Concrete for common slabs and post holes
Each row includes 10% extra. A slab 4 in thick is the usual patio or walkway; garage floors and driveways run 5 to 6 in.
| Size | Cubic yards | 80-lb bags | 60-lb bags |
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| 10 × 10 ft, 4 in thick | 1.36 | 62 | 82 |
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| 10 × 12 ft, 4 in thick | 1.63 | 74 | 98 |
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| 12 × 12 ft, 4 in thick | 1.96 | 88 | 118 |
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| 16 × 16 ft, 4 in thick | 3.48 | 157 | 209 |
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| 20 × 20 ft, 4 in thick | 5.43 | 245 | 326 |
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| 24 × 24 ft, 6 in thick | 11.73 | 528 | 704 |
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| 8 in post hole, 2 ft deep | 0.03 | 2 | 2 |
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| 10 in post hole, 3 ft deep | 0.07 | 3 | 4 |
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| 12 in post hole, 4 ft deep | 0.13 | 6 | 8 |
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How this calculator works
A rectangle is length × width × thickness; a round hole or column is π × radius² × depth. Divide cubic feet by 27 for cubic yards, then add the extra for waste.
Bag counts divide that volume by what each bag yields once mixed: 0.60 cu ft for an 80-lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb, the yields QUIKRETE uses in its own quantity calculator. Counts round up to whole bags.
Above 1 cubic yard the tool suggests ready-mix, the same point at which the concrete slab cost calculator switches from bags to a truck, and it flags the short-load fee most plants charge below 6 cubic yards.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 in thick is 33.3 cubic feet. With 10% extra that is 1.36 cubic yards: 62 bags of 80 lb, or 82 bags of 60 lb. At that size ready-mix is usually easier than mixing bags.
How much concrete is in an 80-lb bag?
An 80-lb bag of concrete mix yields about 0.60 cubic feet, so a cubic yard takes 45 bags. A 60-lb bag yields 0.45 cu ft (60 per yard) and a 40-lb bag 0.30 cu ft (90 per yard), before any extra for waste.
Should I use bags or ready-mix concrete?
Bags suit anything under about 1 cubic yard: post holes, steps, small pads. Above that, the number of bags to mix by hand grows fast, and a ready-mix truck is the usual choice. Plants usually sell in quarter-yard steps, and most add a short-load fee below 6 cubic yards.
How much concrete do I need for a fence post?
A 10 in hole 3 ft deep holds 1.64 cubic feet. With 10% extra that is 3 bags of 80 lb or 5 bags of 50 lb per hole. The post itself takes up part of the hole, so that count leaves a little to spare.
Why add 10% extra concrete?
A base is never perfectly flat, forms bow slightly under the weight of wet concrete, and some is lost to spills and the mixer. Running short in the middle of a pour leaves a cold joint, which is weaker and shows. Ten percent is a common allowance; a carefully graded base can use less.
