How Much Concrete for a 6×6 Post?
A 6×6 post in a 16-inch hole 36 inches deep takes 3.56 ft³ of concrete: 10 bags of 50-lb, 8 bags of 60-lb or 6 bags of 80-lb mix per post.
- Concrete per hole3.56 ft³
- 50-lb bags10
- 60-lb bags8
- 80-lb bags6
Concrete needed
3.56 cubic feet
0.132 yd³ · 0.101 m³ · no extra added
You need about 3.56 cubic feet of concrete, or 6 bags of 80 lb mix.
Bags of premix
- 1240 lb bags
- 1050 lb bags
- 860 lb bags
- 680 lb bags
- —20 kg bags
- —25 kg bags
- Before extra0.13 yd³ (3.6 ft³)
- Weight of bags480 lb
- Per hole3.56 ft³ · 6 × 80 lb
- Ready-mix costAdd a price per yd³
- Bagged costAdd a price per bag
One 80 lb bag makes about 0.60 ft³ (manufacturer's published yield). Bags are rounded up.
Bagged mix makes sense at 0.13 yd³. A ready-mix truck for a load this small would usually come with a minimum-order or short-load charge.
Estimate only, not a quote or engineering advice. Disclaimer
Six-by-six posts (5.5 × 5.5 inches actual) carry gates, pergolas, carports and decks, so their holes are wider and deeper than a fence post’s.
Bags per 6×6 post by hole size
| Hole diameter × depth | ft³ | 50-lb | 60-lb | 80-lb |
|---|---|---|---|---|
| 12 × 30 in | 1.44 | 4 | 4 | 3 |
| 12 × 36 in | 1.73 | 5 | 4 | 3 |
| 14 × 36 in | 2.58 | 7 | 6 | 5 |
| 16 × 42 in | 4.15 | 12 | 10 | 7 |
| 18 × 48 in | 6.23 | 17 | 14 | 11 |
How the post changes the answer
A 6×6 post takes up 0.210 ft³ for every foot it sits in the hole, so the calculator subtracts it. In a 16 × 36 in hole the post displaces 0.63 ft³, the yield of about 1.7 bags of 50-lb mix.
Hole size
QUIKRETE's fence-post directions call for a hole about three times the post's width and about a third of the post's length deep. For a 6×6 that is roughly a 17-inch hole. For gates, decks and anything that carries a structure, check local code and frost depth, which may require deeper or wider footings.
For a whole fence, set the number of holes in the calculator above. See also concrete for a 4×4 post.
Frequently asked questions
How many bags of concrete for a 6x6 post?
In a 16-inch hole 36 inches deep: 10 bags of 50-lb, 8 bags of 60-lb or 6 bags of 80-lb mix. Bigger holes take more; see the table.
What size hole for a 6x6 post?
A common rule, from QUIKRETE’s directions, is a hole about 3 times the post width (about 17 inches for a 6×6) and about one-third of the post’s length deep. Local code can require more for structural posts.
Do I put gravel under a fence post?
Many builders put a few inches of gravel in the bottom for drainage. If you do, enter its depth in the calculator and it is left out of the concrete.
Other posts
Related
Sources
- QUIKRETE: Concrete Mix No. 1101 data sheet (Revised Oct 2022; checked Nov 11, 2026)