Concrete Calculator
Area 1
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Area __n__
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No result yet. Enter length, width and thickness.
Buying options
Starts at 10%, the top of the 4% to 10% that NRMCA, the ready-mix industry association, says to order over the plan dimensions. That is one source, so treat 10% as a starting value and change it to suit the job.
Bag sizes are US bags, sold by the pound.
How this is calculated
How concrete volume is worked out for each shape
Each part of a pour is one of six shapes, and every shape comes down to a volume in cubic feet.
- Slab: length × width × thickness.
- Footing: length × width × depth. Enter each straight run of a footing as its own area.
- Column or post hole: a cylinder, π × (diameter ÷ 2)² × depth.
- Steps: width × run × rise × n × (n + 1) ÷ 2, where n is the number of steps.
- Round slab: π × (diameter ÷ 2)² × thickness, for a round pad such as the base of a hot tub or a fire pit.
- Wall: length × height × thickness. Enter each straight stretch of wall as its own area; the footing under it is a separate footing area.
The steps formula counts the flight as one solid block without a landing. Step one is one rise high, step two is two rises high, so the heights add up to n × (n + 1) ÷ 2 rises.
These are geometry, not published rules. No bag maker or concrete association prints a formula for a cylinder or a flight of steps, so the calculator shows its arithmetic under the result instead of pointing to a citation.
The volumes of all parts are added first. Waste goes on the total, and the total is divided by what one bag of ordinary concrete mix makes. Fast-setting and other special mixes make a different amount, so check the yield on the bag you buy.
Examples
Strip footing: 20 ft × 1 ft 4 in × 8 in
- Area: 20 ft × 1 ft 4 in = 26.7 sq ft = 2.48 m²
- Volume: 26.7 sq ft × 8 in = 17.8 cu ft = 0.66 cu yd
- With 10% waste: 17.8 cu ft + 10% = 19.6 cu ft = 0.72 cu yd
- Ready-mix order: 19.6 cu ft ÷ 27, rounded up to 0.01 cu yd = 0.73 cu yd
- Bags: 19.6 cu ft ÷ 0.60 cu ft per bag, rounded up = 33
- Weight of the bags: 33 bags × 80 lb = 2,640 lb
33 80 lb bags with 10% waste. Ready-mix order: 0.73 cu yd. Weight of the bags: 2,640 lb.
Post hole: 1 ft × 36 in
- Area: π × (1 ft ÷ 2)² = 0.8 sq ft = 0.07 m²
- Volume: π × (1 ft ÷ 2)² × 36 in = 2.4 cu ft = 0.09 cu yd
- With 10% waste: 2.4 cu ft + 10% = 2.6 cu ft = 0.10 cu yd
- Ready-mix order: 2.6 cu ft ÷ 27, rounded up to 0.01 cu yd = 0.10 cu yd
- Bags: 2.6 cu ft ÷ 0.45 cu ft per bag, rounded up = 6
- Weight of the bags: 6 bags × 60 lb = 360 lb
- Cost: 6 bags × $5.25 = $31.50
6 60 lb bags with 10% waste. Ready-mix order: 0.10 cu yd. Weight of the bags: 360 lb. Estimated cost: $31.50.
Porch steps with a landing: Area 1: 3 steps × 4 ft × 7 in × 11 in; Area 2: 4 ft × 3 ft × 4 in
- Area 1: 3 × 4 ft × 11 in = 11 sq ft = 1.02 m²
- Area 1 volume: 4 ft × 11 in × 7 in × 3 × (3 + 1) ÷ 2 = 12.8 cu ft = 0.48 cu yd
- Area 2: 4 ft × 3 ft = 12 sq ft = 1.11 m²
- Area 2 volume: 12 sq ft × 4 in = 4.0 cu ft = 0.15 cu yd
- Total volume: 12.8 cu ft + 4.0 cu ft = 16.8 cu ft = 0.62 cu yd
- With 10% waste: 16.8 cu ft + 10% = 18.5 cu ft = 0.69 cu yd
- Ready-mix order: 18.5 cu ft ÷ 27, rounded up to 0.01 cu yd = 0.69 cu yd
- Bags: 18.5 cu ft ÷ 0.60 cu ft per bag, rounded up = 31
- Weight of the bags: 31 bags × 80 lb = 2,480 lb
31 80 lb bags with 10% waste. Ready-mix order: 0.69 cu yd. Weight of the bags: 2,480 lb.
Garden wall: 16 ft × 3 ft × 8 in
- Area: 16 ft × 3 ft = 48 sq ft = 4.46 m²
- Volume: 48 sq ft × 8 in = 32.0 cu ft = 1.19 cu yd
- With 10% waste: 32.0 cu ft + 10% = 35.2 cu ft = 1.30 cu yd
- Ready-mix order: 35.2 cu ft ÷ 27, rounded up to 0.01 cu yd = 1.31 cu yd
- Bags: 35.2 cu ft ÷ 0.60 cu ft per bag, rounded up = 59
- Weight of the bags: 59 bags × 80 lb = 4,720 lb
59 80 lb bags with 10% waste. Ready-mix order: 1.31 cu yd. Weight of the bags: 4,720 lb.
Bags for common pours
| Shape and size | Volume | 40 lb | 60 lb | 80 lb |
|---|---|---|---|---|
| Slab: 10 ft × 10 ft × 4 in | 1.36 cu yd | 123 | 82 | 62 |
| Footing: 20 ft × 1 ft 4 in × 8 in | 0.73 cu yd | 66 | 44 | 33 |
| Column / post hole: 8 in × 48 in | 0.06 cu yd | 6 | 4 | 3 |
| Column / post hole: 1 ft × 36 in | 0.10 cu yd | 9 | 6 | 5 |
| Steps: 3 steps × 4 ft × 7 in × 11 in | 0.53 cu yd | 48 | 32 | 24 |
| Steps: 4 steps × 4 ft × 7 in × 11 in | 0.88 cu yd | 79 | 53 | 40 |
Bags needed for each bag size, with 10% waste. The volume includes the waste.
Sizing a post hole
- Hole diameter: 3 times the width of the post. Sources: Quikrete; Sakrete Fast Setting Concrete Mix.
- Hole depth: One third of the length of the post. Sources: Quikrete; Sakrete Fast Setting Concrete Mix.
- Gravel in the bottom of the hole: 4 to 6 in. Sources: Quikrete; Sakrete Fast Setting Concrete Mix.
Questions
How much concrete does a post hole take?
Choose "Column / post hole" and enter the diameter and depth of the hole. The calculator counts the hole as a plain cylinder. A post standing in it takes up some of that space, so the answer runs a little high, which leaves you a margin rather than a shortfall.
What do the steps numbers assume?
A solid block of steps with no landing. The first step is one rise high, the second two rises, and so on up the flight. If there is a landing at the top, add it as a slab with "Add another area". Filling the inside of the form with clean rubble, as Quikrete's step instructions describe, uses less mix than the solid figure.
Can one calculation cover a whole project?
Yes. Add an area for each part, each with its own shape. Three post holes are three areas. The breakdown lists the volume of every part, then the total the bag count is based on.
How deep and wide should my footing be?
That is set by your local building code, the load and the frost depth, and none of the sources this site uses gives a figure. Ask your building department before you dig. The calculator only turns the dimensions you were given into a quantity.
Why does the column depth field take inches?
Post holes and footings are normally measured in inches, so depth is entered that way. For a tall column, multiply the feet by 12. An 8 ft column is 96 in.
When does it make sense to order a truck instead of bags?
The second line of the result is the same volume in cubic yards, rounded up to 0.01, the unit ready-mix suppliers quote in. Switch "Buy as" to ready-mix and enter the supplier's price per cubic yard to see that cost, then switch back to compare it with bags. The guide on bags and ready-mix works through the comparison.
How heavy is the load?
With bags, the weight line is the bag count times the weight printed on the bag, so 45 bags of 80 lb are 3,600 lb. With ready-mix it is the volume times 150 lb per cubic foot, within the 140 to 155 lb that FHWA gives for normal-weight concrete. Check it against the rating of your trailer or truck.
Related tools
Guides
Sources
Every volume on this page is plain geometry, a box for slabs and footings, a cylinder for columns and post holes, stacked boxes for steps. No manufacturer or trade association publishes those formulas, so none is cited. Bag yields and the post hole sizing rules are each published by both Quikrete and Sakrete. The 4% to 10% ordering allowance comes from NRMCA alone. The ready-mix weight uses 150 lb per cubic foot, inside the 140 to 155 lb FHWA gives and the figure an Ohio University table of building material weights lists.
- Quikrete Concrete Mix, Product No. 1101, data sheet (October 2022), section Yield, accessed October 7, 2026
- Sakrete High Strength Concrete Mix, product page, yield per bag, accessed October 7, 2026
- NRMCA, CIP 31, Ordering Ready Mixed Concrete, accessed October 8, 2026
- NRMCA, CIP 8, Discrepancies in Yield, accessed October 8, 2026
- Quikrete, Setting Posts, project instructions, accessed October 8, 2026
- Sakrete Fast Setting Concrete Mix, technical data sheet, Rev. 04/16 (copy hosted by The Home Depot), accessed October 8, 2026
- FHWA, HRT-05-057, Appendix C, Proposed Revisions to the AASHTO Standard Specifications for Highway Bridges, unit weight of normal weight concrete, accessed October 8, 2026
- Ohio University, H-20, Weights of Building Materials, Agricultural Commodities, and Floor Loads for Buildings, Table 1, accessed October 8, 2026
- NIST Handbook 44 (2024), Appendix C, General Tables of Units of Measurement, accessed October 8, 2026
- NIST Special Publication 811, Appendix B.9, conversion factors listed by kind of quantity, accessed October 8, 2026
Last reviewed: October 8, 2026