Stem Wall Foundation
Measure the total length of the wall using your architect scale set at the corresponding scale factor indicated on the building plan sheet.
Multiply the footing height by its width to find the cross sectional footing area. These dimensions will be noted on the plan sheet near the footing itself. It may be easier to calculate in inches, then convert to square feet by dividing the product by 144 (number of inches in a square foot). Repeat this process for the area of the wall above.
Add the products from Step 2 together to find the total cross sectional area of the foundation. You may have to break the wall up into sections if walls are at different heights.
Multiply the cross sectional foundation area by the length (length sections for different height walls) to calculate the total wall volume in cubic feet.
Divide the total volume of foundation walls by 27 (number of feet in a cubic yard) to find the total yards of concrete needed for the foundation.
Find the surface area of the foundation slab by multiplying its length by its width. For a slab with an irregular shape, you may want to break it up into more manageable sections and add together later. Also measure the total perimeter of the slab; this will give the length of the slab footing.
Multiply the slab's surface area by its depth (this will be noted on the plan sheet and is usually 4 or 6 inches) to find the total volume of the slab in cubic feet. For 6 inch slabs, simply multiply the area by 0.5 to calculate cubic foot volume. For 4 inch slabs, you can simply divide the total slab area by 3.
Multiply the length of the slab footing found in Step 1 by its cross sectional dimensions (these dimensions will be noted on the building plan sheet) to find the total footing volume in cubic feet.
Add the products of Steps 2 and 3 to find the total amount of concrete in cubic feet. Divide this amount by 27 (number of feet in a cubic yard) to find the total yards of concrete needed for the foundation.