Concrete/Slab Volume Calculator
Enter your pour's dimensions to find out how much concrete you need — in cubic yards for ready-mix, or bags for a DIY pour.
How Concrete/Slab Volume Calculator Works
Concrete is ordered by volume, not area, so a slab's square footage alone doesn't tell you how much to buy -- thickness matters just as much as length and width. This calculator converts your pour's dimensions into cubic yards (how ready-mix is sold and priced) and into a bag count (how a small DIY pour is typically bought), with a waste allowance built in.
Formula & Method
For a rectangular slab or footing: Volume = Length × Width × (Thickness ÷ 12) × Quantity, with thickness entered in inches and converted to feet before multiplying. For a round slab or column: Volume = π × radius² × (Thickness ÷ 12) × Quantity. Either way, the raw volume is then increased by your waste allowance percentage to get the final cubic-foot total, which is divided by 27 (cubic feet per cubic yard) for the ready-mix figure, and divided by standard bag yields (0.30 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb) and rounded up for the bag counts, since you can't buy a fraction of a bag. Estimated weight uses a typical 150 lb per cubic foot for normal-weight concrete.
Worked Example
A 10ft × 10ft slab at 4in thick, with a 10% waste allowance: raw volume is 10 × 10 × (4÷12) = 33.33 ft³, which becomes 36.67 ft³ after waste -- that's about 1.36 cubic yards (order at least 1.5 yards, since ready-mix is typically sold in ¼-yard increments), roughly 5,500 lb, and would need 123 bags at 40 lb, 82 bags at 60 lb, or 62 bags at 80 lb.
Frequently Asked Questions
- Why does a 4-inch-thick slab use inches while length and width use feet?
- This mirrors how concrete pours are conventionally specified on job sites and in supplier order forms -- slab footprint (length/width) is almost always discussed in feet, while thickness is almost always discussed in inches, since it's a much smaller dimension. The calculator converts thickness to feet internally before computing volume.
- Should I order ready-mix or buy bags?
- As a rule of thumb, pours over about 1 cubic yard are usually more practical (and often cheaper per unit) as ready-mix delivery, since most suppliers have a minimum order (often 1 yard) anyway. Smaller pours -- a fence post footing, a small patch -- are typically more practical with bagged concrete you mix yourself.
- Why add a waste allowance at all?
- Real pours never go exactly to the theoretical volume -- uneven subgrade, over-excavation, and ordinary spillage during placement all use up extra material. A 5-10% allowance (10% is this calculator's default) is a commonly used buffer; irregular shapes or rough ground warrant going higher.
- How accurate is the weight estimate?
- It assumes a typical normal-weight concrete density of 150 lb per cubic foot, but actual density varies roughly 140-160 lb/ft³ depending on the specific mix design (aggregate type, air content, etc.) -- treat the weight figure as a planning estimate for things like delivery logistics, not an exact spec.
Add 5–10% extra for spillage, uneven subgrade, and over-excavation — 10% is a commonly used default; go higher for irregular shapes or rough ground. Round columns/footings (e.g. fence posts) commonly use Quantity for multiple identical holes. Bag yields are based on standard Quikrete/Sakrete-style figures — 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³ per bag — check your product's label, as yields vary slightly by brand. For pours over about 1 cubic yard, ready-mix delivery is usually more practical than bagged concrete; most suppliers have a minimum order (often 1 yard) and sell in ¼-yard increments. Weight estimate assumes a typical 150 lb per cubic foot for normal-weight concrete (actual density varies roughly 140–160 lb/ft³ by mix design).