Start with the dimensions that contain concrete
Measure the inside of the forms or use the dimensions on the project plan. A slab uses length, width and thickness. A strip footing uses the total run, width and concrete depth. A round form uses its inside diameter and concrete fill height. The same footprint can need very different quantities when depth changes.
For an L-shaped patio, split the footprint into non-overlapping rectangles. Record each section’s dimensions, calculate its volume, and add the net volumes. Count a shared corner only once. If the sections have different thicknesses, calculate them separately rather than applying one average thickness without checking the geometry.
Calculate concrete yards for a slab
When length and width are in feet and thickness is in inches, divide thickness by 12 before multiplying. The result is cubic feet. Divide by 27 to obtain cubic yards. Square footage alone cannot tell you the concrete quantity because the depth is still missing.
| Example thickness | Net ft³ | Net yd³ | At chosen 10% allowance |
|---|---|---|---|
| 4 in | 33.33 | 1.235 | 1.358 yd³ |
| 5 in | 41.67 | 1.543 | 1.698 yd³ |
| 6 in | 50.00 | 1.852 | 2.037 yd³ |
Footing and round-form worked examples
A rectangular footing 24 ft long, 18 in wide and 6 in deep has a net volume of 24 × 1.5 × 0.5 = 18 ft³. Divide by 27 to obtain 0.667 yd³. A chosen 10% allowance gives 0.733 yd³. These dimensions illustrate arithmetic; the calculator does not select suitable footing dimensions.
For one round form with a 12 in inside diameter and a 4 ft fill height, the radius is 0.5 ft. Its net volume is π × 0.5² × 4 = 3.142 ft³, or 0.116 yd³. Four identical forms contain 12.566 ft³ before allowance. Enter only the height filled with concrete.
Calculate concrete for round forms with diameter, height and count
Convert a metric concrete estimate
A 3 × 4 m area filled to a specified 100 mm depth has a net volume of 3 × 4 × 0.10 = 1.20 m³. With a chosen 5% allowance the planning volume is 1.26 m³, approximately 1.648 yd³. Keep millimetres, centimetres and metres consistent before multiplying.
One cubic yard equals 27 cubic feet and 0.764554857984 cubic metres. Use those conversions when comparing quotes offered in different units; an amount per cubic metre and an amount per cubic yard refer to different quantities.
How many bags of concrete do I need?
Divide planning volume by the mixed yield printed on your exact bag, then round up. The 10 × 10 ft, 4 in slab example contains 36.6667 ft³ after its chosen 10% allowance. At 0.60 ft³ per bag it needs ceiling(36.6667 ÷ 0.60) = 62 bags. At a different yield, the answer changes even if the package weight looks familiar.
Apply allowance once. If your calculator result already includes it, do not add the same percentage again before calculating bags. For delivered concrete, ask the supplier about order increments and minimum loads separately from your geometric estimate.
Compare whole bag quantities using your product’s mixed yield
Check the estimate before ordering
Check input units, measured thickness, repeated sections, openings and any displacement. A round post can occupy part of a round hole, but a square post needs different geometry. The post-hole tool’s optional subtraction applies only to a round post occupying the full entered concrete depth.
Use approved project dimensions and confirm your product yield and supplier ordering rules. The example thicknesses above are calculation inputs, not recommendations for a driveway, foundation or load-bearing slab.
Sources and further reading
- NIST: SI Units – Length ↗Exact international length definitions underpin the volume conversions used in these examples.
This article is general quantity-planning information. Confirm product requirements, project specifications, and local rules with the appropriate supplier or qualified professional.