Figuring out a load of concrete for foundations, walls or slabs can seem like an endless math assignment. The following formulas are shortcuts to the conversions from square inches to cubic yards. For footings, multiply the cross section in square inches by .000257 to get the cubic yards in one lineal foot. Multiply that result by the length of the footing for your total. For example: Find the number of cubic yards of concrete in a footing 14 in. wide, 17 in. deep and 43 ft. long.
14 x 17 x .000257 x 43 = 2.63 cu. yd.
This formula also works for columns. Just think of them as vertical footings.
For walls and slabs, it’s a little different. Multiply the slab or wall area in square feet times the thickness in inches times .0031. For example: How many cubic yards of concrete are there in a wall 4 ft. high, 82 ft. long and 9 in. thick?
4 ft. x 82 ft. x 9 in. x .0031 = 9.15 cu. yd.
Charles Fockaert, Eureka, CA
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Not meaning to be too flippant, but if you can't do the math to estimate concrete volume, perhaps this is not your thing.
1 CY = 27 CF = 46,656 CI
1 ft -> 1/3 = 0.333 Yards
1 in -> 1/36 = 0.0278 Yards
Pay attention to units and the rest is easy.
I think it's just easier to to make the inches into fractions of feet i.e. (x inches/12) then divide by 27.
All these odd number conversions remind me of why I like the metric system where it's all in decimals.