Concrete Calculator

Enter the size of a slab, footing or column to estimate the volume of concrete in cubic yards, plus the number of bags if you know the yield per bag.

Shape

A box shape suits slabs, footings and walls. A cylinder suits columns, piers and posts.

Length

Width

Thickness

Each size has its own unit, so you can mix feet and inches (for example 10 ft × 12 ft × 4 in).

Quantity of identical pieces (1 to 10,000)

Allowance for waste (0 to 100%; empty = 0%)

Added on top of the net volume. No typical figure is suggested; decide it yourself.

Yield per bag (optional)

The volume of wet concrete one bag makes, as stated by the manufacturer. It is not the weight of the bag or the volume of the dry mix. Leave it empty to skip the number of bags.

Show the main result in

Concrete needed (with the allowance) —

Details
Needed, in cubic yards	—
Needed, in cubic feet	—
Needed, in cubic meters	—
Needed, in liters	—
Net volume (without the allowance)	—
Bags (rounded up)	—
Left over from the bags	—

This is an estimate, not a structural calculation. The real amount depends on formwork, uneven ground, spillage, pump lines and how the work is done.

The number copied is the volume needed, in the unit chosen for the main result.

How it is calculated

Volume of one piece. A box shape is length × width × thickness. A cylinder is π × (diameter ÷ 2)² × height.
Net volume = volume of one piece × quantity.
Volume needed = net volume × (1 + allowance). The allowance is added on top of the net volume.
Bags = volume needed ÷ yield per bag, rounded up (only when you enter the yield).

Every size is first converted exactly into meters from its own unit (1 ft = 0.3048 m, 1 in = 0.0254 m, 1 yd = 0.9144 m), so mixed units such as feet and inches are handled exactly. 1 cubic yard = 27 cubic feet = 0.764554857984 m³. The number of bags is rounded up once, from the exact volume, not from the rounded number on the screen. A cylinder uses an approximation of π with 60 decimal places.

Mixed units

Many slabs are measured in feet but poured to a thickness in inches. Choose a unit for each size: for a 10 ft × 12 ft slab that is 4 in thick, choose ft for the length and width, and in for the thickness. There is no need to convert 4 in to 0.3333 ft yourself.

Example

A slab 10 ft × 12 ft × 4 in, an allowance of 10%, and a bag that yields 0.6 ft³ (an example value only; use the yield printed on your bag):

Net volume: 10 × 12 × (4 ÷ 12) = 40 ft³ = about 1.4815 yd³
With the allowance: 44 ft³ = about 1.6296 yd³
Bags: 44 ÷ 0.6 = 73.33 → 74 bags

What is not included

Mix design, strength, reinforcing steel, formwork and prices are not covered.
Ready-mix orders are often placed in cubic yards or cubic meters with a minimum load. Ask the supplier how they measure and round.
The yield of a bag is set by the manufacturer for a given amount of water, so check the label. Weight is not converted to volume by this tool.

Sources

The length definitions come from NIST SP 811 Appendix B.8 and B.9 (foot, inch and yard rows read word for word). NIST Handbook 44 Appendix C (opened and searched) lists 27 cubic feet as 1 cubic yard and 1 cubic yard as 0.764 554 857 984 cubic meters, checked on November 1, 2026. The concrete formula is a general calculation, not taken from a source.

Frequently asked questions

How many cubic yards is a 10 x 12 slab that is 4 inches thick?

10 × 12 × (4 ÷ 12) = 40 ft³, which is about 1.48 cubic yards before any allowance.

How do I convert cubic feet to cubic yards?

Divide by 27.

Can I use feet for the length and inches for the thickness?

Yes. Each size has its own unit selector.

What is the yield per bag?

The volume of mixed concrete one bag makes. It is printed on the bag or the manufacturer's data sheet. This tool does not guess it from the weight of the bag.