How much concrete you need
Find the volume from area and thickness: cubic yards = length × width × (thickness in inches ÷ 12) ÷ 27. A 10 × 10 ft slab at 4 inches thick is 1.23 cubic yards (33.33 cubic feet). Ready-mix is ordered in cubic yards. If you only have the slab area, convert it with the square feet to cubic yards calculator.
How to use this calculator
Enter slab length, width and thickness in inches. The result shows cubic yards and cubic feet. For beds and paths, see the mulch volume method, which uses the same depth-over-area logic. Most slabs need a reinforcing grid, which the rebar calculator lays out.
Order a little extra
Add about 5–10% for spillage, uneven subgrade and over-excavation. Standard slabs are 4 inches thick; driveways often 5–6 inches. Need wall finishes after the pour? See the paint calculator.
Ready-mix suppliers sell in quarter-yard increments and usually set a minimum load, so round your figure up to the next quarter yard rather than ordering an exact decimal. Below about half a cubic yard, bagged mix is often more practical. Check the subgrade before you order too: a base that sits half an inch lower than planned across a large slab quietly adds several percent to the volume you actually pour. Have the site, forms and helpers ready before the truck arrives, since ready-mix will not wait.
Frequently asked questions
- How do I calculate concrete?
- Multiply length × width × thickness in feet, then divide by 27 for cubic yards. Convert thickness in inches to feet by dividing by 12.
- How many cubic feet in a cubic yard?
- 27 cubic feet make one cubic yard, which is why the formula divides by 27.
- How thick should a concrete slab be?
- Patios and walkways are usually 4 inches; driveways and garage floors are 5–6 inches.
- Should I order extra concrete?
- Yes, about 5–10% for spillage and uneven subgrade. You cannot add to a short pour later.
- How many bags of concrete equal a cubic yard?
- About 60 sixty-pound bags, since each yields roughly 0.45 cubic foot. Eighty-pound bags yield about 0.6 cubic foot, so you would need around 45 of them.