Estimate how much concrete a slab may need before you buy materials or order ready-mix. Enter slab length, width and thickness, calculate from known square footage, multiply identical slabs or enter a known volume, then add a practical waste allowance and review cubic yards, cubic feet, cubic metres and an optional concrete-only cost.
Select the input method that matches the information you already have. All numeric entries start at zero so the result is based entirely on your own project measurements.
Your result will appear after you press Calculate Slab Concrete.
The PDF includes the slab input method, measurements, waste allowance, base cubic yards, rounded cubic-yard quantity, cubic feet, cubic metres and concrete-only cost.
A concrete slab needs three dimensions. The thickness is converted from inches to feet, then length × width × thickness gives cubic feet. Divide by 27 to convert cubic feet to cubic yards.
The tool keeps slab area, thickness, volume, waste allowance, order rounding and material cost separate so each number is easy to check.
Calculate from length × width or enter known square footage directly.
Use the required thickness from your project documents or design.
See the main ready-mix planning quantity in cubic yards.
Compare the same slab volume in metric units.
Add your own percentage for real site variation and small placing losses.
Enter your own rate per cubic yard for a simple material-only estimate.
A Concrete Slab Calculator turns slab length, width and thickness into a three-dimensional concrete volume. The most common mistake in slab estimating is to stop at square footage. Square feet describe the surface area, but concrete is ordered by volume, so thickness must be included before the quantity can be converted to cubic yards or cubic metres.
This calculator gives you four ways to start. Use Length × Width when you know the slab dimensions, Area + Thickness when you already know the square footage, Multiple Slabs when several identical slabs share the same dimensions, or Known Volume when another drawing or calculator has already provided the concrete quantity.
Cubic Yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27
For example, a 10 ft × 10 ft slab at 4 inches thick contains 33.33 cubic feet. Divide that result by 27 and the base volume is approximately 1.23 cubic yards. If you add a 10% allowance, the planning quantity becomes about 1.36 cubic yards before final order rounding.
Thickness affects every square foot of the slab. Increasing a 1,000 ft² slab from 4 inches to 5 inches adds one extra inch of concrete over the entire area. That increases the base volume from about 12.35 yd³ to about 15.43 yd³, a difference of more than 3 cubic yards.
| Slab Area | 4 Inches | 5 Inches | 6 Inches | 8 Inches |
|---|---|---|---|---|
| 50 ft² | 0.62 yd³ | 0.77 yd³ | 0.93 yd³ | 1.23 yd³ |
| 100 ft² | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ | 2.47 yd³ |
| 250 ft² | 3.09 yd³ | 3.86 yd³ | 4.63 yd³ | 6.17 yd³ |
| 500 ft² | 6.17 yd³ | 7.72 yd³ | 9.26 yd³ | 12.35 yd³ |
| 1,000 ft² | 12.35 yd³ | 15.43 yd³ | 18.52 yd³ | 24.69 yd³ |
If you already know the slab area, select Area + Thickness. Enter square feet and thickness in inches. The calculator converts the thickness to feet, multiplies by the area to find cubic feet, then converts the result to cubic yards.
Cubic Yards = Slab Area (ft²) × Thickness (in) ÷ 12 ÷ 27
At 4 inches thick, 100 ft² needs about 1.23 yd³ before waste. At 5 inches it needs about 1.54 yd³. At 6 inches it needs about 1.85 yd³. The correct thickness should come from the project design rather than from a generic online recommendation.
At 4 inches thick, 500 ft² needs about 6.17 yd³ before waste. At 5 inches, approximately 7.72 yd³. At 6 inches, approximately 9.26 yd³. Large areas make even small thickness changes important, so check the design and measure the prepared base carefully.
Use Multiple Slabs when several identical pads or slabs have the same length, width and thickness. The calculator works out one slab and multiplies it by the quantity. If the slabs have different sizes, calculate each one separately instead of using an average.
Known Volume is useful when a drawing, takeoff or another calculator already gives you concrete quantity. Enter cubic yards, cubic feet or cubic metres and the calculator converts that number into cubic yards before applying waste and order rounding.
There is no single waste percentage that is correct for every slab. Carefully installed forms over a level prepared base can be measured relatively accurately. Uneven excavation, low spots, form movement and placing losses can increase the actual quantity.
The calculator leaves waste at zero by default so you make the decision deliberately. A small planning allowance may be appropriate where site measurements are not perfectly predictable, but it should not replace proper measurement.
| Base Volume | 5% Allowance | 10% Allowance | 15% Allowance |
|---|---|---|---|
| 1.00 yd³ | 1.05 yd³ | 1.10 yd³ | 1.15 yd³ |
| 2.00 yd³ | 2.10 yd³ | 2.20 yd³ | 2.30 yd³ |
| 5.00 yd³ | 5.25 yd³ | 5.50 yd³ | 5.75 yd³ |
| 10.00 yd³ | 10.50 yd³ | 11.00 yd³ | 11.50 yd³ |
The price field lets you enter your own concrete rate per cubic yard. The calculator multiplies the volume with waste by that rate to create a concrete-only material estimate.
Concrete-Only Cost = Cubic Yards With Waste × Price Per Cubic Yard
This does not automatically include delivery, short-load fees, waiting time, pump hire, reinforcement, base preparation, formwork, labor or finishing. Use the Concrete Cost Calculator for broader budgeting and the Concrete Order Calculator when supplier minimums and truck loads are important.
| Slab Example | Dimensions | Base Cubic Yards | With 5% | With 10% |
|---|---|---|---|---|
| Small Pad | 8 ft × 8 ft × 4 in | 0.79 yd³ | 0.83 yd³ | 0.87 yd³ |
| 10 × 10 Slab | 10 ft × 10 ft × 4 in | 1.23 yd³ | 1.30 yd³ | 1.36 yd³ |
| Patio | 12 ft × 16 ft × 4 in | 2.37 yd³ | 2.49 yd³ | 2.61 yd³ |
| Garage Slab | 20 ft × 20 ft × 5 in | 6.17 yd³ | 6.48 yd³ | 6.79 yd³ |
| Larger Slab | 30 ft × 40 ft × 6 in | 22.22 yd³ | 23.33 yd³ | 24.44 yd³ |
Concrete slabs are used for many different projects, but the volume calculation follows the same principle: area multiplied by thickness. What changes from project to project is the required structural design, site preparation and how accurately the finished space can be measured before the pour.
A rectangular patio is straightforward to measure. Use the inside dimensions of the forms and the required slab thickness. If the patio includes curves or separate sections, divide it into simple shapes and calculate each volume individually. This is more accurate than averaging a complicated outline.
Garage slabs can be large enough that small errors matter. A one-inch difference in thickness across hundreds of square feet can add several cubic yards. Use the project design for thickness and remember that perimeter thickenings, grade beams or local load-bearing sections can require extra concrete beyond the main slab field.
Driveways may contain several widths, aprons or turning areas. Calculate each section separately where dimensions differ. A dedicated Driveway Concrete Calculator can also help when the project is primarily a driveway rather than a general slab.
Small pads can often be measured very accurately after forms are set. This makes them good candidates for a low uncertainty estimate, but you should still check the base level and actual depth in several places. Do not reduce required thickness simply because the volume result seems high.
Cubic feet are the intermediate imperial volume used before converting to cubic yards. If a slab measures 12 ft × 12 ft × 4 in, its thickness is 4 ÷ 12 = 0.3333 ft. The volume is 12 × 12 × 0.3333 = 48 cubic feet. Dividing by 27 gives approximately 1.78 cubic yards.
The calculator also shows cubic metres. One cubic yard is approximately 0.7646 m³. This is useful when a project mixes imperial dimensions with metric product information or when you want to compare a U.S. ready-mix quantity with a metric reference.
| Cubic Yards | Cubic Feet | Approx. Cubic Metres |
|---|---|---|
| 0.50 yd³ | 13.5 ft³ | 0.38 m³ |
| 1.00 yd³ | 27 ft³ | 0.76 m³ |
| 2.00 yd³ | 54 ft³ | 1.53 m³ |
| 5.00 yd³ | 135 ft³ | 3.82 m³ |
| 10.00 yd³ | 270 ft³ | 7.65 m³ |
| 20.00 yd³ | 540 ft³ | 15.29 m³ |
A slab may be 4 or 5 inches thick across most of its area but significantly deeper around the perimeter. If the project includes a thickened edge, calculate the extra edge volume separately rather than treating the entire slab as the deeper thickness.
Integrated beams or ribs can add substantial volume. Use the beam dimensions from the project documents and calculate each additional rectangular section. Avoid double-counting the slab thickness that is already included in the main slab volume.
If the slab intentionally changes thickness, calculate separate zones where possible. For a gradual slope, an average thickness may be used for rough planning only when that approach is appropriate to the geometry. Complex structural shapes should be taken from project drawings rather than simplified online assumptions.
The Concrete Yard Calculator handles several project shapes and focuses broadly on cubic yards. This slab page is more focused on flat slab work and includes area-based and multiple-slab inputs. Use whichever tool matches the information you have.
Small pads and repairs may be completed with packaged concrete. If the cubic-yard quantity is small and you want to convert it into whole bags, use the Concrete Bags Needed Calculator and enter the manufacturer-stated finished yield for the exact product.
Calculate once during budgeting, again after dimensions are confirmed, and once more after the base and formwork are ready. The final calculation should use the best available measurements immediately before ordering concrete.
For U.S. concrete technical education and industry resources, visit the American Concrete Institute. For workplace information about respirable crystalline silica, review OSHA silica guidance. Project-specific work should always follow applicable plans, specifications, local requirements and supplier instructions.
The calculator estimates concrete-only cost from the rate you enter, while a complete slab budget can include many other items.
Use related calculators to check cubic yards, supplier ordering and packaged concrete options.
Answers to common questions about slab square footage, thickness, cubic yards, multiple slabs, waste allowance and concrete cost.
Multiply slab length by slab width by slab thickness, using compatible units. If thickness is in inches, convert it to feet before multiplying; if thickness is in millimetres, convert it to metres.
Calculate the slab volume in cubic feet and divide by 27. Add your chosen waste allowance and round upward to the supplier's accepted order increment.
Yes. Select Area + Thickness and enter the slab area in square feet plus thickness in inches.
Yes. Use the Multiple Slabs option and enter length, width, thickness and the number of identical slabs.
Yes. Select Known Volume and enter cubic yards, cubic feet or cubic metres. The calculator will apply waste and rounding.
There is no universal percentage. The allowance should reflect the accuracy of the prepared base and forms, local depth variations and practical placing losses.
No. It estimates volume only. Slab thickness, reinforcement, concrete strength and structural design must come from the relevant plans, specifications or qualified professionals.
The calculator starts at zero so your estimate is based only on the measurements, waste allowance and price you intentionally enter.
Yes. The result area shows cubic yards, cubic feet and cubic metres so you can compare imperial and metric quantities.
Yes. Enter your own concrete rate per cubic yard. The result is a concrete-only estimate and may not include delivery, pump hire, labor or other project costs.
Yes. After calculating, you can download a PDF with slab inputs, waste allowance, cubic-yard quantity, cubic feet, cubic metres and cost.
Use current project documents, supplier information and official safety guidance alongside any online slab estimate.
Concrete technical education, standards and industry resources.
Visit ACIU.S. workplace information about respirable crystalline silica.
Read OSHA GuidanceConfirm cubic-yard increments, minimum loads, delivery access and current pricing.
Use the required slab thickness, reinforcement, strength and construction details for the actual project.