How we worked it out
Updated as you change the numbers.
- Volume = length × width × thickness = 10 ft × 10 ft × 0.333 ft (4 in) = 33.333 cu ft.
- Add 10% extra: 33.333 × 1.1 = 36.667 cu ft.
- Cubic yards = cubic feet ÷ 27 = 36.667 ÷ 27 = 1.358 yd³.
- 80 lb bags = 36.667 cu ft ÷ 0.60 cu ft per bag = 61.11, rounded up to 62 bags.
Method and assumptions
Concrete is ordered by volume. The calculator finds the volume of each piece (length × width × thickness for a slab, π × radius² × height for a round column), multiplies by the number of pieces and adds the extra you chose. One cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft).
Bag counts use the yield printed on standard concrete mix bags: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag 0.45 cubic feet and a 40 lb bag 0.30 cubic feet. High-strength and fast-setting mixes can yield slightly differently, so check the bag. Counts are rounded up to whole bags.
Ground is rarely perfectly flat, so the actual depth of a slab often runs a little over the form height, and some concrete is always lost to spills and the mixer. The waste allowance depends on the site: about 5% for a formed slab on flat, compacted ground, 10% for a typical job, and 15% for uneven or soft ground, hand-dug footings or many small pours.
The reverse questions run the same math backwards: the bags you have (or your budget ÷ the bag price) give a volume; after the waste allowance, dividing by the thickness gives the area it covers.
Tips
- Measure thickness in inches. A 4 in slab is 0.333 ft, not 0.4 ft: the calculator converts for you.
- For post holes, the concrete volume is the hole minus the post. Leaving the post in the calculation gives a small safety margin.
- Ready-mix is usually sold in steps of a quarter or half yard, so round your order up.
Questions
How much will 20 bags of 80 lb concrete pour?
20 bags make 12 cubic feet. At 4 in thick, with a 10% allowance for waste, that covers about 32.7 sq ft: a slab about 5.7 ft square.
How many 80 lb bags of concrete are in a cubic yard?
About 45. A cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags. With 60 lb bags it is 60, and with 40 lb bags 90.
How much concrete do I need for a 10 × 10 slab?
At 4 in thick, a 10 × 10 ft slab is 33.3 cubic feet, or 1.23 cubic yards. With 5% extra, that is 1.30 cubic yards, or 59 bags of 80 lb.
How thick should a concrete slab be?
4 in is the usual thickness for patios, walkways and shed floors. Driveways and slabs for heavier loads are commonly 5 to 6 in. Follow your local building code and any engineer’s drawings for structural work.
Sources
- QUIKRETE Concrete Mix (No. 1101): product data and yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft
- NIST Handbook 44, Appendix C: unit conversions
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.