How to use the concrete calculator
- Choose US units (feet and inches) or metric (meters and centimeters).
- Pick what you're pouring: a rectangular slab, footing or wall, a round column or post hole, a circular slab, or a set of steps.
- Enter the dimensions. Thickness is in inches (or centimeters) because that's how slabs are specified.
- Enter how many identical pieces you need, such as eight post holes.
- Set the extra for waste. The default of 10% covers spillage, uneven ground and forms that bow a little.
- Choose a bag size and, if you like, enter a price per bag or a ready-mix price to see the cost.
How concrete volume is calculated
Each shape has its own volume formula. The calculator converts everything to cubic feet, adds the extra, then converts to cubic yards and cubic meters.
For steps, n is the number of steps. The formula treats the stairs as solid concrete: the bottom step is one rise tall, the next is two rises, and so on, so the heights add up to 1 + 2 + … + n = n(n + 1) ÷ 2 rises. One cubic yard is 27 cubic feet, and one cubic meter is 35.31 cubic feet.
Worked example
The defaults are a 10 ft × 10 ft slab, 4 inches thick, with 10% extra:
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.33 cu ft
- With 10% extra: 33.33 × 1.10 = 36.67 cu ft
- In cubic yards: 36.67 ÷ 27 = 1.36 cubic yards (1.04 m³)
- 80 lb bags at 0.60 cu ft each: 36.67 ÷ 0.60 = 61.1, so 62 bags
The same pour takes 82 bags of 60 lb, 123 bags of 40 lb or 87 bags of 25 kg. At $6.50 per 80 lb bag, that's 62 × $6.50 = $403.
For three steps, each 7 inches high, 11 inches deep and 4 feet wide, the volume is 4 × (11 ÷ 12) × (7 ÷ 12) × 6 = 12.83 cu ft, or 14.12 cu ft with 10% extra: 0.52 cubic yards.
Bag yields
| Bag | Typical yield | Bags per cubic yard | Bags per m³ |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 118 |
| 60 lb | 0.45 cu ft | 60 | 79 |
| 80 lb | 0.60 cu ft | 45 | 59 |
| 25 kg | 0.012 m³ | 64 | 84 |
The 40, 60 and 80 lb yields are those printed by major US makers of standard concrete mix, such as Quikrete's Concrete Mix data sheet. Metric 25 kg bags yield roughly 0.011 to 0.012 m³ depending on the brand; the calculator uses 0.012 m³. High-strength, fast-setting and lightweight mixes can yield a little more or less, so check the label.
Bags or ready-mix?
Bags make sense for post holes, small pads and repairs. Past about 1 cubic yard, mixing becomes hard work: 45 bags of 80 lb weigh 3,600 lb, and the first batches start to set before the last ones are placed. A ready-mix truck delivers the whole pour at once, and many suppliers will send part loads for a short-load fee. Ask the supplier how they round orders, since many sell in quarter- or half-yard steps.
Frequently asked questions
How thick should a concrete slab be?
Four inches (10 cm) is standard for patios, walkways and shed floors. Driveways and garage floors are usually 5 to 6 inches (12 to 15 cm) because they carry vehicles. Footings depend on the load and your local code, so check with your building department.
How much extra concrete should I order?
Add 5% to 10% for a typical pour. Go toward 10% or more if the ground is uneven, the forms are long, or you're filling post holes dug by hand, which are rarely the exact size planned. Running short mid-pour leaves a weak cold joint, so it's better to have a little left over.
How many 80 lb bags make a cubic yard?
About 45. A cubic yard is 27 cubic feet, and each 80 lb bag yields about 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags. With 60 lb bags you need 60, and with 40 lb bags you need 90.
Does the calculator account for rebar or gravel base?
No. Rebar and mesh take up so little volume that they don't change the order. A gravel base sits under the slab and is ordered separately; the gravel calculator works out how much you need.
Related calculators
- Square Footage Calculator — Add up the area of up to six rectangles, circles, triangles, trapezoids or L-shapes in sq ft or m², with cost.
- Gravel Calculator — Volume and weight of gravel, crushed stone, sand or topsoil for an area and depth, in tons or tonnes, with cost.
- Stair Calculator — Number of steps, riser height, total run, stringer length and angle, checked against US IRC stair limits.
- Volume Calculator — Volume of a cube, box, cylinder, cone, sphere, pyramid, capsule or tube, in cubic units, liters and gallons.
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