YardCalculate concrete
Bag counts use the standard yields on our Methodology page: an 80 lb bag makes about 0.6 cu ft of concrete, a 60 lb bag 0.45 cu ft and a 40 lb bag 0.3 cu ft. Estimates only - confirm your order with your supplier.
How to use the concrete calculator
- Pick the shape: a rectangle for slabs and footings, a circle for a round slab, or a cylinder for sonotubes, columns and post holes.
- Enter the size. Thickness is in inches for slabs; a post hole is measured by its diameter in inches and its depth in feet.
- Set how many you are pouring, and keep the 10% extra unless your base is perfectly level and measured.
- Read the result two ways: cubic yards if a ready-mix truck is coming, or bags if you are mixing yourself. Add prices to compare the two honestly.
Concrete formula
Concrete is just volume. For a slab: Length (ft) x Width (ft) x Thickness (in) / 12 = cubic feet, and cubic feet / 27 = cubic yards. For a round shape, the area is pi x radius squared. Bag counts come from the yield printed on the bag: an 80 lb bag makes about 0.60 cubic feet of concrete, a 60 lb bag 0.45 and a 40 lb bag 0.30, which is why one cubic yard takes about 45, 60 or 90 bags.
Worked example
A 10 ft x 10 ft patio slab, 4 inches thick: 10 x 10 x 4 / 12 = 33.3 cubic feet. 33.3 / 27 = 1.23 cubic yards. In 80 lb bags that is 33.3 / 0.60 = 55.6, so 56 bags. With 10% extra you would order about 1.36 cubic yards ready-mix, or 62 bags.
How thick should concrete be?
| Project | Typical thickness | Notes |
|---|---|---|
| Patio or walkway | 4 in | The standard for foot traffic, on a compacted gravel base. |
| Driveway | 4 to 6 in | Go thicker where heavy vehicles turn; reinforcement matters more than extra inches. |
| Shed or garage floor | 4 in | 6 in where vehicles or machinery sit. |
| Footings | Per local code | Footings must reach below the frost line; your building department sets the depth. |
These are common rules of thumb, not engineering specs. The gravel base under a slab matters too: size it with the gravel calculator.
Bags or ready-mix?
Bags win on small jobs: a few post holes, a small pad, anything under about half a cubic yard. Past roughly 1 cubic yard (45 bags of 80 lb), mixing by hand becomes slow, heavy work, and a ready-mix truck is usually cheaper per yard and far more consistent. Remember ready-mix trucks have a minimum load, and short loads often carry a fee, so a job just over a yard is the awkward middle: price both ways with the cost fields above. For the general bags-versus-bulk maths, see the bag vs bulk buying guide.
Tips before you pour
- Order once, order enough. Running short mid-pour leaves a cold joint and a visible weak line in the slab.
- Check the base, not just the top. The thickness you calculate is only real if the gravel base is level; a 1 inch dip across a slab quietly eats 25% more concrete.
- Plan the pour for the morning, have help lined up, and keep water for curing ready. Concrete waits for nobody.
- Results are planning estimates. For structural work, footings and anything needing a permit, follow local code and your inspector, not a calculator.
Frequently Asked Questions
How many bags of concrete do I need?
Divide your cubic feet by the yield of the bag: 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb, then round up. A 10 x 10 ft slab 4 inches thick is 33.3 cubic feet, so 56 bags of 80 lb, or 62 with 10% extra. The calculator above does this for any size.
How many 80 lb bags of concrete make a cubic yard?
About 45 bags. One cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 cubic feet, so 27 / 0.60 = 45 bags. For 60 lb bags the answer is 60, and for 40 lb bags it is 90.
When should I order ready-mix instead of bags?
Around 1 cubic yard is the usual tipping point. Below half a yard, bags are simpler and cheaper. Above a yard, a ready-mix truck is normally cheaper per yard, much faster and more consistent than mixing 45 or more bags by hand. Watch for minimum-load and short-load fees on small truck orders.
How thick should a concrete driveway be?
4 inches is the minimum for cars on a good compacted base, and 5 to 6 inches is safer where heavy vehicles turn or park. Reinforcement (rebar or mesh) and a proper gravel base matter as much as thickness.
How much extra concrete should I order?
Add 10%. Bases are never perfectly level, forms bulge slightly, and some concrete is always lost to spillage and the wheelbarrow. Ordering 10% extra is far cheaper than a second delivery or a yard of bags to finish a pour.
How many bags of concrete do I need for a post hole?
A typical 12 inch wide, 3 ft deep hole holds about 2.4 cubic feet, roughly 4 bags of 80 lb mix, before you subtract the space the post itself takes up. Deep or wide holes in soft soil can take double that, so measure your actual holes and use the cylinder option above.
How long does concrete take to cure?
You can usually walk on it after 2 to 3 days and drive on it after about 7 days, but full design strength takes about 28 days. Keep it moist for the first week for the strongest result, and follow the bag or supplier instructions.
How much does a cubic yard of concrete weigh?
About 2 tons (roughly 4,000 lb) once placed. That weight is why a yard of bagged mix, 45 bags of 80 lb, is not a one-person Saturday job, and why ready-mix wins on bigger pours.