How much concrete do I need?
Work out the volume in cubic feet, divide by 27 to get cubic yards, and add a little extra. Ready-mix is sold by the cubic yard. Bagged mix is sold by weight, and each bag yields a set volume.
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
A 10 × 10 ft patio at 4 inches: 10 × 10 × 4 ÷ 12 = 33.3 cubic feet, or 1.23 cubic yards. With 10% extra for spillage and an uneven base, plan on 1.36 yards. That's 62 bags of 80 lb mix, or 1.5 yards from a ready-mix plant.
The calculator handles slabs, footings, pads and round columns. Add each pour as its own area, or switch to sonotubes for post holes and piers.
Sonotube concrete calculator
For a round form or post hole, the volume is π × radius² × depth. Pick "Sonotubes or post holes only", enter the number of tubes, the diameter and the depth. The slab sizes above are ignored in that mode. Pick "Slab plus sonotubes" to price both in one order.
Cubic feet per tube = 3.1416 × (diameter in ÷ 24)² × depth (ft)
Four 12 in tubes, 4 ft deep: 3.1416 × 0.5² × 4 = 3.14 cubic feet each, 12.57 in total. With 10% extra that's 13.8 cubic feet, or 24 bags of 80 lb mix.
| Tube diameter | Cubic feet per foot of depth | 80 lb bags per foot | 60 lb bags per foot |
|---|---|---|---|
| 6 in | 0.20 | 0.33 | 0.44 |
| 8 in | 0.35 | 0.58 | 0.78 |
| 10 in | 0.55 | 0.91 | 1.21 |
| 12 in | 0.79 | 1.31 | 1.75 |
| 16 in | 1.40 | 2.33 | 3.10 |
| 18 in | 1.77 | 2.95 | 3.93 |
| 24 in | 3.14 | 5.24 | 6.98 |
Footing size and depth for decks and posts are set by your local code and frost depth. The calculator only tells you how much concrete fills the hole you plan to dig.
How many bags of concrete do I need?
Divide the cubic feet by the bag yield and round up. Standard concrete mix typically yields 0.60 cubic feet per 80 lb bag, 0.45 per 60 lb bag and 0.30 per 40 lb bag. Check the yield printed on the bag you buy, because blends vary.
| 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 |
80 lb bags are the cheapest per cubic foot. 40 and 60 lb bags are easier to carry and mix by hand.
Bag concrete calculator
Set "You'll buy it" to the bag size you want and the shopping list switches to bags. The panel below the list always shows all three bag counts and what ready-mix would cost, so you can compare.
Bags usually make sense below about 1 cubic yard. Above that you're mixing 45 or more 80 lb bags by hand or in a small mixer, which is slow and hard to finish before the first batch starts to set.
Concrete calculator cost
Ready-mix is priced per cubic yard, and most plants add a short-load fee when you order less than a minimum load. The minimum and the fee vary a lot by plant, so enter both in the price fields. Plants typically sell in quarter-yard steps, and the calculator rounds your order up to the next quarter yard.
Default prices are rough 2026 figures. Replace them with your local quote before relying on the total.
Slab thickness
4 inches is typical for patios, walks and shed floors. Driveways are usually 4 to 6 inches, and thicker where heavy trucks park. Footings depend on the load and your local code, so take the size from your plans or building department.
Pricing a concrete job
Most concrete contractors price material at cost plus markup, pass the short-load fee and any pump through at cost, and add labor for forming, placing and finishing. Switch to contractor mode to set your labor rate and markup, add base gravel or rebar from the other calculators to the same job, and send it as an estimate.
Questions
How many bags of concrete are in a yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix, based on typical yields of 0.60, 0.45 and 0.30 cubic feet per bag.
How many 80 lb bags of concrete do I need for a 10x10 slab?
At 4 inches thick, a 10 x 10 slab is 1.23 cubic yards, which is 56 bags of 80 lb mix. With 10% extra, buy 62.
How much concrete does a 12 inch sonotube need?
A 12 inch tube holds about 0.79 cubic feet per foot of depth. A 4 ft tube takes 3.14 cubic feet, or about 5.2 bags of 80 lb mix, so buy 6.
When should I order ready-mix instead of bags?
Above about 1 cubic yard, ready-mix is usually easier and often cheaper, even with a short-load fee. Below that, bags are usually simpler.
What is a short-load fee?
A charge ready-mix plants add when an order is below their minimum load. The minimum and the fee vary by plant, so ask when you order.