Concrete Calculator

Concrete volume is just the space the pour fills. For a slab or rectangular footing that is length × width × thickness, with thickness converted from inches to feet by dividing by 12. For a round footing, pier, or post hole it is the cylinder formula, π × radius² × depth, where the radius is half the diameter in feet.

The result comes out in cubic feet. Suppliers quote in cubic yards, so it is divided by 27, and the metric equivalent multiplies cubic feet by 0.0283168. The waste allowance is applied to the total before any bag count, because you want the extra material in the order, not left as a shortfall on the ground.

• Slab volume (cu ft) = length (ft) × width (ft) × [thickness (in) ÷ 12]
• Round volume (cu ft) = π × [diameter (in) ÷ 12 ÷ 2]² × depth (ft)
• Total volume = unit volume × number of pours × (1 + waste % ÷ 100)
• Cubic yards = cubic feet ÷ 27
• Cubic metres = cubic feet × 0.0283168
• Bags = round up(cubic feet ÷ bag yield), where yield is 0.30, 0.45, or 0.60 cu ft for 40, 60, and 80 lb bags

The cubic-yard figure is what you order; the bag counts are the same volume expressed in premix. Compare the two costs before deciding — under about a yard bags usually win, above it ready-mix almost always does.

Common mistakes and assumptions

• Leaving thickness in inches without converting to feet, which inflates the volume roughly twelve times.
• Ordering the exact calculated volume, so an uneven subgrade leaves you short mid-pour.
• Assuming every brand's bag yields the same; the 0.30/0.45/0.60 cu ft figures are typical estimates, not guarantees.

A homeowner is pouring a 24 ft × 24 ft garage pad at 4 inches thick, with a 10% waste allowance and a quoted ready-mix price of $165 per cubic yard.

1. 1Convert the thickness: 4 in ÷ 12 = 0.333 ft.
2. 2Volume = 24 ft × 24 ft × 0.333 ft = 192 cubic feet.
3. 3Add 10% waste: 192 × 1.10 = 211.2 cubic feet.
4. 4Cubic yards = 211.2 ÷ 27 = 7.82 cu yd (5.98 cubic metres).
5. 5Ready-mix cost = 7.82 cu yd × $165 = about $1,291, before delivery.
6. 6The same pour in 80 lb bags is 211.2 ÷ 0.60 = 352 bags — around 8 tons of mixing, which is why anything this size gets delivered.

How much concrete do I need for a 24x24 slab?

At 4 inches thick, a 24 ft × 24 ft slab is 192 cubic feet, or 7.11 cubic yards. With a 10% waste allowance you would order about 7.8 cubic yards, and most plants round to the nearest quarter yard, so 8 yards is the practical order.

How many 80 lb bags of concrete make a cubic yard?

An 80 lb bag yields roughly 0.60 cubic feet mixed, and a cubic yard is 27 cubic feet, so it takes about 45 bags per yard. A 60 lb bag yields about 0.45 cu ft (60 bags per yard) and a 40 lb bag about 0.30 cu ft (90 bags per yard).

How thick should a concrete slab be?

Four inches is typical for patios, sheds, and walkways. Driveways, garage pads, and anything carrying vehicles are usually 5-6 inches, often with reinforcement. Thickness drives volume directly — going from 4 to 6 inches adds 50% to the concrete order.

Is it cheaper to mix bags or order ready-mix?

Bags are usually cheaper for small pours — post holes, small footings, and repairs under about a cubic yard. Above that, ready-mix is almost always less expensive per yard and vastly less labour, though plants add a short-load fee on small deliveries.

How much extra concrete should I order?

Five to ten percent over the calculated volume is the common allowance, and more on rough or over-dug ground where the subgrade is uneven. Ordering short is the expensive mistake: a partial second load or a cold joint costs far more than a little leftover mix.

How do I calculate concrete for a round post hole?

Use the cylinder formula π × radius² × depth, with the radius as half the hole diameter in feet. A 12 inch hole 3 feet deep is π × 0.5² × 3 = 2.36 cubic feet, about four 80 lb bags per hole before waste. Select the round option above and the calculator handles the conversion.

Does this include rebar, gravel base, or forms?

No. The figures cover the concrete volume only. Base gravel, reinforcement, forms, and finishing are separate line items — you can size the gravel bed with the Construction Material Quantity Calculator and price the whole package with the Construction Material Markup Calculator.