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Estimate volume, waste, bag count, formwork, reinforcement, and a simple purchase list for slabs, footings, walls, steps, or round columns.
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Concrete calculator guide
A concrete estimate starts with the space you are filling: length, width, and finished depth. This calculator converts the depth from inches to feet, multiplies the three dimensions to find cubic feet, adds your waste allowance, and shows the result in cubic yards. That sequence matters because ready-mix concrete is commonly ordered by the cubic yard, while the measurements on a tape measure are often a mix of feet and inches. Use the planned concrete depth, not the thickness of the gravel base below it. For a slab, measure the inside length and width of the pour area. For a footing or path, use the actual run of concrete. The shape selector also handles footings, walls, steps, and round columns, and optional fields add extra sections, formwork, reinforcement, and material prices. The result is a planning estimate, so confirm access, minimum delivery quantities, product yield, and site conditions with the supplier before placing an order.
Base volume (ft³) = length (ft) × width (ft) × depth (in) ÷ 12. Adjusted volume (yd³) = base volume (ft³) × (1 + waste ÷ 100) ÷ 27. Bags = adjusted volume (ft³) ÷ bag yield (ft³/bag), rounded up.
Worked example with the default slab: a 10 ft × 10 ft pour that is 4 in deep has a base volume of 10 × 10 × (4 ÷ 12) = 33.33 ft³, which is 33.33 ÷ 27 = 1.23 yd³ before waste. Adding the default 10% allowance gives 33.33 × 1.10 = 36.67 ft³, and 36.67 ÷ 27 = 1.36 yd³, the recommended order the calculator displays. The metric line shows 36.67 × 0.0283168 = 1.04 m³. The bag row divides the same 36.67 ft³ by the default 0.6 ft³ yield of an 80 lb bag: 36.67 ÷ 0.6 = 61.1, rounded up to 62 bags. Actual bag yield varies by product, so change that field to match the label.
Enter length and width in feet and depth in inches, as labeled. Four inches is 4 ÷ 12 = 0.333 ft. Do not multiply feet by inches without converting first. One cubic yard equals 27 cubic feet, and the calculator uses 0.0283168 cubic meters per cubic foot for the metric display. Bag yield is entered in cubic feet per bag, reinforcement in linear feet, and formwork in square feet.
The concrete calculator returns a quantity to buy, not a guarantee of coverage. Substrate condition, waste from cuts, spillage, and settling all push real consumption above the theoretical figure, which is why most trades order a margin on top. Check the coverage rate printed on the product you actually purchase, because it varies between brands and mixes. On this page the figure rests entirely on project shape, length / diameter, width / height, depth / thickness, step count, step run, step rise, additional rectangular sections, additional section length, additional section width, additional section depth, optional formwork area, formwork allowance, optional reinforcement length, reinforcement stock length, reinforcement allowance, waste allowance, 80 lb bag yield, ready-mix price, 80 lb bag price, delivery cost and material tax, so start there if the concrete calculator returns something you did not expect.
Waste covers small measurement errors, spillage, uneven forms, and site loss; it does not correct a wrong depth or an undersized form, so the single largest error on this page is entering the depth of the gravel base or the total excavation instead of the finished concrete thickness. Measure the finished pour, keep all dimensions in their labeled units, and round a purchase up rather than down when a shortage would stop the job. For steps and walls, remember the calculator treats them as planning shapes, not engineered designs.
Construction quantities are planning estimates. Confirm product yield, waste, site conditions, structural requirements, and local code with the supplier or qualified professional.
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How it works
Estimate volume, waste, bag count, formwork, reinforcement, and a simple purchase list for slabs, footings, walls, steps, or round columns. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Project shape = slab · Length / diameter = 10 ft · Width / height = 10 ft · Depth / thickness = 4 in · Step count = 3 · Step run = 1 ft → 1.36 yd³ (recommended slab volume including waste). Change an input above and this example updates with your numbers.
Common questions
At 4 inches deep, a 10 ft × 10 ft slab needs about 1.23 cubic yards before waste and about 1.36 cubic yards with the default 10% waste allowance. Enter your own depth and allowance for a closer estimate, then round the order up to the supplier’s minimum increment.
Multiply length by width by depth in feet to get cubic feet, apply any waste allowance, then divide by 27 because a cubic yard is 27 cubic feet. Convert inches to feet first: a 4-inch slab is 0.333 ft deep, not 4 ft.
A modest waste allowance covers uneven ground, spillage, and small measuring errors; 5–10% is a common planning range for simple slabs. The right percentage depends on the shape and site, so confirm the final quantity with the concrete supplier before the truck is dispatched.
Yes, but convert the inches to feet first. The calculator accepts feet for length and width and inches for depth, then performs that conversion for you before multiplying the three dimensions together.
Divide the adjusted cubic feet by the bag yield and round up. This page uses 0.6 ft³ per 80 lb bag as a sample yield, so 36.67 ft³ becomes 62 bags; check your product label because yields differ between mixes.