About the Concrete Calculator
This concrete calculator tells you how much concrete to order or mix for a job. Choose a slab (patios, driveways, shed bases, footings) or a round column (sonotubes, deck piers and fence post holes), enter the dimensions and the number of identical pours, and it returns the volume in cubic yards, cubic feet and cubic meters plus the number of premixed bags.
It is meant for homeowners and contractors deciding between bagged concrete and a ready-mix truck. As a rule of thumb, jobs under about one cubic yard are usually mixed from bags, while larger pours are cheaper and more consistent from a ready-mix supplier, who sells by the cubic yard.
Thickness is entered in inches because that is how slabs are specified (4 inches is typical for patios and walkways, 5–6 inches for driveways). A waste allowance of 5–10% covers uneven subgrade, spillage and form bulge; bag counts use the typical yields of 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags.
With the default inputs, the concrete needed is 1.36 yd³. Change any value above to recalculate instantly.
How to use the concrete calculator
- 1Choose a rectangular slab or a round column / post hole.
- 2Enter the dimensions — length and width in feet, thickness or diameter in inches.
- 3Set how many identical slabs, footings or holes you are pouring.
- 4Add a waste allowance (5–10% is typical).
- 5Pick a bag size to see how many bags to buy, or use the cubic yards to order ready-mix.
Formula and method
For a rectangular slab or footing the volume in cubic feet is length × width × thickness, with thickness converted from inches to feet by dividing by 12. For a round column or post hole it is π × radius² × height, where the radius in feet is the diameter in inches ÷ 24. The result is multiplied by the number of identical pours and by (1 + waste %).
Cubic feet are converted to cubic yards by dividing by 27 (3 ft × 3 ft × 3 ft). Bag counts divide the total cubic feet by the typical yield of a premixed bag — about 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb — and round up, since you cannot buy part of a bag.
- L, W
- Slab length and width (ft)
- T
- Slab thickness (in)
- D, H
- Column diameter (in) and height (ft)
- V
- Volume in cubic feet
Worked examples
10 × 10 ft patio, 4 inches thick
The slab is 10 × 10 × (4 ÷ 12) = 33.33 ft³. Adding 10% waste gives 36.67 ft³, or 1.36 cubic yards. That is 62 bags of 80 lb concrete, or about $204 of ready-mix at $150 per yard.
Four 12-inch sonotubes, 4 ft deep
Each tube holds π × 0.5² × 4 = 3.14 ft³, so four tubes need 12.57 ft³. With 5% extra that is 13.19 ft³ (0.49 yd³), which works out to 30 bags of 60 lb mix.
24 × 20 ft driveway, 6 inches thick
A 24 × 20 ft slab at 0.5 ft thick is 240 ft³. With 8% waste you need 259.2 ft³ — exactly 9.6 cubic yards, about $1,440 of ready-mix at $150/yd³. Bagging it would take 432 bags of 80 lb, which is why driveways are poured from a truck.
Frequently asked questions
How many bags of concrete are in a cubic yard?+
A cubic yard is 27 cubic feet, so it takes about 45 bags of 80 lb concrete (0.60 ft³ each), 60 bags of 60 lb, or 90 bags of 40 lb.
How thick should a concrete slab be?+
Patios, walkways and shed bases are usually 4 inches thick. Driveways that carry cars are typically 5–6 inches, and slabs for heavy trucks or equipment are thicker and should be engineered.
How much extra concrete should I order?+
Most contractors add 5–10% for uneven subgrade, spillage and forms that bow out. Running short mid-pour creates a cold joint, so it is better to have a little left over.
Should I use bags or order ready-mix?+
Bags make sense for small jobs under roughly one cubic yard, such as post holes or a small pad. Larger pours are usually cheaper and more uniform from a ready-mix truck, though many suppliers charge a short-load fee under a minimum volume.
How do I calculate concrete for a round post hole?+
Multiply π by the radius squared by the depth, with all measurements in feet. A 12-inch hole 4 ft deep is π × 0.5² × 4 ≈ 3.14 cubic feet, or about 5–6 bags of 80 lb mix.