How the concrete calculator works
It multiplies length × width × thickness to get the volume in cubic feet, then divides by 27 to get cubic yards, which is how ready-mix suppliers sell concrete. For a round pad it uses the area of a circle instead. The waste percentage is added on top, because forms bulge, the ground is never perfectly level and running short halfway through a pour is far worse than having a little left over.
How many bags of concrete per cubic yard?
Bagged concrete is sold by weight, but what matters is how much space it fills once mixed. These are typical yields for standard concrete mix:
| 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 |
Check the yield printed on the bag you buy; fast-setting and high-strength mixes can differ slightly.
Bags or ready-mix?
Bags make sense for small jobs such as fence posts, steps or a shed base under about 1 cubic yard. Beyond that, mixing 45 or more 80 lb bags by hand is slow and hard to keep consistent, and a ready-mix delivery is usually easier. Many suppliers have a minimum order, so ask before booking a truck for a small pour.
Frequently asked questions
How thick should a concrete slab be?
A patio, shed base or walkway is usually 4 inches thick. Driveways that carry cars are often 5 to 6 inches, and anything carrying heavy vehicles needs a design from a professional.
How much does a cubic yard of concrete weigh?
Normal concrete weighs about 150 pounds per cubic foot, so a cubic yard is roughly 4,000 pounds, or about 2 US tons.
How much extra concrete should I order?
Add 5 to 10 percent. Use the higher figure when the ground is uneven or the forms are long and may bow outwards.