How to use this calculator
This tool is a volume estimate for a rectangle. Length and width are in feet; thickness is in inches. The result is planning math—not a mix design, rebar schedule, or dispatcher quote.
- Measure the slab length and width in feet (use the same rectangle you will form).
- Enter thickness in inches. Sidewalks are often about 4 inches; many residential drives are 4–6 inches over a compacted base—confirm what your project needs.
- Set a waste percent if you expect over-excavation or spillage. The default is 8%; it is not a required “correct” allowance—edit it for your site.
- Check bag yields if you are bagging: defaults are 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag. Change them to match the bag label.
- Read cubic yards for ready-mix and bag counts for DIY mixing. Edit ready-mix price per yard to match a local quote.
How this calculation works
- Volume in cubic feet is length × width × thickness in feet (inches ÷ 12).
- That volume is multiplied by (1 + waste%). Cubic yards divide by 27.
- Bag counts divide cubic feet by the yield printed on the bag (defaults are common bagged-mix yields). Counts round up.
ft³ = L × W × (t_in/12) × (1+waste%); yd³ = ft³/27; bags = ceil(ft³ / bag yield)
Worked example
A 10 ft × 10 ft pad, 4 inches thick, with the default 8% waste and bag yields of 0.60 cu ft (80 lb) and 0.45 cu ft (60 lb)—the same starting values as the form.
- Thickness in feet = 4 ÷ 12 ≈ 0.333 ft.
- Net volume before waste = 10 × 10 × (4÷12) ≈ 33.33 cu ft ≈ 1.23 yd³.
- With 8% waste: 33.33 × 1.08 = 36.0 cu ft ≈ 1.33 yd³.
- 80 lb bags = ceil(36.0 ÷ 0.60) = 60. 60 lb bags = ceil(36.0 ÷ 0.45) = 80.
Result: 1.33 cubic yards (1.23 net before waste), 60 of the 80 lb bags, or 80 of the 60 lb bags—the same numbers this calculator returns for those inputs.
Common mistakes
- Entering thickness in feet in a field that expects inches (for example typing 0.33 instead of 4). Thickness here is inches.
- Mixing units across length and width (feet on one side, inches on the other). Convert everything to the field units first.
- Treating an L-shaped or curved pour as one rectangle. Break the shape into rectangles, run the calculator for each, and add the yards—or reshape the forms.
- Ignoring field conditions entirely, or treating the waste field as a guaranteed correct order quantity. Waste is your editable allowance; the plant may still round the dispatch size.
- Ordering bags from an assumed yield without reading the bag. Mix designs vary—edit the yield fields to match the product you will buy.
What to do next
- For anything bigger than a few bags, ready-mix is usually more consistent. Ask about short-load fees.
- Confirm thickness with the use: sidewalks ~4 in; many residential drives 4–6 in over base.
- Plan control joints and reinforcement with someone who will warranty the slab.
Pouring a drive instead of a small pad? Use the driveway calculator for area and an installed or material cost range by surface. For a sitting area, the patio calculator covers paver or slab takeoff. Need base rock under the slab? Try the gravel calculator.
FAQ
- How much concrete do I need?
- For a rectangle, multiply length × width × thickness (in feet), convert cubic feet to cubic yards by dividing by 27, then apply any waste percent you choose. A 10×10 ft slab at 4 inches is about 1.23 yd³ before waste; with the default 8% waste it is about 1.33 yd³.
- How do I calculate concrete volume?
- Volume (ft³) = length (ft) × width (ft) × (thickness in inches ÷ 12). Yards = ft³ ÷ 27. This page also multiplies by (1 + waste%) and converts the result into bag counts using the yields you enter.
- How many cubic yards of concrete do I need?
- Divide cubic feet by 27. Enter your slab size above to see yards after your waste percent. Ready-mix plants often round the order; ask how they dispatch short loads.
- How thick should a concrete slab be?
- It depends on the use and the base under it. Sidewalks are often about 4 inches; many residential driveways are 4–6 inches over compacted base. This calculator does not pick a thickness for you—enter the thickness your project requires.
- How do I calculate concrete for an irregular area?
- This tool is built for a rectangle. Split an irregular pour into rectangles, run the calculator for each piece, and add the cubic yards (and bags). Curved edges still need a practical rectangular takeoff or a contractor’s form plan.
- How many 80-pound bags in a yard?
- At 0.60 cubic feet per bag, 27 ÷ 0.60 = 45 bags per yard before waste. Always round up, and verify yield on your bag. Change the 80 lb yield field if your product differs.
Sources
- yd³ = length × width × (thickness in / 12) / 27
- Typical bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³ (check the bag; mixes vary)
Cost defaults are typical US planning bands, last reviewed 2026-08-25. Edit the price fields on this page rather than treating them as a local market index.