Concrete Calculator
Volume of concrete and number of bags for slabs, footings and columns
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Calculates how much concrete a slab, strip footing, round column or post hole, round slab or set of stairs needs, in cubic yards, cubic feet and cubic meters, and how many pre-mixed bags that is. Use it before ordering ready-mix or buying bags.
How to use
- Under "Units", choose "ft / in" or "m / cm / mm".
- Choose a "Shape" and enter its dimensions and "How many". Use "Add another pour" for more parts, and switch on "Subtract this" for an opening that is left empty.
- Set "Extra for waste and spillage" (10% by default) to see "Concrete needed, including extra".
- The bag table shows how many 40, 50, 60 or 80 lb bags are needed. For another product, enter "Your bag's yield" from the bag.
How it works
- Volume is length × width × thickness; a round column is π × (diameter ÷ 2)² × depth. Stairs are treated as a solid block on flat ground. 1 cubic yard = 27 cubic feet.
- Bag yields are the approximate figures from the QUIKRETE Concrete Mix (No. 1101) data sheet, checked in October 2026: 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags.
- Only quantities are calculated. Slab thickness and reinforcement are not covered.
Questions and answers
- How many cubic yards for a 10x10 slab 4 inches thick?
- 10 ft × 10 ft × 4/12 ft = 33.3 cubic feet, which is about 1.23 cubic yards before any extra.
- How much extra concrete should I order?
- 5 to 10 percent is common. Add more if the ground is uneven or the forms may bulge. Change it in "Extra for waste and spillage".
- How many 80 lb bags of concrete are in a yard?
- One cubic yard is 27 cubic feet. With 0.60 cubic feet per 80 lb bag, that is 45 bags before extra for waste.
- Can I add several post holes at once?
- Yes. Choose "Round column or post hole" and enter the diameter, the depth and "How many" to get the total for all holes.