Concrete calculator
This concrete calculator turns the size of a slab, footing, round column or set of steps into cubic yards of ready-mix and a count of 40, 60 or 80 lb bags, with extra added for waste.
- Slabs, footings, round columns and steps
- Cubic yards for a ready-mix order
- Bag counts from the manufacturer’s yields
Last updated
How to use the concrete calculator
- Pick the shape: a slab or footing (any rectangle), a round column or post hole, or a set of steps.
- Enter the measurements. Length and width are in feet; slab thickness, column diameter and step sizes are in inches.
- Enter how many of that shape you are pouring, and the extra for waste. 5 to 10 percent covers spills and an uneven base.
- Read the cubic yards for a ready-mix order, or the number of 80, 60 or 40 lb bags if you are mixing it yourself.
How to calculate concrete
Concrete is ordered by volume. Find the volume in cubic feet, add a little for waste, then divide by 27 because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet.
cubic yards = length (ft) × width (ft) × thickness (in ÷ 12) ÷ 27
A round column or post hole is a cylinder: π × radius² × height, with the diameter in inches divided by 24 to give the radius in feet. A flight of steps poured solid is a stack of blocks: the bottom step is one rise tall, the next two rises, and so on, so the volume is width × run × rise × (1 + 2 + … + number of steps).
For bags, divide the cubic feet by the bag’s yield. QUIKRETE’s data sheet gives about 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. Always round bags up.
Worked example
A 20 ft × 12 ft patio slab, 4 inches thick, with 10% extra.
- Volume: 20 × 12 × (4 ÷ 12) = 80 cubic feet.
- Add 10% for waste: 80 × 1.10 = 88 cubic feet.
- Cubic yards: 88 ÷ 27 = 3.26 cubic yards.
- Bags: 88 ÷ 0.60 = 146.7, so 147 bags of 80 lb mix (or 196 bags of 60 lb, or 294 bags of 40 lb).
At that size a ready-mix truck is the practical choice. Round up when you order, and ask the plant what increments it sells in and whether small loads carry an extra fee.
Coverage by thickness
How far one cubic yard goes at common thicknesses. To find it for any thickness, divide 324 by the thickness in inches.
| Thickness | Sq ft per cubic yard | Typical use |
|---|---|---|
| 3 in | 108 | Thin pads, overlays |
| 4 in | 81 | Patios, sidewalks, shed floors |
| 5 in | 64.8 | Heavier patios, hot tub pads |
| 6 in | 54 | Driveways, garage floors |
| 8 in | 40.5 | Footings, thickened edges |
The uses are common starting points, not a design. Your local code and the loads on the slab decide the real thickness.
Bags per cubic yard
Bag yields from the QUIKRETE Concrete Mix (No. 1101) data sheet, and the bags needed for one cubic yard (27 cubic feet), rounded up.
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Other brands are close but not identical, so check the yield printed on the bag you buy.
Common questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 inches thick is 33.3 cubic feet, which is 56 bags of 80 lb mix before waste. With 10% extra it is 36.7 cubic feet, or 62 bags of 80 lb mix (1.36 cubic yards).
How many 80 lb bags of concrete make a yard?
45 bags. An 80 lb bag yields about 0.60 cubic feet, and 27 ÷ 0.60 = 45. With 60 lb bags it is 60, and with 40 lb bags it is 90.
How do I calculate cubic yards of concrete?
Multiply length × width in feet by the thickness in feet (inches ÷ 12), then divide by 27. A 12 × 12 ft slab at 4 inches is 48 cubic feet, or 1.78 cubic yards.
How much concrete do I need for a post hole?
Use the round column option with the hole’s diameter and depth. A 10 inch hole 3 feet deep holds 1.64 cubic feet; subtract the post’s own volume if you want to be exact. That is about three 80 lb bags.
How much extra concrete should I order?
5 to 10 percent is typical. Use the higher end when the base is uneven or the forms might bow, because running short in the middle of a pour leaves a cold joint.
How much concrete do I need for steps?
Use the steps option. Three steps 4 ft wide with a 7 inch rise and 11 inch run hold 4 × 0.917 × 0.583 × 6 = 12.8 cubic feet, or 0.48 cubic yards before waste (22 bags of 80 lb mix).
Sources
- QUIKRETE Concrete Mix (No. 1101) data sheet. Bag yields: 40 lb 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60. Last checked 28 October 2026
See the methodology page for how YardageHQ builds and checks its calculators.