A 12 × 10 ft patio poured 4 inches thick takes 1.48 cubic yards of concrete. With a 10 % allowance for an uneven base and spillage that becomes 1.63 yards: 74 bags of 80 lb premix, or a 1¾-yard ready-mix order. The calculator above does the same arithmetic for any slab, round pier or set of steps, in feet and inches or in metres.
How the calculator works
Pick what you are pouring, enter the dimensions, then choose how you buy the concrete. Everything is converted to one unit system inside, so you can mix feet, inches and fractions freely: 12' 6½" is read exactly, and the result follows the market you pick in the header.
Slab or footing
Volume is length × width × thickness. Thickness is entered in inches because that is how slabs are specified: 4" for patios, walkways and shed bases, 5 to 6" for driveways and garage floors. The IRC sets 3½ inches as the minimum for a concrete floor slab on the ground (R506.1); the calculator flags anything thinner. The same shape works for a strip footing: enter its length, width and depth.
Round column or pier
A round pier or sonotube holds π × (D/2)² × H. A 12-inch tube 4 feet deep is π cubic feet, about 3.14 ft³ or 0.12 yd³: six 80 lb bags once you add 10 %. Enter the number of identical piers to get the total for a deck or a carport.
Steps
Steps poured on the ground are treated as solid blocks, each one from the ground up. For n steps of riser R, tread T and width W the volume is W × R × T × n(n+1)/2. Three steps with 7" risers and 11" treads, 4 feet wide, hold 12.83 ft³ (0.48 yd³), or 24 bags of 80 lb with waste. The IRC limits risers to 7¾" (R311.7.5.1) and requires treads of at least 10" (R311.7.5.2); the calculator checks both.
Known volume
If the volume comes from a drawing or a plan takeoff, enter it directly. You still get the bag count, the truck order and the weight.
Bags or a ready-mix truck?
Premixed concrete in bags is sold by weight. Manufacturers publish the yield of each size: an 80 lb bag makes about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. So one cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb. All three yields work out to the same ratio, 133⅓ lb of dry mix per cubic foot; the calculator uses it for metric bag sizes too, so a 25 kg bag counts as about 11.7 litres.
Below about one cubic yard, bags usually win: no short-load fee and no truck access to plan. Above two yards a truck is cheaper per yard and the pour is done in an hour instead of a day of mixing. Ready-mix is ordered in quarter-yard steps, so the calculator rounds the order up: 1.63 yd³ becomes 1¾. A typical mixer truck carries 8 to 11 yards; the drawing panel shows how much of a 10-yard load your pour fills.
Which strength to ask for
Concrete strength is specified in psi. The IRC sets minimums by use and by weathering, the risk of freeze–thaw damage where you build (Table R402.2):
| Use | Negligible | Moderate | Severe |
|---|---|---|---|
| Foundations and walls not exposed to weather | 2,500 | 2,500 | 2,500 |
| Interior and basement slabs | 2,500 | 2,500 | 2,500 |
| Walls and vertical work exposed to weather | 2,500 | 3,000 | 3,000 |
| Porches, steps, carport and garage slabs | 2,500 | 3,000 | 3,500 |
Choose the use and weathering in the third step and the result card shows the minimum. Exposed concrete in moderate and severe areas also needs air entrainment; ask your supplier, and check whether your local code asks for more.
How much extra to order
The waste allowance covers an uneven subgrade, forms that bow, spillage and the concrete left in the mixer. 5 % is enough for a formed slab on a screeded base, 10 % is the usual default, and 15 % suits hand-dug footings where the trench is never quite the size on paper. Running short in the middle of a pour costs far more than a few spare bags; stores take back unopened bags.
How heavy is it?
Normal-weight concrete weighs about 150 lb per cubic foot (2,400 kg/m³). The 12 × 10 ft patio with waste is 44 ft³, about 6,600 lb. That matters when a pickup or a trailer carries the bags, and when the slab sits on a deck or a suspended floor rather than on the ground.
Common mistakes
- Mixing inches and feet: 4" is 0.33 ft, not 0.4 ft. Enter thickness in inches and let the calculator convert.
- Forgetting the thickened edge of a monolithic slab or the footing under it. Add it as a separate footing.
- Assuming every bag size yields the same: in metric markets bags are 25 kg, and the yield follows the mass, about 11.7 litres each.
- Ordering ready-mix to the exact volume. Trucks round to quarter yards and a short load is expensive.
Planning a roof over the slab? The pitch, angle and rafter length are in the Roof Pitch Calculator.