What this concrete calculator does
Ordering too little concrete forces a cold joint in the middle of your pour, a classic source of cracks. Ordering too much wastes money and leaves you with a heavy disposal problem. This calculator gives you the exact volume of concrete for a slab, footing, wall or round column in seconds, then turns it into a shopping list: cement bags, sand and gravel, water, premix bags and even wheelbarrow loads.
Pick the shape, enter the dimensions in feet, inches, yards or metric units, choose the mix and a waste allowance, and every result updates in real time. You can display the volume in cubic yards, cubic feet or cubic meters.
How to calculate concrete volume
Slabs, footings and walls (rectangular)
Volume = length × width × thickness
All three dimensions must use the same unit. The usual trap is thickness: 4 inches is 0.333 ft, not 4 ft. Divide cubic feet by 27 to get cubic yards, the unit ready-mix suppliers use in the US. The calculator handles every conversion for you.
Columns, piers and sonotubes (cylindrical)
Volume = π × (diameter ÷ 2)² × height
A 12-inch sonotube that is 8 ft deep holds π × 0.5² × 8 ≈ 6.28 cubic feet, or about 0.23 cubic yards. Enter the number of identical piers in the “number of identical pours” field to get the total.
Waste allowance
Subgrades are never perfectly flat, forms bow slightly and some concrete always stays in the mixer or the chute. Add 5% for a well-prepared slab, and up to 10% for uneven ground or trench footings.
Example: a 20 ft × 12 ft patio slab
For a slab of 20 × 12 ft, 4 inches thick, with a 590 lb/yd³ (350 kg/m³) mix and 5% waste:
| Material | Quantity |
|---|---|
| Concrete volume | 3.11 cubic yards (84 cubic feet) |
| Cement | about 1,835 lb, or 20 bags of 94 lb |
| Or premixed concrete | 141 bags of 80 lb |
| Sand | about 3,566 lb (1.8 short tons) |
| Gravel | about 6,162 lb (3.1 short tons) |
| Water | about 110 gallons |
| Fresh concrete weight | about 12,300 lb |
At just over 3 cubic yards, a ready-mix truck is the obvious choice: mixing 141 bags by hand in one session is exhausting and rarely consistent.
Choosing the right mix
The cement content of a mix drives its strength. US suppliers often describe it in “bags per yard” or by compressive strength (psi):
| Cement content | Typical use |
|---|---|
| 420 lb/yd³ (250 kg/m³) | Lean concrete, fence posts, non-structural fill |
| 505 lb/yd³ (300 kg/m³) | Footings, foundations |
| 590 lb/yd³ (350 kg/m³) | Slabs, patios, driveways, general structural work |
| 675 lb/yd³ (400 kg/m³) | Columns, beams, heavily loaded slabs |
A 3,000 psi mix is common for sidewalks and patios, while driveways in freeze-thaw climates usually call for 4,000 psi with air entrainment. The comparison table below the results shows how cement, bags and water change across mixes for your own dimensions.
The recipe for one cubic yard
For a general-purpose mix, one cubic yard of concrete contains roughly:
- 590 lb of Portland cement (about 6.3 bags of 94 lb);
- 1,150 lb of sand;
- 1,980 lb of gravel (3/8 to 3/4 inch);
- 35 gallons of water, a water-to-cement ratio of 0.5 by weight.
On site, many builders use the 1 : 2 : 3 rule: one part cement, two parts sand and three parts gravel by volume. Adding extra water makes concrete easier to place but weakens it and increases shrinkage cracks, so use a plasticizer instead if the mix is too stiff.
Bags, mixer or ready-mix?
- Under about 0.4 cubic yards (setting posts, a small step): premixed 60 lb or 80 lb bags are simplest. An 80 lb bag yields about 0.6 cubic feet, so a cubic yard takes 45 bags.
- From 0.4 to 2 cubic yards: renting a portable mixer and buying cement, sand and gravel separately is usually the cheapest option.
- Above 2 cubic yards: order ready-mix. It guarantees consistent strength and lets you pour in one go. Many suppliers add a short-load fee for orders under 3 to 4 cubic yards, and a pump truck may be needed when access is difficult.
Tips for a successful pour
- Prepare the base: remove topsoil, compact 4 to 6 inches of gravel and add a vapor barrier under interior slabs.
- Reinforce: wire mesh or rebar on chairs improves flexural strength and controls cracking.
- Plan control joints at intervals of roughly 2 to 3 times the slab thickness in inches, expressed in feet (8 to 12 ft for a 4-inch slab).
- Watch the weather: avoid pouring below 40 °F or in strong heat and wind, and keep the slab moist for several days to cure properly.
- Be patient: you can walk on a slab after 24 to 48 hours, but concrete reaches its design strength after about 28 days.
Quantities are estimates based on common proportions. For structural work such as house foundations, retaining walls or suspended slabs, follow your engineer’s specifications and local building codes.