How it works
Three inputs, an instant answer you can take straight to the hardware store or the ready-mix supplier.
Enter your slab size
Type the length and width in feet and the thickness in inches. Add how many identical slabs you are pouring.
Add a waste allowance
We suggest 5–10% extra for spillage and uneven ground. Adjust it to match how confident you are in your subgrade.
Get yards and bags
Instantly see the cubic yards for ready-mix and the number of 60 lb and 80 lb bags for a DIY pour.
Estimating concrete the simple way
Concrete is sold two ways, and which one you need depends on the size of your pour. Ready-mix, delivered by truck, is priced by the cubic yard. Bagged pre-mix from the hardware store is priced by the bag. This calculator gives you both so you can compare before you buy.
The math is straightforward. Volume equals length × width × thickness. The only trick is keeping your units consistent: length and width are in feet, but thickness is almost always given in inches, so we divide the thickness by 12 to convert it to feet. That gives cubic feet, and dividing by 27 converts to the cubic yards a supplier will ask for.
How many bags is that?
A standard 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet once mixed; a 60 lb bag yields about 0.45 cubic feet. So a full cubic yard takes roughly 45 of the 80 lb bags or 60 of the 60 lb bags — which is exactly why anything much over a yard is usually cheaper and far easier to order as ready-mix.
Why the waste allowance matters
Real ground is never perfectly flat and forms are never perfectly tight. Order the exact volume and you risk running short mid-pour, which for concrete means a cold joint and a weak spot. Adding 5–10% is cheap insurance; for a rough or over-excavated subgrade, lean toward 10%.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab poured 4 inches thick needs about 1.24 cubic yards, which is roughly 56 bags of 80 lb pre-mix or 74 bags of 60 lb. Add your waste allowance on top.
How many 80 lb bags are in a cubic yard?
About 45 bags. Each 80 lb bag yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet.
What thickness should my slab be?
Patios and walkways are typically 4 inches. Driveways and slabs that carry vehicles are usually 5–6 inches. Always follow your local code and any engineer's spec.
Should I use bags or order ready-mix?
Under about 1 cubic yard, bags are usually simpler and cheaper. Over a yard, ready-mix delivered by truck saves money and a lot of mixing labor.
Does this include rebar or gravel base?
No — this estimates the concrete volume only. Budget separately for a compacted gravel base, rebar or mesh, and forms.
More material calculators
Estimates are for planning only and assume standard material yields. Always confirm quantities with your supplier and follow local building codes before ordering or pouring.