Order enough concrete without paying for a second truck.

Multiply length, width and depth in feet to get cubic feet, then divide by 27. The trap is mixing units: a footing 20 feet long, 24 inches wide and 12 inches deep is 20 × 2 × 1 = 40 cubic feet, which is 1.48 cubic yards, not 480 of anything. Ready-mix is ordered in quarter-yard steps and almost always carries a short-load charge below about 3 yards, so round up rather than risk a second truck.

An 80 lb bag yields about 0.60 cubic feet, so a cubic yard takes 45 bags — roughly 3,600 pounds you have to carry, open and mix. Below about a quarter yard (12 bags) bag mixing is clearly right. Above half a yard it becomes a long, hot day with an uneven result, because bag-to-bag water content varies and cold joints form while you mix the next batch. Anything structural and continuous should be one pour from one source.

If you have a geotechnical report, use its number and nothing else. Without one, IBC Table 1806.2 gives presumptive values: 1,500 psf for clay and silty clay, 2,000 psf for sandy silt and clayey sand, 3,000 psf for sandy gravel and gravel, and 12,000 psf for crystalline bedrock. This calculator defaults to 2,000 psf. Note that presumptive values assume undisturbed natural soil — engineered fill, organic soil and expansive clay all fall outside the table and need testing.

For bearing, divide the service load by the allowable soil pressure and take the square root for a square footing. A 40,000 lb column on 2,000 psf soil needs 20 square feet, so about 4 ft 6 in square. That is only the bearing check: the footing also has to be thick enough to resist punching shear around the column, and reinforced for the bending it sees as a cantilever off the column face. Those two usually set the thickness, and they are engineer territory.

The traditional first pass is span over twelve for depth and half of that for width. In imperial units it is memorable: depth in inches equals the clear span in feet, so a 20 ft span starts at a 20 by 10 in beam. Treat that as a placeholder for coordination, not a design. Deflection limits, concentrated loads, continuity over supports and the space the reinforcement actually needs all push the real number around, sometimes by a lot.

As a budgeting figure, a solid reinforced slab carries 2.0 to 3.1 pounds of steel per square foot, a joist-and-block slab 1.4 to 2.0, and a lightweight or voided slab 1.6 to 2.5. A 400 square foot solid slab therefore lands between 820 and 1,230 pounds of steel. Add 5 to 10% for laps and offcuts. These ratios are useful for pricing and for checking that a supplier quote is in the right universe; they are not a bar schedule.

A modular clay brick with standard 3/8 inch joints gives 6.86 units per square foot of single-wythe wall — the figure the Brick Industry Association publishes and every US supplier quotes. Queen size gives 5.76, utility brick 3.0, and an 8 by 8 by 16 inch concrete block 1.125. A double-wythe solid brick wall is two leaves, so about 13.7 per square foot of face. Add 5 to 10% for breakage and cuts on top of any of them.

About 7 cubic feet of mortar per thousand modular brick with standard joints, which is roughly 875 pounds of wet mortar. Concrete block takes less per square foot of wall because there are fewer, larger joints, but the joints themselves are thicker. Waste is high with mortar — what boards up, what stiffens past working time, and what falls — so 10 to 15% over the theoretical figure is normal on a real wall.

A single wythe of modular brick runs about 35 to 40 pounds per square foot of face including mortar; a double-wythe wall is over 60; an 8 inch hollow concrete block wall about 38 to 55 depending on whether the cores are grouted. That number matters twice: the footing has to carry it, and on a retrofit the existing structure may not have been sized for it. Swapping a stud wall for a masonry one without checking the foundation is a classic and expensive mistake.

Almost always for anything structural. Footings, beams, retaining walls and load-bearing masonry are permitted work in essentially every US jurisdiction, and most require sealed drawings from a licensed engineer or architect. The inspection sequence matters too — footing excavations and reinforcement are typically inspected before the pour, and concrete placed before that inspection can be ordered removed. Check with the local building department before excavating, not after.

The conversions used here are: 1 cubic meter is 35.31 cubic feet or 1.308 cubic yards; concrete at 2,400 kg per cubic meter is 150 pounds per cubic foot; 1 kg per square meter of reinforcement is 0.2048 pounds per square foot; and 150 kN per square meter of soil bearing is 3,133 psf. Where a metric source constant and a published US figure disagree — as they do slightly on bricks per unit area — this calculator uses the US figure and shows the metric one alongside it.

Because the underlying rule of thumb this calculator ports assumes ten 50 kg sacks of cement per cubic meter, which is 500 kg per cubic meter or 31.2 pounds per cubic foot. That is a rich site mix aimed at small hand-batched footings where compaction is poor. Structural ready-mix is typically proportioned at 300 to 350 kg per cubic meter and reaches higher strength with less cement because it is properly graded, dosed and vibrated. If you are ordering ready-mix, specify a strength class and let the plant proportion it.