Concrete calculator
Pick a slab, a wall footing or round post holes, and enter the size, the thickness or depth and how many. The calculator gives the volume in cubic yards and cubic metres and the number of 40, 60 or 80 lb bags, and draws the pour with a section through it.
Concrete calculator
Add your own allowance for spillage, uneven ground and form flex; your supplier can suggest one.
Bag yields from the QUIKRETE Concrete Mix data sheet: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³.
Concrete to order
2.47 yd³
- 80 lb bags
- 112
- 60 lb bags
- 149
- 40 lb bags
- 223
- 80 lb bags weigh
- 8,960 lb
- Pass: 4 in thick Within what the QUIKRETE Concrete Mix data sheet allows: pours 2 in (50 mm) thick or more.
Slab from above, and its section
Bags, by size
- 40 lb bags223 bags
- 60 lb bags149 bags
- 80 lb bags112 bags
Bags or ready-mix? Ready-mix plants set their own minimum loads and short-load fees, so ask your local supplier where the switch pays for your pour.
The formula
- Slab or footing: length × width × thickness (or depth), all in feet. A 4 in slab is 4 ÷ 12 = 0.333 ft thick.
- Round hole or column: π × (diameter ÷ 2)² × depth, times the number of holes.
- Waste: add your own allowance for spillage, uneven ground and forms that bow.
- Cubic yards: cubic feet ÷ 27. Cubic metres: cubic feet × 0.0283.
- Bags: the volume divided by one bag's yield, rounded up. Buy one bag size for the job.
Worked example
A 20 × 10 ft patio slab, 4 in thick: 20 × 10 × 0.333 = 66.7 ft³, which is 66.7 ÷ 27 = 2.47 yd³ or 1.89 m³. In 80 lb bags that is 66.7 ÷ 0.60 = 111.1, so 112 bags; in 60 lb bags, 149; in 40 lb bags, 223. With 10% waste added it is 73.3 ft³, 2.72 yd³ and 123 bags of 80 lb. Eight post holes 10 in across and 3 ft deep take 13.1 ft³: 22 bags of 80 lb.
| Slab | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 33.3 | 1.23 | 56 | 75 |
| 12 × 12 ft | 48.0 | 1.78 | 80 | 107 |
| 20 × 10 ft | 66.7 | 2.47 | 112 | 149 |
| 20 × 20 ft | 133.3 | 4.94 | 223 | 297 |
| 24 × 24 ft (two-car garage) | 192.0 | 7.11 | 320 | 427 |
| 20 × 20 ft at 6 in | 200.0 | 7.41 | 334 | 445 |
| Hole | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 6 in × 2 ft deep | 0.39 | 1 | 1 | 2 |
| 8 in × 2 ft deep | 0.70 | 2 | 2 | 3 |
| 10 in × 3 ft deep | 1.64 | 3 | 4 | 6 |
| 12 in × 3½ ft deep | 2.75 | 5 | 7 | 10 |
| 12 in × 4 ft deep | 3.14 | 6 | 7 | 11 |
Bags or ready-mix?
A cubic yard is 45 bags of 80 lb: about 3,600 lb to lift, carry and mix by hand. Ready-mix trucks deliver it mixed, but each plant sets its own minimum load and charges its own fee for a short load, so the point where a truck pays off is a question for your local supplier. Ask before you buy a pallet of bags for anything bigger than a small pad.
- Footings and slabs under a house are sized by your building code and inspector, not by this calculator: depth below frost, width for the soil, and reinforcement.
- Gravel base under a slab is a separate order; it is not concrete volume.
- Post holes: the calculator does not take off the post's own volume, so the count errs a little high.
Lay it out first
For a slab, "Open the slab as a plan" builds it as a room in FloorGen (one room per slab, up to 30), where you can place it beside the house, check clearances and see it in 3D. A footing or post holes are not rooms, so for those the button opens a blank plan to draw on. See how to draw a site plan for placing a patio or pad on the lot, and garage dimensions for slab sizes.
Questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 in thick, 33.3 ft³ (1.23 yd³): 56 bags of 80 lb, 75 bags of 60 lb or 112 bags of 40 lb, using QUIKRETE's yields of 0.60, 0.45 and 0.30 ft³ a bag. Add your waste allowance on top.
How many 80 lb bags of concrete make a yard?
45. A cubic yard is 27 ft³ and an 80 lb bag yields about 0.60 ft³, so 27 ÷ 0.6 = 45 bags. It takes 60 bags of 60 lb or 90 bags of 40 lb.
How do I calculate concrete for a post hole?
π × (diameter ÷ 2)² × depth, in feet. A 10 in hole 3 ft deep is 3.1416 × 0.417² × 3 = 1.64 ft³, which is 3 bags of 80 lb.
How thick should a concrete slab be?
It depends on what the slab carries and the ground under it, and anything structural follows your local code and inspector. The calculator takes any thickness; bagged concrete mix is made for pours at least 2 in thick, per the QUIKRETE data sheet.
How much extra concrete should I order?
There is no single published figure. Uneven ground, forms that bow and spillage all eat concrete, so add an allowance your supplier or contractor suggests, and type it in the waste field.