A 6x6 post is 5 1/2" square once it is dressed. Set in a round hole 12" across with 36" of concrete, it takes 1.73 cubic feet of concrete: the hole holds 2.36 cubic feet and the post fills 0.63 of it. That is 4.6 bags of 50 lb fast-setting mix, 3.8 bags of 60 lb or 2.9 bags of 80 lb, so one post takes 5, 4 or 3 whole bags.
For a fence line, add up all the posts before you round. Ten 6x6 posts in the same holes take 17.26 cubic feet: 47 bags of 50 lb, 39 of 60 lb or 29 of 80 lb, rather than 5 whole bags for every post.
How the numbers are worked out
The hole is a cylinder: π × (diameter ÷ 2)² × depth = 3.1416 × 6² × 36 = 4,072 cubic inches. The post takes out 5.5 × 5.5 × 36 = 1,089 cubic inches, which leaves 2,983 cubic inches. Divide by 1,728 cubic inches per cubic foot: 1.73 cubic feet per post.
Bags = cubic feet ÷ the bag yield: about 0.375 cubic feet for a 50 lb bag of fast-setting mix, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag of regular mix. 1.73 ÷ 0.375 = 4.6, so 5 bags of 50 lb for one post. These are the same numbers the fence calculator gives.
Bags per 6x6 post by hole size
Bags per 6x6 post by hole size| Hole (across × deep) | Cu ft per post | 50 lb bags | 60 lb bags | 80 lb bags |
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| 10" × 30" | 0.84 | 3 | 2 | 2 |
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| 10" × 36" | 1.01 | 3 | 3 | 2 |
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| 10" × 42" | 1.17 | 4 | 3 | 2 |
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| 12" × 30" | 1.44 | 4 | 4 | 3 |
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| 12" × 36" | 1.73 | 5 | 4 | 3 |
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| 12" × 42" | 2.01 | 6 | 5 | 4 |
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| 16" × 30" | 2.97 | 8 | 7 | 5 |
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| 16" × 36" | 3.56 | 10 | 8 | 6 |
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| 16" × 42" | 4.15 | 12 | 10 | 7 |
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Bags for a row of 6x6 posts (12" × 36" holes)
Bags for a row of 6x6 posts (12" × 36" holes)| Posts | Cubic feet | 50 lb bags | 60 lb bags | 80 lb bags |
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| 5 | 8.63 | 24 | 20 | 15 |
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| 10 | 17.26 | 47 | 39 | 29 |
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| 15 | 25.89 | 70 | 58 | 44 |
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| 20 | 34.52 | 93 | 77 | 58 |
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| 30 | 51.78 | 139 | 116 | 87 |
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Before you dig
A common rule is a hole about 3 times the post width across and deep enough for 1/3 to 1/2 of the post's height above ground, below the frost line where your local code requires it. Call 811 before you dig. Leave any gravel in the bottom out of the concrete depth.
A square post only fits a round hole if its diagonal is smaller than the hole: 5 1/2" × 1.414 = 7 3/4" for a 6x6, and the smallest hole in the table is 10". For deck posts and anything else that carries a load, the footing size and depth come from your plans and local code, not from this page.
Frequently asked questions
How many 50 lb bags of fast-setting concrete per 6x6 post?
4.6 bags in a 12" hole with 36" of concrete, so buy 5 for one post. Ten posts take 47 bags, because the part bags add up.
What size hole for a 6x6 post?
A common rule is about 3 times the post width across, 16 1/2" for a 6x6, and 1/3 to 1/2 of the post's height above ground deep, below the frost line where your code requires it. This page works out a 12" × 36" hole; the table has the other sizes.
How many 80 lb bags for ten 6x6 posts?
29 bags for 17.26 cubic feet with 12" × 36" holes. With 50 lb fast-setting mix it is 47 bags.
Estimates only. Check measurements, product labels and local building codes before you buy or build.