Concrete Slab & Footings Calculator (Cubic Yards & Bags)
Turn slab or footing dimensions into cubic yards of concrete and the number of 60 lb or 80 lb pre-mixed bags to buy, with a waste allowance included.
Formulas, units and sources
Slab volume (ft³) = Length (ft) × Width (ft) × Thickness (in) ÷ 12
Footing volume (ft³) = Run length (ft) × Width (in) ÷ 12 × Depth (in) ÷ 12
Total volume = Net volume × Number of pours × (1 + Waste % ÷ 100)
Cubic yards = Total ft³ ÷ 27
Cubic meters = Cubic yards × 0.7646
Bags to buy = ceil(Total ft³ ÷ Yield per bag)
- Units: one cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet, the unit ready-mix suppliers in the US sell by.
- Bag yield: pre-mixed concrete bags state their yield on the label. Typical values are about 0.45 ft³ for a 60 lb bag and about 0.60 ft³ for an 80 lb bag. Brands differ, so the two yield fields are editable.
- Waste: the waste percentage is your own allowance, not a code value. Rough subgrade, spillage and forms that move all add volume.
- Limits: the tool assumes a rectangular slab with uniform thickness and a strip footing with a constant cross-section. It does not model sloped surfaces, thickened edges, piers, columns or voids. Round columns can be entered as separate calculations by hand.
How to estimate and pour a concrete slab or footing
Follow these steps with the Concrete Slab & Footings Calculator. Slab thickness, footing size, depth and reinforcement must come from your approved plans or local building department, not from this page.
- Get the required dimensions. Take thickness, footing width and depth from your plans, engineer or local building department. Building codes are adopted and amended locally, so confirm what applies to your address.
- Measure the area. Measure slab length and width in feet, or the total footing run along its centerline. Break L-shaped areas into rectangles and run each one as a separate pour.
- Enter dimensions and a waste allowance. Type the sizes into the calculator and keep 10% waste unless your site calls for more or less.
- Choose between bags and ready-mix. Compare the bag count with the cubic yard total. Many small pours are done with bags; larger volumes are usually ordered from a local ready-mix supplier by the cubic yard.
- Prepare the forms and base. Build the forms to the planned height, prepare and compact the base as your plans specify, and place any reinforcement shown on them before concrete arrives.
- Place, finish and cure. Place the concrete in the forms, level it with a screed board, finish the surface, and keep it damp or covered while it cures, following the product label instructions.
Concrete mix, mixing and finishing supplies (Home Depot & Amazon)
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Once you know your bag count, compare pre-mixed concrete and the tools for mixing and finishing it. Check the yield and weight printed on each bag against the values you entered above.
Pre-mixed concrete bags at The Home Depot
Browse 60 lb and 80 lb bagged concrete mixes. Compare the yield printed on each label with the bag yield fields in the calculator.
Bagged concrete mix on Amazon
Search bagged concrete and fast-setting mixes sold on Amazon. Confirm the bag weight and stated yield before you order.
Wheelbarrows and concrete mixers at The Home Depot
Hand mixing is slow beyond a few bags. See wheelbarrows and electric mixers that can help you place concrete before it starts to set.
Concrete floats, trowels and edgers on Amazon
Search hand tools for leveling and finishing a slab: magnesium floats, steel trowels, edgers and screed boards.
Stakes and form accessories at The Home Depot
Search stakes, form boards and fasteners for building the edge forms of a slab or footing.
Gloves, boots and knee pads on Amazon
Wet concrete is caustic to bare skin. Search waterproof gloves, rubber boots and knee pads for finishing work.
Frequently asked questions
How many cubic yards of concrete do I need for a slab?
Multiply length (ft) by width (ft) by thickness (in) divided by 12 to get cubic feet, then divide by 27. A 10 ft by 10 ft slab that is 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards before waste and 1.36 cubic yards with a 10% allowance.
How many 80 lb bags of concrete make a cubic yard?
A cubic yard is 27 cubic feet. If one 80 lb bag yields about 0.60 cubic feet, you need about 45 bags per cubic yard. With a 60 lb bag yielding about 0.45 cubic feet, you need about 60 bags. Check the yield on your bag, since brands vary.
Why add 10% to a concrete order?
Ground is rarely perfectly flat, forms can bow, and some concrete is lost to spillage. Running short mid-pour can leave a cold joint, so a waste allowance is a common planning habit. The percentage in this tool is editable, and you should adjust it to your site.
How do I calculate concrete for a footing?
Switch the calculator to strip footing mode and enter the total run length in feet, plus the footing width and depth in inches. For example, 100 ft of footing that is 16 inches wide and 8 inches deep is 88.9 cubic feet, about 3.3 cubic yards before waste.
Should I use bags or order ready-mix concrete?
Compare the two totals. Bags suit small pours because you can mix as you go, but the number of bags grows quickly with volume. For larger volumes, ask a local ready-mix supplier about delivery, minimum loads and pricing for your area.
How thick should a concrete slab be?
This calculator does not decide thickness. Required thickness, base preparation and reinforcement depend on the use of the slab, the soil and your local building department. Take those values from your plans or a qualified professional, then enter them here to get the volume.
Limits and project considerations
- Volume only: the tool converts dimensions to volume and bag counts. It gives no structural, code-compliance or engineering advice, and it does not check whether your dimensions are acceptable.
- Estimate, not a quote: real volume depends on subgrade, form accuracy and mix. Confirm quantities with your supplier before ordering.
- Safety: wet concrete can burn skin and eyes. Wear gloves, eye protection and a dust mask when emptying bags, and follow the label directions.