Estimate how much concrete you need for a floor slab before you order. Enter the floor dimensions and thickness, add a practical waste allowance, compare cubic feet, cubic yards and cubic metres, and build a quick material-only concrete budget. The concrete floor calculator supports rectangular floors, square floors, round floors and area-only calculations for garages, basements, sheds, workshops, patios and other slab-on-ground projects.
Select the floor type that matches your measurements. All numeric entries start at 0. The calculator shows the result only after you enter valid dimensions and press Calculate Concrete Floor.
Your result will appear here after you press Calculate Concrete Floor.
The PDF includes the floor type, dimensions, floor area, base volume, waste allowance, rounded order quantity, cubic metres, optional bag estimate and concrete-only cost.
A floor starts as square footage, but concrete is a volume. The slab thickness adds the third dimension. Convert inches to feet, multiply length × width × thickness, then divide cubic feet by 27 to get cubic yards.
The calculator separates floor area, exact geometric volume, waste allowance, rounded order quantity, optional bag quantity and an editable concrete-only cost estimate.
See the slab surface area before thickness is applied.
Convert the floor dimensions into the volume commonly used for ready-mix orders.
Add a planning allowance for small site and measurement differences.
See the same concrete floor quantity in metric volume.
Enter a local material rate per cubic yard for a quick budget estimate.
Save the dimensions and main result for project planning or quote comparison.
A concrete floor calculation starts with three measurements: length, width and thickness. Length and width tell you the surface area of the floor, while thickness turns that area into a three-dimensional volume. For U.S. projects, ready-mix concrete is commonly estimated in cubic yards, so the most useful workflow is to calculate cubic feet first and then divide by 27.
The basic formula is simple, but accurate measurements matter. A 4-inch slab is not the same volume as a 5-inch slab, and a floor that is slightly larger than the plan can require noticeably more material. Concrete floors may also include thickened edges, turned-down beams, pads, steps or separate sections that should be calculated individually rather than hidden inside a rough percentage.
Cubic Yards = Length (ft) × Width (ft) × [Thickness (in) ÷ 12] ÷ 27
Measure the inside dimensions of the area that will actually be filled with concrete. For a simple rectangular garage, workshop, basement or shed floor, multiply length by width to get square feet. A 20 ft × 15 ft floor has an area of 300 ft².
Concrete floor thickness is usually described in inches, while length and width are commonly measured in feet. Divide inches by 12 before multiplying. A 4-inch thickness is 0.3333 ft. A 5-inch thickness is about 0.4167 ft. A 6-inch thickness is 0.50 ft.
| Floor Thickness | Thickness In Feet | Concrete Per 100 ft² | Concrete Per 500 ft² |
|---|---|---|---|
| 3 in | 0.2500 ft | 0.926 yd³ | 4.630 yd³ |
| 4 in | 0.3333 ft | 1.235 yd³ | 6.173 yd³ |
| 5 in | 0.4167 ft | 1.543 yd³ | 7.716 yd³ |
| 6 in | 0.5000 ft | 1.852 yd³ | 9.259 yd³ |
| 8 in | 0.6667 ft | 2.469 yd³ | 12.346 yd³ |
One cubic yard contains 27 cubic feet. After multiplying floor length, width and thickness in feet, divide the cubic-foot result by 27. For a 20 ft × 15 ft floor at 4 inches thick, the exact volume is 100 cubic feet. Divide 100 by 27 and the result is approximately 3.704 cubic yards before any waste allowance.
20 × 15 × (4 ÷ 12) = 100 ft³ → 100 ÷ 27 = 3.704 yd³
There is no single waste percentage that is correct for every concrete floor. Carefully built forms over a consistent compacted base can be measured more accurately than an irregular excavation. Small losses can also occur from spillage, low spots, over-excavation, uneven subgrade or concrete left in placing equipment.
The calculator lets you enter your own waste or contingency percentage instead of forcing a fixed number. Recheck the forms and prepared base before ordering, then use an allowance that matches the actual condition of the project and the ordering rules of your supplier.
Thickness has a direct linear effect on concrete volume. A 6-inch floor contains 50% more concrete than a 4-inch floor over the same area because 6 inches is 1.5 times 4 inches. That does not mean one thickness is structurally better for every project; it simply explains how strongly thickness changes the quantity.
The calculator does not select the correct slab thickness. A residential basement, garage, warehouse, industrial floor, equipment pad or structural slab can have very different loading and design requirements. Use the thickness specified in the project documents or by the appropriate qualified professional.
If you already know the total square footage, choose the Area Only option. Multiply square feet by thickness in feet to get cubic feet, then divide by 27 for cubic yards. This is useful when architectural plans already state the net floor area or when several small rectangles have already been combined into one square-foot total.
For example, 1,000 ft² at 4 inches thick is approximately 12.346 yd³ before waste. At 5 inches thick, the same 1,000 ft² becomes approximately 15.432 yd³. At 6 inches thick, it becomes approximately 18.519 yd³.
| Floor Area | 4 in Thick | 5 in Thick | 6 in Thick | 8 in Thick |
|---|---|---|---|---|
| 100 ft² | 1.235 yd³ | 1.543 yd³ | 1.852 yd³ | 2.469 yd³ |
| 250 ft² | 3.086 yd³ | 3.858 yd³ | 4.630 yd³ | 6.173 yd³ |
| 500 ft² | 6.173 yd³ | 7.716 yd³ | 9.259 yd³ | 12.346 yd³ |
| 750 ft² | 9.259 yd³ | 11.574 yd³ | 13.889 yd³ | 18.519 yd³ |
| 1,000 ft² | 12.346 yd³ | 15.432 yd³ | 18.519 yd³ | 24.691 yd³ |
A garage slab is usually easy to estimate geometrically because it is often rectangular. Measure the actual slab length and width, enter the specified thickness and calculate the base volume. If the garage design has a thickened perimeter, foundation wall, grade beam or isolated footing, calculate those elements separately and add them to the slab volume.
Do not assume a particular thickness just because another garage used it. Vehicle loads, soil conditions, reinforcement, local practice and structural design can all affect the slab specification. The calculator is a quantity tool, not a design tool.
Basement floors can include columns, sumps, drains, recessed areas, steps and irregular wall lines. For a simple basement, use the rectangle or area-only option. For an irregular basement, divide the layout into smaller rectangles, calculate each area and add the results. Openings that will not be filled with concrete can be deducted where appropriate.
Small workshops, sheds and utility buildings often use slab-on-ground floors, but the intended loads can differ significantly. A lightweight storage shed and a workshop carrying vehicles or heavy machinery are not equivalent design cases. Enter the specified floor thickness rather than selecting one from a generic chart.
A circular floor uses the area of a circle. Divide the diameter by two to get the radius, calculate π × radius², then multiply by thickness in feet. The calculator handles this automatically and can also multiply the result when several identical round floors or pads are required.
Cubic Yards = π × Radius² × [Thickness (in) ÷ 12] × Quantity ÷ 27
A square floor is simply a rectangle with equal length and width. The dedicated square option reduces the number of fields you have to enter. Multiply the side length by itself, multiply by thickness in feet, then divide the resulting cubic feet by 27.
Bagged concrete can be practical for small floors, patches and projects where a ready-mix truck is not suitable. To estimate bags, you need the manufacturer's finished yield per bag. Enter that yield in cubic feet in the optional bag field. The calculator divides the concrete volume after your waste allowance by the stated bag yield and rounds up to whole bags.
For a larger floor, the bag count can become extremely high. If the quantity is substantial, compare bagged concrete with ready-mix based on material price, delivery access, mixer capacity, labor, working time and the need to place the slab continuously. Use our concrete bags to cubic yards calculator when you already know how many bags you have and want to see their total volume.
The calculator includes a concrete rate per cubic yard. Multiply the rounded planning quantity by your entered rate to get a concrete-only material estimate. This figure is useful for comparing supplier quotes, but it is not the complete project price.
A finished concrete floor can also require excavation, grading, compacted base material, vapor barrier, insulation, reinforcement, dowels, formwork, pumping, placing, screeding, finishing, curing, saw cutting, sealers, testing and labor. Delivery or short-load fees may also be separate from the quoted price per cubic yard.
Suppose the planning quantity after waste and order rounding is 4.2 yd³ and the concrete rate is $180 per cubic yard. The concrete-only material estimate is $756. That does not include any of the site preparation, reinforcement, labor, pump or finishing costs mentioned above.
This concrete floor calculator estimates material volume. It does not determine bearing capacity, slab thickness, reinforcement, fiber dosage, concrete strength, joint spacing, vapor barrier, insulation, curing, subbase thickness, frost protection or structural performance. Those decisions must come from the applicable plans, specifications, local requirements or qualified professionals.
For related quantity planning, use the concrete slab calculator, concrete area calculator, cubic feet to cubic yards converter and concrete cost calculator.
The calculator tells you how much concrete the dimensions represent. Structural and construction requirements still need to come from the project specification.
Surface area tells you how large the floor is, but not how much concrete it contains.
Thickness creates the third dimension and strongly affects the total volume.
Cubic yards make it easier to compare the result with ready-mix supplier quotes.
Strength, reinforcement, joints and subbase are separate design decisions.
The calculator estimates concrete material only. A completed floor can involve several additional items that are not included in a simple $/yd³ number.
These examples show geometry only. Use the floor thickness and construction specification required for your actual project.
| Floor Example | Dimensions | Thickness | Base Volume | With 10% Allowance |
|---|---|---|---|---|
| Small Utility Floor | 10 ft × 10 ft | 4 in | 1.235 yd³ | 1.358 yd³ |
| Single Garage Example | 12 ft × 20 ft | 4 in | 2.963 yd³ | 3.259 yd³ |
| Workshop Example | 20 ft × 30 ft | 5 in | 9.259 yd³ | 10.185 yd³ |
| Large Floor Example | 30 ft × 40 ft | 6 in | 22.222 yd³ | 24.444 yd³ |
| Round Floor Example | 16 ft Diameter | 4 in | 2.482 yd³ | 2.730 yd³ |
Simple answers about floor thickness, square footage, cubic yards, bagged concrete, waste and cost.
For a rectangular floor, multiply length by width by thickness after converting all measurements to the same unit. In feet and inches, convert thickness to feet, calculate cubic feet, then divide by 27 to get cubic yards.
At 4 inches thick, a 10 ft by 10 ft floor has about 33.33 cubic feet of concrete, which is about 1.235 cubic yards before any waste allowance.
Thickness depends on the intended use, loads, subgrade, reinforcement and project design. The calculator does not choose structural thickness; use the project drawings, local requirements or qualified design guidance.
Yes. You can enter a waste or contingency percentage. The result separates the exact geometric volume from the volume after the allowance is added.
Yes. Use the rectangular floor option for a simple garage slab. Enter the inside length, width and specified thickness, then add any thickened edges or grade beams separately if they are part of the design.
Yes. Enter the measured floor dimensions and the specified slab thickness. Irregular basement layouts can be split into multiple rectangular areas and added together.
Yes. Choose the round floor option, enter the diameter and thickness, and the calculator will use the circle area formula multiplied by the slab depth.
Square feet measure surface area. Cubic yards measure volume. A concrete floor needs a thickness as well as an area, so square footage alone is not enough to calculate the amount of concrete.
The answer depends on the manufacturer's yield per bag. If an 80 lb bag yields 0.60 cubic foot, then 45 bags equal one cubic yard. Always use the exact published yield for the product.
Concrete is normally ordered in practical increments and real site dimensions are rarely perfect. The calculator can add a waste allowance and round the resulting order quantity upward to a selected increment.
Yes. Enter your local concrete rate per cubic yard and the calculator will multiply it by the planned order volume. Delivery, pumping, reinforcement, labor, finishing and other project costs are separate.
Yes. After calculating, use the PDF button to save the floor dimensions, waste allowance, base volume, planned order volume and material-only cost.
Use project specifications, current product information and authoritative concrete resources alongside any online floor estimate.
Technical resources, codes, standards and education for concrete design and construction.
Visit ACIIndustry education and ready-mixed concrete resources for contractors, producers and owners.
Visit NRMCAManufacturer quantity guidance for packaged concrete and common project shapes.
Visit QUIKRETE CalculatorWorkplace information about respirable crystalline silica hazards associated with concrete and masonry work.
Read OSHA Guidance