Enter the size of your slab, footing, wall or round column to see how much concrete to order in cubic yards, or how many 40, 60 or 80 lb bags to buy.
How to use the concrete calculator
- Pick the shape. Slabs, footings, walls and steps are rectangles; Sonotube piers, deck footings and post holes are round.
- Enter the measurements in any unit. Thickness is usually easiest in inches and length in feet; the calculator converts everything.
- Set how many identical pours you have, such as eight deck footings of the same size.
- Keep an extra allowance unless your forms and subgrade are exact, then read the cubic yards to order or the bag count to buy.
Concrete volume formulas
Everything is converted to feet first, so the volume comes out in cubic feet. Divide by 27 to get cubic yards, because a cubic yard is 3 ft × 3 ft × 3 ft.
Slab, footing or wall: volume (ft³) = length (ft) × width (ft) × thickness (ft) Round column or hole: volume (ft³) = π × (diameter ÷ 2)² × height Cubic yards = cubic feet ÷ 27 With extra = volume × (1 + extra % ÷ 100) Bags = volume with extra ÷ bag yield, rounded up
A thickness of 4 inches is 4 ÷ 12 = 0.333 ft. Bag yields come from QUIKRETE’s data sheet for its Concrete Mix (No. 1101); other brands of standard concrete mix list similar yields per pound, but check the bag.
Worked example: a 10 × 10 ft patio slab
- Volume: 10 ft × 10 ft × 0.333 ft = 33.33 ft³.
- Add 10% extra: 33.33 × 1.10 = 36.67 ft³.
- Cubic yards: 36.67 ÷ 27 = 1.36 yd³, so order 1.5 yd³ of ready-mix.
- Bags: 36.67 ÷ 0.60 = 62 bags of 80 lb, or 82 bags of 60 lb.
Four 12-inch round piers, 4 ft deep, take 12.57 ft³ before extra and 0.51 yd³ with 10%
extra: 24 bags of 80 lb mix.
How many bags make a cubic yard?
| Bag size | Yield per bag | Bags per cubic yard | Weight per cubic yard |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 3,600 lb |
| 50 lb | 0.375 ft³ | 72 | 3,600 lb |
| 60 lb | 0.45 ft³ | 60 | 3,600 lb |
| 80 lb | 0.60 ft³ | 45 | 3,600 lb |
| 90 lb | 0.675 ft³ | 40 | 3,600 lb |
A cubic yard of bagged concrete is 45 bags of 80 lb mix, or 3,600 lb to lift, mix and place by hand. That is the point where many people compare bag prices with a ready-mix delivery, including any short-load fee your supplier charges for small orders.
Concrete per 100 square feet
For a quick check, here is the exact volume of a 100 ft² slab at common thicknesses, before any extra:
| Thickness | Cubic yards per 100 ft² | 80 lb bags per 100 ft² |
|---|---|---|
| 3 in | 0.93 | 42 |
| 4 in | 1.23 | 56 |
| 5 in | 1.54 | 70 |
| 6 in | 1.85 | 84 |
| 8 in | 2.47 | 112 |
Tips for an accurate order
- Measure thickness in several places. A slab that averages 4½ inches thick instead of 4 takes 12.5% more concrete.
- Enter inside form dimensions. Lumber forms are measured on the inside faces, where the concrete goes.
- Split odd shapes. Treat an L-shaped patio as two rectangles and add the results, or use the square footage calculator first.
- Plan the mixing water. QUIKRETE lists a starting point of 6 pints of water per 80 lb bag; the concrete bag calculator totals it for the whole job.
Questions
How many cubic yards is a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab is 1.23 cubic yards before any extra, and 1.36 yd³ with 10% extra.
How many 80 lb bags of concrete are in a yard?
45. Each 80 lb bag yields about 0.60 ft³, and 27 ÷ 0.60 = 45.
Can I use this for footings, walls and steps?
Yes. Footings and walls are rectangles: length × width × height. For steps, add one rectangle per step, using each step’s full height from the ground.
Why add extra concrete?
Excavations are rarely perfectly flat and forms can bow under the weight of wet concrete. Running short in the middle of a pour creates a cold joint, so a small surplus is cheaper than a second delivery.
Sources
- Concrete Mix Product No. 1101 data sheet, QUIKRETE.
Used for: Bag yields (40 lb ≈ 0.30 ft³ to 90 lb ≈ 0.675 ft³) and mixing water per bag. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact inch and foot definitions; cubic foot, cubic yard and gallon conversions.