Concrete Calculator
Work out how much concrete a slab, footing, round column or post hole needs, in cubic yards, cubic feet and cubic meters, and how many premixed bags to buy.
A 10 ft by 10 ft slab 4 in thick needs 1.36 cubic yards of concrete with 10% extra for waste, or 62 bags of the 80 lb size. To work it out, multiply length by width by thickness, with every measure in feet, to get cubic feet (33.33 here, 36.67 with waste), then divide by 27 for cubic yards.
This calculator estimates material quantities only. It is not structural or engineering advice: slab thickness, reinforcement, base preparation and footing depth depend on the load, soil, climate and your local building code, so check those with a qualified professional or your building department before you pour.
Concrete needed, with waste
1.36 cubic yards
- Cubic feet
- 36.67
- Cubic meters
- 1.04
- Before waste, cubic feet
- 33.33
Premixed bags (rounded up)
- 40 lb (18.1 kg) bags, 0.30 cu ft each
- 123
- 50 lb (22.6 kg) bags, 0.375 cu ft each
- 98
- 60 lb (27.2 kg) bags, 0.45 cu ft each
- 82
- 80 lb (36.2 kg) bags, 0.60 cu ft each
- 62
- 90 lb (40.8 kg) bags, 0.675 cu ft each
- 55
Bag yields are from the QUIKRETE Concrete Mix No. 1101 data sheet; check the yield on the bag you buy. For a big pour, ready-mix is usually ordered by the cubic yard or cubic meter.
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About the Concrete Calculator
Concrete is sold by volume, so the job is to measure the space you will fill. For a slab, footing or path, convert the thickness from inches to feet first, because 4 in is a third of a foot, then multiply length by width by thickness. For a round column or post hole, the volume is pi times the radius squared times the depth, so four 12 in tubes 4 ft deep hold 12.57 cubic feet, or 0.47 cubic yards, before any waste. Bases are rarely perfectly flat and forms bow a little, so the calculator adds 10% by default, which you can change. Ready-mix trucks deliver by the cubic yard or cubic meter, so a large pour is usually ordered that way. For small jobs, premixed bags list their yield: the QUIKRETE Concrete Mix No. 1101 data sheet states 0.60 cubic feet for an 80 lb bag and 0.45 for a 60 lb bag. The calculator divides your total by each yield and rounds up, since you cannot buy part of a bag.
Frequently asked questions
How much concrete do I need for a 10 by 10 slab?+
At 4 in thick, a 10 ft by 10 ft slab is 33.33 cubic feet, or 1.23 cubic yards. With 10% added for waste it comes to 36.67 cubic feet, or 1.36 cubic yards, which is 62 of the 80 lb bags or 82 of the 60 lb bags.
How many cubic feet are in a cubic yard?+
27, because a cubic yard is 3 ft long, 3 ft wide and 3 ft high. Divide cubic feet by 27 to get cubic yards, which is the unit ready-mix suppliers quote.
How many bags of concrete make a cubic yard?+
It depends on the bag size and the brand's stated yield. Using the yields on this page, a cubic yard takes 45 bags of the 80 lb size, 60 of the 60 lb size or 90 of the 40 lb size. Check the yield printed on the bag you buy, because mixes differ.
Should I use bags or order ready-mix?+
Bags suit small pours such as fence posts, steps or a small pad, where you can mix by hand or in a small mixer. A larger slab usually needs more concrete than one person can mix by hand, so ready-mix delivered by truck is often the practical choice. A local supplier can tell you their minimum order and whether a pump or a short-load fee applies.
Bag yields: QUIKRETE Concrete Mix data sheet, Product No. 1101 (revised October 2022), section Yield: 40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 and 90 lb 0.675 cubic feet per bag. Volumes use exact unit definitions: 27 cubic feet per cubic yard and 0.3048 m per foot. ManyTools is not affiliated with or endorsed by QUIKRETE.
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