CONCRETE BASICS
How to estimate concrete for a slab
To estimate concrete, first find the slab volume in cubic feet, then convert that figure to cubic yards. Measure the length and width in feet and the thickness in inches. Convert thickness to feet by dividing it by 12, multiply the three dimensions to get cubic feet, and divide by 27 because one cubic yard contains 27 cubic feet.
For example, a 12 ft by 10 ft slab that is 4 in thick needs about 1.48 cubic yards before any overage. Selecting the 10% waste buffer raises that estimate to about 1.63 cubic yards. Round the order quantity up to a practical amount and confirm the final requirement with your concrete supplier.
Approximate bag coverage
Bag yields are estimates; mix design and manufacturer instructions determine actual coverage. The figures below use common approximate yields and should be checked against the product label.
| Bag weight | Approx. yield per bag | Approx. bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Frequently asked questions
Why add a waste buffer?
A small buffer can help account for spillage, minor measurement variation, and an uneven base. Actual site conditions differ, so use your judgment and supplier guidance.
Are the bag counts exact?
No. Yields vary between brands and mixes. This calculator uses common approximations and rounds bag quantities up; always verify coverage on the packaging.
Can I use this for a footing or column?
This tool estimates a rectangular slab. For irregular forms, footings, or columns, divide the shape into simpler sections and calculate each volume separately, or ask a qualified professional.
Estimates are for planning only and are not structural, engineering, or purchasing advice. Confirm dimensions, mix requirements, and ordering quantities with a qualified professional or supplier.