Concrete Calculator

How to calculate the volume of concrete

Concrete is bought by volume, so every estimate starts with the volume of the space you want to fill. For a slab, footing or wall the shape is a box: multiply length by width by thickness, with all three measured in the same unit. A 10 ft by 10 ft slab that is 4 inches thick needs 10 × 10 × (4 ÷ 12) = 33.33 cubic feet. Divide by 27 to turn cubic feet into cubic yards, the unit most suppliers quote in the United States: 33.33 ÷ 27 = 1.235 cubic yards. In metric the same slab is 0.944 cubic metres.

The calculator converts every length exactly before it multiplies, so mixing feet, inches, yards, metres and centimetres in one section is safe. Thickness is usually the awkward one, because it is given in inches while length and width are in feet. Use the unit selector beside each field instead of converting by hand, and the result is the same either way. Each shape also has a quantity, which multiplies the volume of one piece by the number of identical pieces.

Round holes, columns and hollow tubes

A round hole or column is a cylinder. Its volume is π × (diameter ÷ 2)² × depth. Four post holes that are 12 inches across and 36 inches deep need 4 × π × 0.5² × 3 = 9.425 cubic feet, which is 0.349 cubic yards. A hollow tube or a ring, such as a concrete pipe wall or a post set inside a sleeve, is the difference between two cylinders: π × ((outer diameter ÷ 2)² − (inner diameter ÷ 2)²) × height. For a 24 inch outer diameter, a 20 inch inner diameter and a height of 4 feet that gives 3.840 cubic feet. The calculator will not accept an inner diameter that is the same as or larger than the outer one.

Cubic yards, cubic feet and cubic metres

One cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. One cubic metre is about 35.315 cubic feet, because a metre is 1 ÷ 0.3048 feet and the volume scales with the cube. A 5 m by 4 m slab that is 10 cm thick is 2.000 cubic metres, which is 2.616 cubic yards and 70.63 cubic feet. The results show all three units side by side so you can match whatever your supplier or bag label uses.

Waste allowance and the number of bags

Real pours use more concrete than the geometry says. The ground is rarely perfectly level, forms can bow, and some mix is lost in the barrow. The waste allowance is a percentage you choose, added on top of the calculated volume. The page opens with 10% so you can see how it works, and you can set it from 0 to 100. With 10% on the slab the order is 36.67 cubic feet, or 1.358 cubic yards.

To count bags, enter the yield that the manufacturer prints on the bag, in cubic feet or litres. Bag yields differ between products, so the calculator never infers one from the weight of the bag. If a bag yields 0.6 cubic feet, the slab with waste needs 36.667 ÷ 0.6 = 61.111 bags, and you cannot buy part of a bag, so the answer is 62 bags. The rounding happens once, at the very end, and it uses exact arithmetic: if the quotient is a whole number, such as exactly 7 bags, the calculator says 7 and not 8. Adding the four post holes to the slab gives 47.03 cubic feet with 10% waste and 79 bags.

Worked example

Bags or ready-mix: comparing the arithmetic

For a small job the choice is between bags and a delivered load, and the calculator can total each one separately from prices you enter. As an illustration only, with bags at 5.50 each the 62 bags cost 341.00, while 1.358 cubic yards of ready-mix at 150.00 per cubic yard would cost 203.70. Those prices are invented for the example. Delivered concrete often has a minimum load charge and a delivery fee, and bagged concrete takes time to mix, so compare real quotes and the work involved, not only the unit price. If you are comparing bag sizes, the unit price calculator shows the cost per unit for each size.

What this calculator does not cover

The calculator estimates volume and the number of bags from the figures you enter. It does not design a concrete mix, choose a strength, size reinforcement, advise on curing, or judge whether a slab or footing is suitable for its load. It also leaves out stairs, curbs and gutters in this version, so those shapes need separate working. For structural work, ask a qualified professional and follow your local building rules. For the rest of a home project, you can also estimate tiles and grout for a finished floor or paint for the walls.

Concrete calculator questions

How do I calculate cubic yards of concrete for a slab?

Multiply length by width by thickness in feet, then divide by 27. A 10 ft by 10 ft slab at 4 inches thick is 33.33 cubic feet, or 1.235 cubic yards before any waste allowance.

How many bags of concrete do I need?

Divide the order volume by the yield printed on the bag and round up to a whole bag. For 36.67 cubic feet and a 0.6 cubic foot bag that is 62 bags. Always use the yield on your own product, because it differs between brands and bag sizes.

How much extra concrete should I order?

That is your decision and depends on how level the base is, how accurate the forms are and how the concrete will be delivered. The calculator opens with 10% so the effect is visible, and you can enter any value from 0 to 100. Ask your supplier what allowance they recommend for your job.

How do I calculate concrete for round post holes?

Choose the round hole or column shape, enter the diameter and depth, and set the quantity to the number of holes. Four holes 12 inches across and 36 inches deep come to 9.425 cubic feet.

Is a yard of concrete the same as a cubic yard?

In ordering, yes. People say yards when they mean cubic yards, which is 27 cubic feet. A cubic metre is a little more than a cubic yard: 1 m³ is about 1.308 yd³.

Can I add several areas to one estimate?

Yes. Add up to ten sections, such as a slab, a footing and some post holes. The calculator adds the volumes exactly, applies the waste allowance once to the total, and then counts the bags for the whole project.