Concrete calculator
How much concrete do you need?
Enter the length, width and thickness of your slab to get the volume in cubic yards, the number of bags if you're buying bagged mix, and an estimated cost either way.
Enter your dimensions
National average is $150–$200 per cubic yard, delivered — edit it to match your local supplier.
Results
- Volume needed
- —
- Recommended order
- —
- Estimated cost
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Bagged mix alternative
- 40 lb bags—
- 60 lb bags—
- 80 lb bags—
Bag counts include your waste % and round up to whole bags.
Estimates only — confirm final quantities with your ready-mix supplier or supply store before ordering.
Quick reference
Concrete for common slab sizes.
At the standard 4-inch thickness for patios, walkways and shed floors. For 5- and 6-inch slabs, see the slab size chart.
| Slab (4″ thick) | Cubic yards | Order (+10%) | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.20 yd³ | 0.22 yd³ | 14 | 10 |
| 8 × 8 ft | 0.79 yd³ | 0.87 yd³ | 53 | 40 |
| 10 × 10 ft | 1.23 yd³ | 1.36 yd³ | 82 | 62 |
| 10 × 12 ft | 1.48 yd³ | 1.63 yd³ | 98 | 74 |
| 12 × 12 ft | 1.78 yd³ | 1.96 yd³ | 118 | 88 |
| 10 × 20 ft | 2.47 yd³ | 2.72 yd³ | 163 | 123 |
| 12 × 20 ft | 2.96 yd³ | 3.26 yd³ | 196 | 147 |
| 16 × 16 ft | 3.16 yd³ | 3.48 yd³ | 209 | 157 |
| 20 × 20 ft | 4.94 yd³ | 5.43 yd³ | 326 | 245 |
| 24 × 24 ft | 7.11 yd³ | 7.82 yd³ | 470 | 352 |
Bag counts include 10% waste and round up to whole bags.
Worked example
A 12 × 12 ft patio, 4 inches thick.
- Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
- Cubic feet: 12 × 12 × 0.333 = 48.0 ft³.
- Cubic yards: 48.0 ÷ 27 = 1.78 yd³.
- Add 10% waste: 1.96 yd³ to order — most suppliers round up to 2 yd³.
- Or in bags: 52.8 ft³ ÷ 0.6 ft³ per 80 lb bag = 88 bags.
How it's calculated
Length × width × thickness, converted to yards.
Concrete is sold by the cubic yard. To find cubic yards, convert your thickness from inches to feet, multiply by length and width in feet to get cubic feet, then divide by 27 (the number of cubic feet in a cubic yard). We add your waste percentage on top and round bag counts up, since suppliers sell whole yards and whole bags.
FAQ
Common questions
How much concrete do I need?
Multiply length × width × thickness (converted to feet) to get cubic feet, then divide by 27 to get cubic yards. The calculator above does this for you — enter your slab's length, width and thickness in inches and it converts straight to cubic yards, plus a recommended order quantity that includes waste.
Why does the calculator add a waste percentage?
Sub-grade isn't perfectly flat, forms flex slightly, and some concrete is lost during placement and finishing. A 5–10% waste allowance is standard for most slabs; use the higher end for irregular shapes or a hand-mixed pour.
Should I order ready-mix or buy bagged concrete?
Ready-mix delivery is almost always cheaper and faster for anything over about 1 cubic yard (roughly a 5×5 ft slab at 4 inches thick). Below that, bagged mix from a home center is usually more practical — the calculator shows both.
What thickness should I use?
4 inches is standard for patios, sidewalks and shed floors. Driveways carrying vehicles typically use 5–6 inches. Structural footings and slabs with heavier loads should follow your local building code or an engineer's spec.
Building something specific?