Concrete volume calculator
Enter the slab or footing size to get the concrete in cubic yards and the number of bags, so you know whether to mix it yourself or order a truck.
Cubic yards of concrete
1.358
36.666 cubic feet, or 62 bags of 80 lb mix
That's 62 bags of 80 lb mix. Above about a yard, ready-mix delivery is usually cheaper and much less work.
How to use it
- Pick a slab or a round footing.
- Enter the dimensions: length and width in feet for a slab, or diameter in inches for a footing, and the thickness or depth in inches.
- Enter how many if you're pouring several identical footings.
- Read the cubic yards for ordering ready-mix, or the bag count for mixing it yourself.
Worked example
A 10 × 10 ft shed pad, 4 inches thick, with 10% extra.
10 × 10 × (4 ÷ 12) = 33.333 cubic feet, plus 10% = 36.666 cubic feet, which is 1.358 cubic yards, or 62 bags of 80 lb mix. That’s a job for a truck, or a very long Saturday.
How this is calculated
A slab is length × width × thickness ÷ 12 in cubic feet, and a round footing is π × (diameter ÷ 24)² × depth ÷ 12. Cubic yards is cubic feet ÷ 27, which is how ready-mix is sold. An 80 lb bag of pre-mixed concrete yields about 0.6 cubic feet and a 60 lb bag about 0.45, with bags rounded up, and the waste option adds 10% for spillage, over-excavation and an uneven subgrade.
Slab: volume = length × width × thickness ÷ 12 (cubic feet)
Round: volume = π × (diameter ÷ 24)² × depth ÷ 12
cubic yards = cubic feet ÷ 27
An 80 lb bag of pre-mixed concrete yields about 0.6 cubic feet; a 60 lb bag about 0.45. Bags are rounded up. The waste toggle adds 10% for spillage, over-excavation and uneven subgrade.
Rates, sources and limits
Last reviewed 13 Sep 2026 by the CalcRabbit team. Estimates only, not financial, tax or medical advice.
Frequently asked questions
Bags or ready-mix?
Under about half a yard, bags are fine: that's 20 to 25 of the 80 lb bags. Above a yard, ready-mix is cheaper per yard and the whole slab cures as one pour. Most suppliers have a one-yard minimum and a short-load fee below a few yards.
How thick should a slab be?
Four inches is standard for patios, walkways and shed pads; six for driveways or anything a vehicle sits on. Footings depend on the frost line in your area, often 36 to 48 inches in northern states.
How many bags does a fence post take?
A 12-inch hole 30 inches deep is 0.65 cubic feet, so about two 50 lb bags of fast-setting mix per post after the post displaces some. Pick round, 12 in diameter, 30 in deep and enter the number of posts.
Why add 10%?
Real ground isn't flat, forms bow slightly, and some concrete stays in the mixer and wheelbarrow. Running short mid-pour is far worse than having a bag left over.