Making New Box

[1976 Silverado]

Well the measurements are 62.5'' in length 7'' on top at 13.5'' tall 11.5'' for bottom since its a truck shape box since its going into my single cab. I just need help figuring out the cft cause i cant do it lol idk why but when i do the trapazoid formula for area it doesnt add up. at 16'' high it can only be 5.5'' until 19'' high where it can be 6'' in length hope you guys understand.. if you need pictures so you can try to understand let me know. also it will be 24'' tall and will sit level with the back window (ill still be able to see out the window)

[1976 Silverado]

Well the measurements are 62.5'' in length 7'' on top at 13.5'' tall 11.5'' for bottom since its a truck shape box since its going into my single cab. I just need help figuring out the cft cause i cant do it lol idk why but when i do the trapazoid formula for area it doesnt add up. at 16'' high it can only be 5.5'' until 19'' high where it can be 6'' in length hope you guys understand.. if you need pictures so you can try to understand let me know. also it will be 24'' tall and will sit level with the back window (ill still be able to see out the window)

since area = 1/2h(a+B.) that means 1/2h would = 12 and a+b = 16.5 so 16.5x12=198? that cant be right.. then i x it by 62.5 since thats the length and get 12375 then / that by 1728 = 7.1cft?.. doesnt seem as if that COULD be right.. any help would be GREATLY appreciated!

[1976 Silverado]

anyone have any ideas at all? I want to start building asap but dont have much money to spend and need to know exactly how much cft i have before i even start building

[basshead4life]

its about 4.5 cubes before displacement. u need to use 2 formulas because your box is 2 shapes like this:

theres a rectangle and a triangle

so u do the rect formula + the triangle formula and then u get 4.5 cubes.