How to use the concrete calculator
Choose slab or footing for anything with a rectangular shape: a patio, a shed base, a sidewalk, a step or a strip footing. Enter the length and width in feet and the thickness in inches. For an L-shaped patio, split it into two rectangles and add the results together.
Choose round holes or columns for fence post holes, deck footings or concrete tube forms. Enter the diameter of the hole in inches, how deep it is in feet, and how many holes you will fill. If you set the posts in the holes first, the result will be slightly more than you need, which leaves a safety margin.
How the calculation works
Slab volume (cu ft) = length × width × (thickness ÷ 12)
Round hole volume (cu ft) = 3.1416 × (diameter ÷ 24)² × depth × number of holes
Cubic yards = cubic feet ÷ 27
Bags = cubic feet (plus waste) ÷ bag yield, rounded up
With the example above, a 10 × 10 ft slab that is 4 inches thick holds 33.3 cubic feet, or 1.23 cubic yards. Adding 10% for waste brings it to 36.7 cubic feet, which is 62 bags of 80 lb concrete.
How much concrete is in a bag?
Bagged concrete mix is sold by weight, but what matters is the volume it makes once mixed. These are the yields printed on standard bags of concrete mix in the US:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Yields can vary a little between brands and products such as fast-setting or high-strength mixes, so check the bag before you buy.
How thick should a concrete slab be?
- 4 inches is the usual thickness for patios, walkways and shed floors that only carry foot traffic and light loads.
- 5 to 6 inches is common for driveways and slabs that will carry cars or heavier equipment.
- Footings, foundations and anything structural depend on your soil and local building code. Check with your local building department or a contractor before you pour.
Why add extra concrete?
The ground under a slab is never perfectly flat, forms bow a little, and some concrete always ends up spilled or left in the wheelbarrow. Running out halfway through a pour is a real problem, because the first part starts to set before the rest arrives. Most people add 5 to 10% to the calculated volume. Use the higher number if the base is uneven or this is your first pour.
Bags or ready-mix delivery?
Mixing bags yourself makes sense for small jobs such as post holes, a few steps or a small pad. Once a job needs a cubic yard or more, that is 45 or more 80 lb bags to lift, mix and place, and a ready-mix truck usually becomes the more practical choice. Many concrete suppliers charge an extra fee for small loads, so call a few local suppliers and compare the delivered price with the cost of bags.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
45 bags. Each 80 lb bag makes about 0.60 cubic feet, and a cubic yard is 27 cubic feet. With 60 lb bags you need 60, and with 40 lb bags you need 90.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab needs 1.23 cubic yards (33.3 cubic feet) before waste. At 6 inches thick it needs 1.85 cubic yards.
How much concrete does a fence post hole take?
A hole 10 inches across and 3 feet deep holds about 1.6 cubic feet, which is close to three 80 lb bags before you account for the post. Enter your own hole size above to check yours.
Can I use this for a concrete wall or steps?
Yes. For a wall, enter its length, its height as the width, and its thickness in inches. For steps, calculate each step as its own small slab and add them up.
This calculator gives an estimate for planning and buying materials. It does not replace engineering advice. Follow your local building code for footings and structural concrete.