How to calculate concrete in cubic yards
Concrete is sold by the cubic yard, which is 27 cubic feet. Multiply length by width by thickness, all in feet, then divide by 27.
For round shapes, use π × radius² × height. For post holes, take the post's own area away from the hole before multiplying by the depth.
How many bags of concrete do I need?
Divide the cubic feet by what one bag of premixed concrete makes, then round up. A bag's yield is printed on the bag, and most brands are close to the figures below.
| Bag size | Makes | Bags per yd³ |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
Concrete slab sizes at 4 inches thick
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 10 × 10 ft | 1.23 | 56 |
| 10 × 12 ft | 1.48 | 67 |
| 12 × 12 ft | 1.78 | 80 |
| 12 × 20 ft | 2.96 | 134 |
| 20 × 20 ft | 4.94 | 223 |
| 24 × 24 ft | 7.11 | 320 |
Concrete for post holes, sonotubes and footings
Fence post holes: a typical hole is about three times the post's width and a third of the post's length deep, below your area's frost line. A 10 in wide, 36 in deep hole around a 4×4 post takes about 1.4 cubic feet, so buy 3 bags at 80 lb per hole. The post hole concrete calculator can suggest a hole size for your fence.
Sonotubes and piers: round tube forms for deck footings and columns. A 12 in tube that is 4 ft tall holds about 3.1 cubic feet, roughly 6 bags at 80 lb. See the sonotube calculator for every common tube size.
Footings: a strip footing under a wall or a square pad under a post is a long, deep slab. The footing calculator takes the width and depth in inches.
Concrete cost: ready-mix vs bags
Ready-mix is priced per cubic yard and delivered by truck, usually with a minimum order and a fee for small loads. Bagged concrete costs more per yard but suits small jobs and needs no delivery. Prices vary a lot by region, so enter your local prices above, or use the concrete cost calculator to include delivery fees.
Before you pour
- Measure in at least two places for length, width and depth. Ground is rarely flat, and the deepest spot sets how much you need.
- Book ready-mix a few days ahead and ask about the minimum order and short-load fees.
- Make sure the truck can get close. A chute only reaches so far, and a pump costs extra.
- Have tools and helpers ready before the truck arrives. Concrete starts setting quickly on hot days.
Common questions
How many bags of concrete make a cubic yard?
About 45 bags at 80 lb, 60 bags at 60 lb, 72 bags at 50 lb, or 90 bags at 40 lb.
Should I use bags or order ready-mix?
Bags are practical for post holes and small pads. Above about one cubic yard, mixing by hand gets slow and tiring, so a ready-mix delivery is usually easier. Ask your supplier about minimum orders.
Why add extra for waste?
Ground is rarely perfectly level, forms bow slightly and some concrete is always lost while pouring. Running short halfway through a pour is far worse than having a little left over.
How much does a cubic yard of concrete weigh?
Roughly 4,000 lb, or about 2 tons. Check what your truck or trailer can carry before you haul it yourself.
How many yards of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 ft slab needs 1.23 cubic yards, or about 1.36 cubic yards with 10% extra. That's 56 bags at 80 lb before the extra.
How much concrete do I need for a driveway?
Driveways are usually 5 to 6 inches thick. A 12×20 ft single-car pad at 5 inches needs about 3.7 cubic yards, or 4.1 with 10% extra, so order ready-mix rather than bags.
How do I work out a round slab?
Choose "Round or column", enter the slab's full width as the diameter, and enter its thickness as the height, for example 0 ft 4 in.