How to Calculate Concrete for a Slab
- Measure the surface. Multiply the slab’s length by its width. Measure inside the forms, in the same unit, and account for each separate rectangular section.
- Convert thickness. Divide inches by 12 to get feet. A 4-inch slab is exactly one-third of a foot thick. Do not round that fraction before calculating.
- Calculate and allow for waste. Multiply area by thickness, divide cubic feet by 27 for cubic yards, then apply your selected allowance.
Worked example: a 10 × 10 ft slab, 4 inches thick
The surface is 100 square feet. Multiplying by 4 ÷ 12 gives 33.3333 cubic feet, or 1.2345679 cubic yards. With 10% extra, the estimate becomes 36.6667 cubic feet or 1.36 cubic yards. At 0.60 cubic feet per 80 lb bag, 36.6667 ÷ 0.60 gives 61.1111 bags. Round up once, at the end, to 62 bags.
For several identical pads, enter their count under quantity. For different dimensions, calculate each pad separately and add the volumes before making a combined order. Do not add already rounded bag estimates if the mix will be shared between sections.
Concrete Slab Formula
Order ft³ = net ft³ × (1 + waste % ÷ 100)
Order yd³ = order ft³ ÷ 27
Bags = round up(order ft³ ÷ bag yield)
One cubic foot equals 0.028316846592 cubic meters. The unit switch converts your current measurements, so the same slab produces the same volume in either system. Square feet describe the surface; cubic yards describe the material filling that surface to a specified depth.
The default 10% allowance is adjustable, including zero. It is a planning assumption for spillage, uneven subgrade and measurement uncertainty. It is not a substitute for checking the actual excavation depth. Ready-mix suppliers may sell in increments larger than the two-decimal result shown here.
Standard Slab Thickness Chart
Use this thickness reference to compare material demand during early planning. The entries are starting points to discuss with your contractor, rather than a structural design. Soil conditions, reinforcement, loads, drainage and local requirements determine the finished specification.
| Use / planning scenario | Reference thickness | What to confirm |
|---|---|---|
| Patio or pedestrian walkway | 4 in | Subgrade and joint layout |
| Small shed pad | 4 in | Building loads and footing requirements |
| Passenger-car driveway | 4–6 in | Vehicle loads and soil support |
| Garage floor | 4–6 in | Local requirements and edge details |
| Heavy vehicle or equipment area | Project-specific | Obtain a designed thickness; compare 8 in only as a volume scenario |
Increasing thickness from 4 to 6 inches uses 50% more concrete for the same area. Increasing it from 4 to 8 inches doubles the volume. A small depth change across a large slab can materially change the order, so measure the prepared base at several points.
How Many Bags of Concrete Do I Need?
This calculator assumes yields of 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. These are approximate concrete-mix yields, not the dry bag’s physical volume. Check the chosen product before purchasing.
One cubic yard contains 27 cubic feet, equivalent to 45 bags yielding 0.60 cubic feet each, without waste. The following table excludes waste and rounds each bag count up using the unrounded volume.
| Slab size | Net yd³ | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 8 ft × 4 in | 0.40 | 18 | 24 |
| 10 × 10 ft × 4 in | 1.23 | 56 | 75 |
| 10 × 10 ft × 6 in | 1.85 | 84 | 112 |
| 12 × 12 ft × 4 in | 1.78 | 80 | 107 |
| 20 × 20 ft × 4 in | 4.94 | 223 | 297 |
Bag counts are alternatives: choose the column matching the size you plan to buy. For larger pours, compare the cost, delivery arrangements, mixing equipment and available working time for bags versus ready-mix. A volume calculator cannot determine which supply method is most practical for your crew.
Yield reference: QUIKRETE Concrete Mix product data. Actual yield depends on product and mixing instructions.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab at 4 inches thick needs 33.33 cubic feet before waste. Using an 80 lb bag yield of 0.60 cubic feet, buy 56 bags without waste or 62 bags with a 10% allowance. At 6 inches thick, the volume is 50 cubic feet and the 10% allowance requires 92 bags of 80 lb mix.
How do I calculate concrete for a slab?
Multiply length by width, then multiply by thickness expressed in the same unit. For feet and inches, divide thickness in inches by 12 first. Divide the resulting cubic feet by 27 for cubic yards. Apply your chosen waste allowance to that unrounded volume, then calculate bag counts and round them upward.
How thick should a concrete slab be?
The thickness depends on the intended load, supporting soil, slab construction and applicable local requirements. Four inches is a useful planning reference for many pedestrian slabs, but a calculator cannot establish a structural specification. Use a contractor’s or designer’s specified thickness in the form and compare volume estimates if you are considering different options.
Does a 4-inch concrete slab need rebar?
Slab thickness alone does not determine reinforcement requirements. Intended loads, soil movement, joint spacing and the relevant construction specification all matter. This calculator estimates concrete volume only and does not select reinforcement. Ask the project designer or local building authority to confirm the appropriate reinforcement before ordering steel or beginning the pour.
How much concrete do I need for a 4×8 slab?
A 4 × 8 ft slab at 4 inches thick contains 10.6667 cubic feet, or approximately 0.40 cubic yards. That is 18 bags of 80 lb mix without waste. Applying 10% to the original volume gives 11.7333 cubic feet, so the purchase estimate becomes 20 bags at a yield of 0.60 cubic feet per bag.
How much does a 20×20 concrete slab cost?
A 20 × 20 ft slab at 4 inches thick needs 4.9383 cubic yards before waste or 5.4321 cubic yards with 10% extra. Multiply the required order quantity by a current local supplier quote, then add delivery, minimum-load fees, reinforcement, preparation and labor. The volume estimate alone does not establish an installed slab price.