How much extra concrete should I order?
A waste allowance can account for uneven subgrade, form variation, spillage, and ordering tolerance. The 10% default is a planning convention, not a rule; confirm final ordering details with your supplier.
Concrete planning tool
Concrete Slab Calculator
Calculate the concrete needed for a rectangular or square slab, including waste-adjusted ready-mix and bag quantities.
Project calculator
Enter your slab dimensions
Use your planned dimensions, not form-board outside dimensions, and adjust waste for site conditions.
Methodology
How we calculate this
Plan Otter converts length and width to feet, multiplies length by width by thickness and the number of identical slabs to find total cubic feet, divides by 27 for cubic yards, and then applies the selected waste allowance without rounding intermediate values.
length × width × thickness × quantity ÷ 27 = cubic yards
Concrete slab thickness
Four inches is a common planning input for some residential slabs, but loads, soil, base preparation, reinforcement, climate, and local requirements determine an appropriate design.
Bags versus ready mix
Compare the displayed ready-mix quantity with all three bag counts. Ask local suppliers about delivery minimums, short-load fees, site access, mixing labor, and pour timing before choosing a supply path.
Quick reference
Common concrete slab quantities
Scroll the table horizontally on smaller screens.
| Slab size | Thickness | Total area | Order with 10% | 80-lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 4 in | 100 sq ft | 1.36 yd³ | 62 |
| 12 × 12 ft | 4 in | 144 sq ft | 1.96 yd³ | 89 |
| 20 × 20 ft | 4 in | 400 sq ft | 5.43 yd³ | 245 |
| 20 × 20 ft | 6 in | 400 sq ft | 8.15 yd³ | 367 |
Worked examples
Concrete Slab Calculator examples
Small slab: 8 ft × 10 ft
At 4 inches thick with 10% waste, the calculated order quantity is 1.09 yd³.
Larger patio: 20 ft × 24 ft
At 4 inches thick with 10% waste, the calculated order quantity is 6.52 yd³.
Common questions
Concrete Slab Calculator FAQ
How do I calculate concrete for a slab?
Multiply slab length by width by thickness after converting measurements to compatible units. Divide cubic feet by 27 for cubic yards, then apply the selected waste allowance.
How much extra concrete should I order?
Extra material may account for uneven grade, form variation, spillage, and ordering tolerance. The calculator defaults to 10%, but project conditions and supplier guidance should determine the final allowance.
How many bags of concrete make a cubic yard?
Using the sourced approximate yields, one cubic yard is about 90 40-lb bags, 60 60-lb bags, or 45 80-lb bags before waste.
Is ready-mix better than bags?
Bags can suit small work and repairs. Larger pours usually require many bags and substantial mixing, so ready-mix delivery may be more practical.
What slab thickness should I use?
Thickness depends on use, loads, soil, base preparation, reinforcement, climate, and local requirements. Confirm structural needs for your project.
Sources
Assumptions and references
- QUIKRETE Concrete Mix No. 1101 Product Data Sheet: Approximate 40, 60, and 80 lb bag yields. Last checked 2026-08-22.
- NIST SI Units – Length: Metric and imperial conversion reference. Last checked 2026-08-22.
Last reviewed: 2026-08-22