or rectangle shape...

PLEASE HELP!!!!

 
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Lets run an example:

Assumptions
- wood is 3/4" mdf
- base is 10"
- height is 12"
- length is 30"
- height of small spot on front before angle 3"
- width of small spot on top before angle 4"

Essentially you need to break the odd shape into ones you know.
- 1 rectangle across the whole bottom (3" x 10")
- 1 rectangle across the back, minus are of other rect (4" x 9") (12"-3"=9")
- 1 triangle across the front minus are of both rect (9" x 6") (12"-3"=9") (10"-4"=6")

Now these measurements are for external volumes, so we need to adjust accordingly. Generally outside walls require you to subtract that 3/4" an inch off.

Area of rec #1:
3" - 3/4" bottom - 0" top x 10" - 3/4" left - 3/4" right
2.25" x 8.5"
19.125 in^2

Area of rec #2:
4" - 3/4" top - 0" bottom x 9" - 3/4" left - 0" right
3.25" x 8.25"
26.813 in^2

Area of triangle:
1/2 * 6" - 3/4" left x 9" - 3/4" top
1/2 * 5.25" x 8.25"
21.656 in^2

Total area of the end is:
21 +27 + 19 = 67.594 in^2

Volume is: 67.594 x (30" - 3/4" end - 3/4" other end)
1926 in^3
1926/12/12/12 = 1.115 cu ft of airspace

 
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Basically, a box like this is a rectangular box with one corner lopped off. So the volume is the volume of the full rectangle (or whatever it's called in 3D) minus the volume of the part that is lopped off.

The part that is lopped off is just a right triangle. That is, it is half of a rectangle. So to get the volume of the part you lop off, you figure out what the volume of that rectangle is. You can figure out the length of the sides of that triangular piece by taking the length of that short little right side and subtracting it from the length of the left side (of the whole thing), and doing the same for the top and bottom.

Now you've got the length of the sides. The area of the triangle is the two lengths multiplied together, divided by two (and the volume is that times the side we can't see in the pic).

This would be much easier to explain with diagrams, but I'm lazy and don't feel like making GIFs... hopefully it all makes sense.

 
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Ya that is an easier way too.

So take the whole square and subtract off the triangle piece.

Saves some steps.

 
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seeeee!!!! its funny how if you aren't in the frame of mind to do something (earlier when I posted this and I was at work) you miss the most obvious thing...like the subtracting the triangle's area...

well, I dont feel like a complete retard, so thats ok...lol