Calculate concrete volume for a slab or footing, in cubic yards and 80-pound bags.
Equivalent bags
Ready-mix vs bagged
Usage Tip
Past roughly half a cubic yard, ordering ready-mix is usually cheaper and easier than mixing dozens of bags by hand.
80-lb bags = cubic feet ÷ 0.6
The result gives cubic yards for ready-mix and the number of 80-pound bags.
How much concrete do I need?
Concrete is figured by volume. Find the volume of the pour in cubic feet, divide by 27 for cubic yards, then add a waste factor because nobody wants to come up a fraction of a yard short with a slab curing in the sun. This calculator handles slabs, footings, piers and tubes, columns, curbs, stairs and round pads, then converts the result into bags — and tells you whether a ready-mix truck makes more sense than lifting eighty bags by hand.
Concrete slab calculator guide
For a slab, multiply length by width by thickness. Keep the thickness in the same units: a 10 by 10 ft slab at 4 in is 10 x 10 x (4/12) = 33.3 cubic feet, about 1.23 cubic yards before waste. Add 5 to 10 percent for spillage, uneven subgrade and over-excavation.
Concrete bags per cubic yard
One cubic yard is 27 cubic feet. Each bag yields a fixed volume, so a yard takes a lot of bags — which is exactly why ready-mix exists.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Concrete coverage chart
How far one cubic yard spreads depends on slab thickness:
| Thickness | Coverage per cubic yard |
|---|---|
| 4 in | 81 sq ft |
| 5 in | 65 sq ft |
| 6 in | 54 sq ft |
| 8 in | 40 sq ft |
Concrete thickness recommendations
| Project | Typical thickness |
|---|---|
| Sidewalk | 4 in |
| Patio | 4 in |
| Shed pad | 4 to 6 in |
| Garage floor | 5 to 6 in |
| Driveway | 6 in (4 in for light cars) |
| Hot tub pad | 6 in |
Bagged or ready-mix?
Bagged concrete wins for small jobs — footings, a few posts, a small pad — up to roughly one cubic yard. Past that, the bag count climbs fast (a yard is 45 of the 80 lb bags) and ready-mix is usually cheaper and far less work, though small loads under three or four yards often carry a short-load fee. The calculator flags the crossover and compares the cost both ways.
Frequently asked questions
How many 80 lb bags of concrete in a yard?
About 45, since each 80 lb bag yields roughly 0.6 cubic feet and a yard is 27 cubic feet.
How much concrete for a 10x10 slab?
At 4 in thick, about 1.23 cubic yards before waste, or roughly 56 of the 80 lb bags. Most people order ready-mix at that size.
How thick should a concrete slab be?
Four inches for sidewalks and patios, five to six for garage floors, and six for driveways and hot tub pads.
How much waste should I add?
Five to ten percent for most pours; more on rough or sloping subgrade. It is much worse to run short mid-pour than to have a little extra.
When should I order ready-mix instead of bags?
Once you pass about one cubic yard. Mixing 45-plus bags by hand is slow and inconsistent; a truck is faster and stronger, watch for short-load fees.
How much does a yard of concrete weigh?
About 3,600 to 4,000 lb. Plan access and subgrade support accordingly.
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Estimates are for planning. Concrete volume varies with subgrade, over-dig, slab thickness tolerance and bag yield; add a waste margin, confirm bag yields and ready-mix minimums with your supplier, and follow local code for thickness, reinforcement and footings.