How to use this concrete calculator
- Choose the project: a slab (patio, walkway, shed base), a footing, round columns or post holes, or a flight of steps.
- Enter the dimensions. Lengths are in feet; thicknesses, widths of footings and diameters are in inches, the way they are usually specified. For 6 inches of length, enter 0.5 ft.
- Set the waste allowance. The default is 10%.
- Read the volume and bag counts, and add your local prices under “Prices” to compare bags with a ready-mix delivery.
Concrete volume formulas
Concrete is ordered by volume. Convert every measurement to feet (inches ÷ 12), multiply to get cubic feet, then divide by 27 to get cubic yards, since a cubic yard is 3 ft × 3 ft × 3 ft.
Slab, patio or walkway
V = L × W × T / 12
- V
- volume in cubic feet
- L, W
- length and width in feet
- T
- thickness in inches
Worked example: a 10 × 10 ft slab, 4 in thick, is 10 × 10 × 4 ÷ 12 = 33.33 cu ft. Adding 10% gives 36.67 cu ft. Divide by 27 for 1.36 cubic yards. In bags, 36.67 ÷ 0.60 = 61.11, so you need 62 bags of 80 lb mix (82 bags of 60 lb or 123 bags of 40 lb). Always round bags up.
Footing
V = L × (W / 12) × (D / 12)
- L
- total length in feet (all sides of a perimeter footing added together)
- W, D
- width and depth in inches
Example: 40 ft of footing 16 in wide and 8 in deep is 35.56 cu ft, or 1.45 cu yd with 10% extra.
Round column, pier or post hole
V = π × (d / 24)² × H × n
- d
- diameter in inches (d ÷ 24 is the radius in feet)
- H
- height or hole depth in feet
- n
- number of columns or holes
Example: 4 tube forms 12 in across and 4 ft tall hold 12.57 cu ft, or 13.82 cu ft (24 bags of 80 lb mix) with 10% extra.
Steps
V = W × (R / 12) × (r / 12) × N(N + 1) / 2
- W
- width of the steps in feet
- R
- run (tread depth) in inches
- r
- rise (height of each step) in inches
- N
- number of steps
Solid steps poured on the ground are a stack of blocks: the top step is one riser tall, the next two risers, and so on, which is where N(N + 1) ÷ 2 comes from. Example: 3 steps 4 ft wide with a 7 in rise and 11 in run need 12.83 cu ft, or 14.12 cu ft with 10% extra. A landing at the top is a separate slab.
Cubic yards of concrete for common slab sizes
Volumes below are exact, with no waste allowance; add your percentage before ordering. Bag counts use 0.60 cu ft per 80 lb bag and are rounded up.
| Slab size | Area | 4 in: cu yd | 4 in: 80 lb bags | 6 in: cu yd | 6 in: 80 lb bags |
|---|---|---|---|---|---|
| 4 × 4 ft | 16 sq ft | 0.2 | 9 | 0.3 | 14 |
| 8 × 8 ft | 64 sq ft | 0.79 | 36 | 1.19 | 54 |
| 10 × 10 ft | 100 sq ft | 1.23 | 56 | 1.85 | 84 |
| 10 × 12 ft | 120 sq ft | 1.48 | 67 | 2.22 | 100 |
| 12 × 12 ft | 144 sq ft | 1.78 | 80 | 2.67 | 120 |
| 10 × 20 ft | 200 sq ft | 2.47 | 112 | 3.7 | 167 |
| 12 × 20 ft | 240 sq ft | 2.96 | 134 | 4.44 | 200 |
| 16 × 16 ft | 256 sq ft | 3.16 | 143 | 4.74 | 214 |
| 20 × 20 ft | 400 sq ft | 4.94 | 223 | 7.41 | 334 |
| 20 × 24 ft | 480 sq ft | 5.93 | 267 | 8.89 | 400 |
| 24 × 24 ft | 576 sq ft | 7.11 | 320 | 10.67 | 480 |
| 30 × 30 ft | 900 sq ft | 11.11 | 500 | 16.67 | 750 |
Need the area of an irregular patio first? The square footage calculator handles circles, triangles and L-shapes; multiply its result by the thickness in feet to get cubic feet.
Bags or a ready-mix truck?
The deciding factors are volume, labor and price. One cubic yard is 45 bags of 80 lb mix, or 3,600 pounds of dry material to haul, lift and mix. Some facts from the QUIKRETE Concrete Mix data sheet help size a bag job:
- Each 80 lb bag needs about 6 pints of water to start, and up to about 9 pints in total.
- A mixer needs at least 1 cubic foot of capacity for each 80 lb bag mixed at one time.
- Slabs should be filled to full depth, working continuously from one end to the other; partial-depth fills can lead to cold joints.
- The mix reaches a typical compressive strength of 4,000 psi at 28 days.
For a bigger pour, get quotes from local ready-mix suppliers and ask about minimum orders and any delivery or short-load charges. Then enter the price per cubic yard and your bag prices under “Prices” in the calculator to compare the two for your project.
Pouring and curing tips
- Slope for drainage. QUIKRETE’s patio guide calls for a slope away from the house of 1/8 inch per foot; its driveway guide suggests 1/4 inch per foot toward the street.
- Cut control joints about 1 inch deep every 6 to 8 feet in a slab, per the Concrete Mix data sheet.
- Cure it. Keep new concrete moist for 5 days in warm weather (70°F or higher) or 7 days at 50 to 70°F, and protect it from freezing for the first 48 hours.
- Check local rules. Footings, driveways that meet a public sidewalk and anything structural are often governed by local building codes.
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb concrete mix at 0.60 cu ft per bag, 60 bags of 60 lb mix at 0.45 cu ft, or 90 bags of 40 lb mix at 0.30 cu ft. Those yields are from the QUIKRETE Concrete Mix data sheet; add your waste allowance on top.
How much concrete do I need for a 10 × 10 slab?
A 10 × 10 ft slab 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards before waste. With 10% extra it is 1.36 cubic yards, which is 62 bags of 80 lb mix. At 6 inches thick the same slab needs 1.85 cubic yards before waste.
How much area does an 80 lb bag of concrete cover?
An 80 lb bag of QUIKRETE Concrete Mix yields about 0.60 cubic feet. Spread 4 inches thick, that covers 1.8 square feet; at 3 inches it covers 2.4 sq ft, and at 6 inches 1.2 sq ft. Divide the yield in cubic feet by the thickness in feet (inches ÷ 12) to get coverage for any thickness.
How thick should a concrete patio or driveway be?
QUIKRETE’s project guides say a 4-inch slab is generally suitable for a patio, and 4 inches is sufficient for a driveway used only by passenger cars. For trucks and other heavy vehicles, they recommend 5 or 6 inches. Its Concrete Mix is designed for pours 2 inches thick or more. Check local building codes, which can set their own requirements.
How much extra concrete should I order?
Order more than the exact volume, because subgrades are rarely perfectly level, forms bow slightly and some concrete is spilled or left in the mixer. This calculator defaults to 10% extra, which you can change. Running short in the middle of a pour is worse than a few leftover bags, since stopping can leave a visible cold joint.
How do I calculate concrete for fence post holes?
Treat each hole as a cylinder: π × (diameter ÷ 2)² × depth, then multiply by the number of holes. QUIKRETE suggests a hole about 3 times the post’s width and a third of its overall height deep. A 12-inch hole 2.67 ft deep holds 2.09 cu ft, about 4 bags of 80 lb mix before subtracting the post itself.
How much does a cubic yard of concrete weigh?
The QUIKRETE Concrete Mix data sheet lists a typical unit weight of about 140 pounds per cubic foot, so a cubic yard of that mix weighs roughly 3,780 pounds. The 45 dry 80 lb bags needed for a yard weigh 3,600 pounds before water is added, which is worth knowing before you load a vehicle.