How to calculate air space in a f/g box?

Im getting ready to build a fiberglass sub box but i need help calculating the air space i know how to on other sub boxes but now with the different shapes like a molded f/g box...any help would be nice...

look at it from the side and measure out the full area and make a scaled down drawling on a piece of paper. Break it down into shapes that can be measured and get as much of the space as possible into the shapes. Then do the same thing from the front and from the top to get your LxWxH and the add all the areas together. Depending on how much space cannot be calculated in this method add a .1cuft or 6 depending on how much untill you feet that its close. Its not a 100% accurate way to measure the space but it can get you close. Theres otherways like filling it with sand and measuring it with a cylender formula but those methods are messy and sand isn't light in bulk so its not always practical."Shiny chrome when used in conjunction with bikini models is particularly effective in inducing brain deficit disorder"

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Use the sand. Also, try searching for this post cause its been asked a lot.dream it, build it, fiberglass it

the easiest method that i have found is taking packing popcorn or packing foam.  You can usually buy big bags of it at office supply stores or any post office.  You can figure out how many cubic feet that you  need for the speaker and convert it from cubic feet to litres.  After that just take a juice jug or something with a litre measurement on it and start tapping that up and dumping the foam into the enclosure jug by jug until you have the required litres.  Remeber that 1 cubic foot of volume is equal to 28.32 litres.

Why would the packing popcorn method make a 5% +/- difference??

the way the packing peanuts settle

there are air gaps in packing materials

but there are airgaps also in the box with the peanuts in the box.  So wat is the difference between airgaps in the litre measuring jug....compared to air gaps in the box?

I would have to agree..... if you take a cardboard box made to 1 cu-ft....  fill it with the peanuts... then that amount of peanuts will alwyas be 1 cu-ft no matter how they settle.... the air gaps you get in the final measurment are present in the first measurment...