A foundation cost calculator, in the contractor's hands, is the concrete calculator applied to each part of the foundation and priced from the contractor's own figures: the footings and stem walls or the slab as cubic yards at the supplier's price per yard, the excavation and backfill at the contractor's rate, the forms, the rebar, the anchor bolts and the drainage as material lines, and the labor at the crew's rate. The owner's version of the same phrase, what a foundation costs per square foot in their county, is a different question for a different person; this page is the contractor's foundation estimate, worked on a 30 by 40 foot footprint. The free concrete calculator on this site does the yards for each part from your dimensions and your supplier's price, with no account.
The concrete foundation cost calculator: footings and stem walls as yards
Footings are long thin slabs: a 30 by 40 foot footprint has 140 feet of perimeter, and a footing 2 feet wide and 1 foot deep is 140 by 2 by 1, 280 cubic feet, 10.4 yards, 11.4 with a 10% allowance. Stem walls are the same sum stood up: 140 feet long, 3 feet high and 8 inches thick is 140 times 3 times 0.667, 280 cubic feet, another 10.4 yards. The concrete for the footings and walls is therefore about 22.8 yards with waste, at the supplier's price per yard; at $150 that is $3,420 of concrete before anything else. The rules on formwork and placing are at 29 CFR 1926 Subpart Q; the yards are the calculator's.
The slab foundation cost calculator: the slab on grade
A slab foundation is the slab worked at its thickness plus the thickened edges worked as footings. The 30 by 40 foot slab at 4 inches is 1,200 times 0.333, 400 cubic feet, 14.8 yards; the thickened edge, 140 feet at 1 foot wide by 1 foot deep beyond the slab's thickness, is another 140 cubic feet, 5.2 yards; together 20 yards, 22 with the 10% allowance, at the supplier's price per yard. The gravel base under the slab is the aggregate sum at its depth, the vapor barrier and the mesh or rebar are material lines, and the finishing is the contractor's own rate for the square footage and the finish specified.
Foundation estimates: the lines beyond the concrete
Excavation and backfill at the contractor's rate per cubic yard moved or per hour of machine; forms, either rented or built, at their cost; rebar by the pound or the foot and anchor bolts by the piece at the yard's prices; the drainage, damp-proofing and insulation the design calls for; the inspections the code requires, published by the International Code Council linked below; and the crew's labor for each stage at the contractor's own rate. Each is a line the owner can read, and the foundation estimate is their sum marked up at the contractor's overhead and profit. A foundation cost calculator that returns one figure per square foot has hidden every one of them.
From the foundation estimate to the schedule of values
The foundation's estimate becomes the foundation's line on the schedule of values when the contract is signed: one value for footings, one for walls or slab, or one for the foundation as a whole, as the owner's form requires. The draw then bills the share complete each period with retainage held, so the estimate's figure and the draw's figure are the same number at different stages. The free construction cost estimator on this site carries the foundation at the contractor's own cost with site work, contingency and overhead and profit shown, and the schedule-of-values draw on this site works the period's payment from it.
Questions people ask about foundation cost calculator
How does a foundation cost calculator work for a contractor?
Each part as cubic yards from its dimensions, times the supplier's price per yard, plus excavation, forms, steel, drainage and labor at the contractor's own rates, marked up at overhead and profit. A 30 by 40 foot footing and stem wall run is about 22.8 yards with waste.
How is a slab foundation calculated differently?
The slab at its thickness plus the thickened edges worked as footings: a 30 by 40 slab at 4 inches with a 1 by 1 foot edge is about 20 yards, 22 with waste, plus the base, the barrier, the steel and the finish.
Why is the owner's foundation cost per square foot a different question?
Because it averages every contractor's price in a county over every design, and the contractor estimating a specific foundation needs their own yards, their supplier's price and their own rates, which this page works.