Concrete Calculator
Calculate the concrete for a slab, patio, footing or fence post. Enter the size and thickness to get cubic yards, plus the number of 60 lb and 80 lb bags if you are mixing by hand.
Short answer: A 10 ft × 10 ft slab at 4 inches thick needs about 1.23 cubic yards of concrete — that is roughly 56 bags of 80 lb mix if you are mixing it yourself.
Fill in the fields above to see your result.
Standard slab thickness is 4 inches. Footings are usually 8–12 inches deep. Order 5–10% extra for waste.
How this concrete calculator works
Every material here comes down to a volume. The area is measured in square feet, the depth is converted from inches to feet, and the result is converted from cubic feet into the unit you actually buy in — cubic yards, tons, or bags.
length (ft) × width (ft) = sq ftsq ft × depth (in) ÷ 12 = cubic feetcubic feet ÷ 27 = cubic yardsBag counts on this page are rounded up to whole bags, because you cannot buy part of a bag. Rounding up also gives you a little spare material, which is what you want on a real job.
Concrete for common slabs at 4 inches thick
| Slab size | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 ft × 8 ft | 0.79 cu yd | 36 bags | 48 bags |
| 10 ft × 10 ft | 1.23 cu yd | 56 bags | 75 bags |
| 12 ft × 12 ft | 1.78 cu yd | 80 bags | 107 bags |
| 20 ft × 20 ft | 4.94 cu yd | 223 bags | 297 bags |
One 80 lb bag yields about 0.60 cu ft and one 60 lb bag about 0.45 cu ft, so a cubic yard takes 45 or 60 bags. Bag counts exclude waste.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
It takes 45 bags of 80 lb concrete mix to make one cubic yard, because each bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet.
How thick should a concrete slab be?
Four inches is standard for patios, shed floors and walkways on well-compacted ground. Use 5 to 6 inches where vehicles will park, and add rebar or mesh either way.
How many bags of concrete do I need for a fence post?
A typical 4x4 post in a 10 inch diameter hole 3 feet deep needs about 1.6 cubic feet of concrete, which is roughly 3 bags of 80 lb mix. Wider or deeper holes take proportionally more.
Should I order ready-mix or mix bags myself?
Ready-mix delivery is usually cheaper and much easier above about one cubic yard. Mixing by hand is practical for small jobs under roughly half a yard, but 45 bags per yard gets old quickly.
How much extra concrete should I order?
Order 5 to 10 percent more than the calculated volume. Running short mid-pour is expensive and leaves a cold joint, and most suppliers charge a short-load fee anyway.
Other calculators
These figures are estimates for planning only. Bag sizes, stone densities and paint coverage vary by product and supplier, so check the label or ask your yard before you order. Last updated 2026-09-30.