How Much Concrete Do I Need?
Estimate concrete volume in cubic feet, cubic yards, or bags before buying or quoting a pour.
Last reviewed: June 2026
Short Answer
To estimate concrete, multiply length x width x thickness in feet to get cubic feet, then divide by 27 for cubic yards. Add a waste factor, usually 5% to 10% for small DIY pours, and verify bag yield or ready-mix requirements before buying.
Use the concrete slab calculatorPractical Explanation
Concrete estimates start with volume. For a rectangular slab, pad, or footing, measure length and width in feet, then convert thickness from inches to feet.
For bagged concrete, divide total cubic feet by the yield printed on the bag. For ready-mix, convert cubic feet to cubic yards. Contractors and small crews can use the same process for quick quote checks, but structural work should be verified against plans and code.
Simple Formula
Volume = length x width x thickness in feet. Cubic yards = cubic feet / 27. Bags = cubic feet / bag yield, rounded up.
Example: 10 ft x 12 ft slab, 4 inches thick
- Convert 4 inches to feet: 4 / 12 = 0.333 ft
- Find volume: 10 x 12 x 0.333 = about 40 cubic feet
- Convert to cubic yards: 40 / 27 = about 1.48 cubic yards
- Add 10% waste: 1.48 x 1.10 = about 1.63 cubic yards
You would plan for about 1.6 cubic yards of concrete, then confirm with your ready-mix supplier or bag yield.
Common Mistakes
- Forgetting to convert inches to feet before multiplying.
- Buying only the raw math amount with no waste factor.
- Using bag counts for a large pour that may be better suited for ready-mix delivery.
- Skipping code, reinforcement, or subbase requirements for structural work.
FAQ
How do I calculate concrete for a slab?
Multiply length by width by thickness in feet. Divide cubic feet by 27 to get cubic yards, then add a waste factor.
Should I order extra concrete?
Usually yes. A small buffer helps account for uneven subgrade, forms, spillage, and measurement differences.
Can this replace engineering plans?
No. Use it for planning quantities. Structural pours, permits, reinforcement, and code requirements should be verified professionally.
Estimate Disclaimer
These guides and calculators provide planning estimates. Bag yields, material density, compaction, waste, site conditions, supplier measurements, and local requirements can vary. Confirm quantities with your supplier, contractor, plans, or local code requirements when needed.
Ready to Estimate Your Project?
Use the calculator to turn your measurements into an approximate material list before you buy, quote, or start work.
Use the concrete slab calculator