Concrete calculator for slabs, footings, columns and steps
Get the concrete to order in cubic yards, cubic metres and 60 lb or 80 lb bags.
Enter all sizes to see the result.
Order
Bag yields (check your bag)
| Bag | Yield per bag | Bags needed | Total weight |
|---|
Buying ready-mix? Round up to your supplier's ordering step.
How this was worked out
These are estimates for ordering material. They assume square, level forms and stairs poured as a solid block. Uneven ground, forms that bulge and spillage all use more, which is what the waste allowance is for. Bag yields are typical values from the QUIKRETE Concrete Mix data sheet (October 2022). The yield printed on your bag is the one to use. This calculator does not design footings, slabs or stairs and does not check building code. Follow your plans and your local code.
How it works
Pick a shape and enter its sizes in inches, feet, millimetres or metres. The calculator works out the volume, adds your waste allowance, and divides by the yield of each bag size, rounding up to whole bags. Every step appears in the "How this was worked out" panel under the result.
How we calculate it
These are the same steps, in the same order, as the panel under the result.
- Dimensions: every size is converted to feet (US) or metres (Metric). 1 in = 25.4 mm and 1 ft = 0.3048 m, both exact.
- Formula for the shape:
- Slab or footing: V = length × width × thickness × count
- Round column or post hole: V = π × (diameter ÷ 2)² × height × count
- Square column: V = side A × side B × height × count
- Stairs, with n steps of rise R and run D, width W and top platform depth P: V = W × D × R × n(n−1)/2 + W × P × n × R. Each lower step is a solid block down to the ground. The top step is the platform. If the platform is the same depth as a tread, this equals W × D × R × n(n+1)/2.
- Cubic yards and cubic metres: cubic yards = cubic feet ÷ 27. Cubic metres = cubic feet × 0.028316846592. In Metric mode the panel starts from cubic metres: cubic yards = m³ ÷ 0.764555.
- Add waste: order volume = volume × (1 + waste ÷ 100).
- Bags: order volume ÷ yield per bag, rounded up to the next whole bag.
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 90 lb | 0.675 ft³ | 40 |
Forms are taken as square and level. Rebar and mesh take up very little volume and are ignored. Concrete shrinks about 2% as it hardens, and ready-mix is sold by fresh volume.
Sources: bag yields from the QUIKRETE Concrete Mix No. 1101 data sheet. The 10% waste default is the top of the range in NRMCA CIP 8, "Discrepancies in Yield". Unit factors from NIST SP 811 Appendix B.8.
Questions
How many 80 lb bags of concrete make a cubic yard?
About 45. A cubic yard is 27 cu ft. The QUIKRETE Concrete Mix data sheet puts an 80 lb bag at 0.60 cu ft, so 27 ÷ 0.60 = 45. A 60 lb bag makes 0.45 cu ft, so it takes 60. Some sites quote 0.66 cu ft for an 80 lb bag. Brands and mixes differ, so use the yield printed on your bag. Type it under "Bag yields" and the counts update.
How much extra concrete should I order?
NRMCA, the ready-mix industry group, says to order at least 4 to 10% more than the measured volume. Uneven ground, forms that bulge and spillage all use concrete. A 1/8 in error on a 4 in slab is already about 3%. The calculator starts at 10%, and you can set anything from 0 to 25%.
How many bags do I need for a 10 × 10 slab, 4 inches thick?
That slab is 33.33 cu ft, or 1.23 cu yd. It takes 56 bags of 80 lb with no extra, or 62 with 10% waste. In 60 lb bags it is 75, or 82 with waste.
How much concrete do I need for steps?
The calculator treats steps as solid blocks poured down to the ground, with the top step as the platform. Three steps with a 7 in rise and 11 in run, 36 in wide, with a 36 in platform, come to 20.56 cu ft (0.76 cu yd). That is 35 bags of 80 lb before waste. If you fill the inside with gravel or rubble, you will need less.
Should I use bags or order a ready-mix truck?
The result shows both. One cubic yard is 45 bags of 80 lb, which is 3,600 lb to lift and mix. Ready-mix is sold by fresh volume, in cubic yards or cubic metres. Take the order volume, round up to your supplier's ordering step, and ask about minimum loads and short-load fees before you decide.