How to use this concrete calculator
Measure the length and width of the area you're pouring and choose how thick the slab will be. If you're pouring several identical pads, such as footings for a deck or a row of fence post pads, set the number of slabs. Leave the waste allowance at 5–10% unless you have a reason to change it: ground is never perfectly flat, forms bow slightly, and some concrete always stays in the mixer or wheelbarrow.
The large number is the total volume to order. Below it you'll see how many bags of premixed concrete that equals, what the job would cost in bags and as ready-mix delivery at the prices you enter, and the approximate weight, which matters if you're hauling bags in a car or a small trailer.
The concrete volume formula
Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
Concrete is sold by the cubic yard in the US, so divide cubic feet by 27 (one cubic yard is 3 ft × 3 ft × 3 ft). In metric, multiply meters by meters by the thickness in meters to get cubic meters directly.
Worked example: a 12 × 10 ft patio, 4 inches thick. 12 × 10 = 120 ft². 120 × 4 ÷ 12 = 40 ft³. 40 ÷ 27 = 1.48 yd³. Adding 10% for waste gives 44 ft³, or about 1.63 cubic yards. In 80 lb bags, which yield about 0.6 ft³ each, that's 44 ÷ 0.6 = 73.3, so you'd buy 74 bags.
How thick should a concrete slab be?
| Project | Typical thickness | Volume per 100 ft² |
|---|---|---|
| Walkways, patios, shed floors | 4 in (10 cm) | 1.23 yd³ |
| Hot tub pads, garage floors | 5 in (13 cm) | 1.54 yd³ |
| Driveways, RV pads | 6 in (15 cm) | 1.85 yd³ |
These are common starting points, not engineering advice. Footings, foundations and anything that carries a structure are governed by your local building code, and may need a permit and inspection.
How many bags of concrete are in a cubic yard?
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Yields vary a little by brand, so check the bag. As a rough guide, once a job needs more than about one cubic yard, around 45 heavy bags, a ready-mix truck usually costs less and saves hours of mixing.
Tips for a better pour
- Compact the base. 4 inches of compacted gravel under the slab prevents settling and cracking. Our gravel calculator works out how much you need.
- Check the forms are level and square before you start. Measure both diagonals; they should match.
- Plan control joints about every 8–12 feet on a 4-inch slab, so cracks form along straight lines instead of at random.
- Keep it damp while it cures. Concrete gains strength slowly. Mist it or cover it with plastic for several days, especially in hot or windy weather.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 ft slab is 100 × 4 ÷ 12 = 33.3 cubic feet, or about 1.23 cubic yards. With 10% extra for waste, plan on about 1.36 cubic yards, or 62 bags of 80 lb mix.
How many 80 lb bags of concrete make a yard?
About 45. An 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags.
Is it cheaper to buy bags or ready-mix concrete?
For small jobs under about a cubic yard, bags are usually cheaper and easier. For larger pours, ready-mix is normally cheaper per yard and far less work, but many suppliers add a short-load fee for small orders. Enter your local prices in the calculator to compare.
Should I add extra concrete for waste?
Yes. Order 5–10% more than the calculated volume. Running short in the middle of a pour can leave a weak cold joint where fresh concrete meets concrete that has already started to set.
Results are estimates. Bag yields vary by brand, and structural work must follow your local building code.