Calculating Cubic Feet

Calculating cubic feet, or volume is necessary for estimating concrete or attic ventilation. The basic formula for calculating cubic feet (CFT) is:


To estimate the cubic feet in a sidewalk you first convert all measurements to feet. A sidewalk that is 30″ wide by 4″ high and 12′ long would then read:

2.5′ X .334′ X 12′

Then multiply the LxWxH:

2.5 X .334 X 12 = 10.02 CFT

To estimate the number of bags of concrete required for this job divide the total cubic feet by the coverage per bag. Most 80# bags of readi-mix concrete cover about 2/3 (.667) of a cubic foot.

10.02 / .667 = 15 bags

Attic volume is calculated the same way – LxWxH. When the roof pitch is the same on both sides of the roof you can multiply the length of the roof section times half the total (building) width times the height of the roof. This works because the two halves make a rectangular whole.

For a roof section 42′ long by 28′ wide by 6.5′ high, the calculation would be:

42 X 14 X 6.5 = 3,822 CFT

Then divide the total cubic feet by the coverage per vent unit to get the total number of vents required.