Ready-mix concrete is sold by the cubic yard, and one cubic yard is 27 cubic feet. Bagged concrete is sold by weight, but what matters is how much volume each bag makes once it is mixed with water: an 80 lb bag yields about 0.6 cu ft of concrete.
How to use this calculator
- Pick the shape: rectangle for slabs, patios, walkways and pads, round for column forms and post holes.
- Enter the length and width, or the diameter, and the thickness. Units can differ: feet for length and inches for thickness is the usual way to measure a slab.
- If you are pouring several identical pieces, such as fence post footings, enter how many.
- Leave the extra for waste at 10 percent or change it. Uneven ground, forms that bow out and spillage all use up concrete, and running short in the middle of a pour is far worse than having some left over.
- Use the cubic yards figure to order ready-mix, or the bag count if you will mix it yourself. Add the price per cubic yard from your supplier to estimate the cost.
Worked examples
Backyard patio
12 ft × 12 ft, 4 in thick
- cubic yards, with 10% extra
- 1.96
- cubic feet
- 52.8
- tons
- 3.7
- bags of 80 lb
- 88
- bags of 60 lb
- 118
- bags of 50 lb
- 141
- bags of 40 lb
- 176
Shed base
8 ft × 10 ft, 4 in thick
- cubic yards, with 10% extra
- 1.09
- cubic feet
- 29.33
- tons
- 2.05
- bags of 80 lb
- 49
- bags of 60 lb
- 66
- bags of 50 lb
- 79
- bags of 40 lb
- 98
Single-car driveway
20 ft × 10 ft, 5 in thick
- cubic yards, with 10% extra
- 3.4
- cubic feet
- 91.67
- tons
- 6.42
- bags of 80 lb
- 153
- bags of 60 lb
- 204
- bags of 50 lb
- 245
- bags of 40 lb
- 306
Fence post footings
8 × 10 in diameter, 36 in deep
- cubic yards, with 10% extra
- 0.533
- cubic feet
- 14.4
- tons
- 1.01
- bags of 80 lb
- 24
- bags of 60 lb
- 32
- bags of 50 lb
- 39
- bags of 40 lb
- 48
Bags of 80 lb concrete for common slab sizes
| Area | 4 in deep | 5 in deep | 6 in deep |
|---|---|---|---|
| 4 × 4 ft | 9 | 12 | 14 |
| 8 × 10 ft | 45 | 56 | 67 |
| 10 × 10 ft | 56 | 70 | 84 |
| 12 × 12 ft | 80 | 100 | 120 |
| 10 × 20 ft | 112 | 139 | 167 |
Number of 80 lb bags, exact volume without extra for waste.
Bags or ready-mix?
Bags make sense for small jobs such as setting posts, pouring a pad for an air conditioner or filling a few column forms. A cubic yard takes 45 bags of the 80 lb size, and every one of them has to be lifted, mixed and placed by hand.
For a patio, a driveway or anything you want to pour in one continuous session, ready-mix delivered by truck is usually the better choice. Ask your supplier about their minimum order and any short-load fee before you book.
How the calculation works
Volume is length × width × thickness, with every measurement converted to feet first. Dividing the cubic feet by 27 gives cubic yards. For a round form or post hole the area is π × radius², multiplied by the depth.
The bag count divides the volume, including the extra for waste, by the yield of one bag and rounds up, because you cannot buy part of a bag. Weight uses about 140 lb per cubic foot, the unit weight a standard bagged mix reaches once placed, so one cubic yard comes to roughly 3,800 lb.
How thick should the slab be?
- A driveway for cars, pickups and SUVs needs at least 4 inches of concrete.
- Where trucks or large RVs park, plan on 5 to 6 inches.
- Walkways and patios that only carry foot traffic are usually no thicker than a car driveway, but local building codes and your soil come first, so check them before you set your forms.
How much concrete do I need for a form tube or round column?
Builder's form tubes come in diameters of 6 to 12 inches, and round column forms in diameters of 6 to 36 inches. Choose the round shape, enter the tube diameter and the depth or height, and set the quantity to the number of tubes. The calculator then gives the cubic yards and the bag count for all of them.
Common questions
How many bags of concrete are in a cubic yard?
It depends on the bag size. With 80 lb bags you need 45 bags per cubic yard, with 60 lb bags 60, and with 40 lb bags 90.
How much does a cubic yard of concrete weigh?
A standard concrete mix weighs about 140 lb per cubic foot once it is placed, so a cubic yard comes to roughly 3,800 lb. That is more than most pickup trucks and small trailers are rated to carry, which is one reason ready-mix arrives in a truck.
How much extra concrete should I order?
The ready-mix industry recommends ordering 4 to 10 percent more than the exact volume, and this calculator uses the upper end. Ground is rarely perfectly level and forms flex a little, and a second delivery or a run to the store halfway through a pour can leave a cold joint in the slab.
How many cubic meters is a cubic yard of concrete?
One cubic yard is 0.765 cubic meter. To convert your result, multiply the cubic yards by that number. One cubic foot is 28.317 liters.
How do I calculate concrete if I only know the square feet?
Change the thickness from inches to feet by dividing by 12 inches. Multiply the area in square feet by that thickness to get cubic feet. Then divide by 27 cubic feet to get cubic yards.
Can I use this calculator for a concrete wall or a curb?
Yes, for a straight wall or curb. Use the rectangle shape and enter the wall's length, its height in place of the width, and its thickness. Stairs are harder because every step is a different size, so work out each step separately and add them up.
How much concrete does a concrete truck hold?
Mixer-truck drums commonly hold 9 to 12 cubic yards, but the weight of a loaded truck limits how much it can actually carry. Ask your supplier for its smallest order, and any fee for a short load, before you book.