Footings will rarely follow the drawing exactly. In rocky soil the footings may collapse when big rocks are excavated.

This was supposed to be a 12"*12" footing, but note how the left side of the footing has collapsed. Calculate the true width.

The excavator may have dug too deep, or it may have rained and footings needed to be dug out deeper to reach firm soil. So it is important to check a variety of spots on your footing and get an average size. Then use the calculator to figure the amount of concrete needed.

House slabs on grade that are 8" out of grade with a 4" slab also have some of the footing above the grade too.

This 12" x 12" footing needs to be calculated 12" x 16" so the footing is figured to go above grade to reach the 4" slab thickness.

Calculating the amount of Concrete Needed For Slabs
(including odd shapes)

Rule of thumb: add ¼" to the thickness of your slab for your slab concrete budget. This assumes you have the job evenly graded to the right depth, and the grade is well compacted.

If you check your grade and one spot is 4", some spots are 4.5" to 5"-the best solution for both the job quality and your concrete budget is to fix the grade. If you don’t fix it, you have to come up with the average depth-them add your ¼" to that thickness.

Odd shapes: Transform odd shapes into rectangles and odd shapes are suddenly easy to figure.


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