Concrete planning tool
Calculate the concrete needed for a rectangular or square slab, including waste-adjusted ready-mix and bag quantities.
Project calculator
Use your planned dimensions, not form-board outside dimensions, and adjust waste for site conditions.
Plan responsibly
This project may be subject to local permit, zoning, site, or structural requirements.
Local requirements control: Model-code references do not confirm the code edition, amendments, zoning rules, permit requirements, or site criteria adopted for your project.
Planning estimate only: Concrete volume does not establish a compliant slab design, reinforcement schedule, joint layout, mixture, drainage plan, or subgrade preparation.
Concrete floors on ground: The IRC contains model-code provisions for concrete floors on ground. Applicability differs for building floors, garages, patios, driveways, and other flatwork.
Source: ICC Digital Codes: 2021 IRC Section R506.1 · Reviewed 2026-08-24
Local requirements may differ. A model-code reference does not confirm the code edition, amendments, zoning rules, or permit requirements adopted where the project is located.
2021 IRC, Section R506.1: Concrete floors on ground. Source: ICC Digital Codes: 2021 IRC Section R506.1. Reviewed 2026-08-24.
Local requirements may differ. Verify the adopted code, amendments, zoning rules, and permit requirements with the local authority.
Methodology
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.
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.
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.
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
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
At 4 inches thick with 10% waste, the calculated order quantity is 1.09 yd³.
At 4 inches thick with 10% waste, the calculated order quantity is 6.52 yd³.
Common questions
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.
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.
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.
Bags can suit small work and repairs. Larger pours usually require many bags and substantial mixing, so ready-mix delivery may be more practical.
Thickness depends on use, loads, soil, base preparation, reinforcement, climate, and local requirements. Confirm structural needs for your project.
Sources
Last reviewed: 2026-08-22
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