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How many 80 lb concrete bags to set your posts.
With these inputs:
| Result | Value |
|---|---|
| Bags to buy | 39 |
| Hole Volume (each) | 1.364 cu ft |
| Post Displacement | 0.213 cu ft |
| Concrete Per Hole | 1.151 cu ft |
| 80 lb Bags Needed | 39 |
| Estimated Cost | $234.00 |
Use a hole 3× post width and 1/3 fence height deep.
The calculator computes the cylinder volume of each post hole, subtracts the volume the post itself takes up, multiplies by the number of posts, then divides by the standard 80 lb bag yield (≈ 0.6 cu ft).
Bags = ⌈(π × (d/2)² × depth − post²×depth) × posts / 1728 / 0.6⌉
At least 1/3 the above-ground fence height plus 6" of gravel for drainage. A 6-ft fence needs about 24–30" deep holes.
Typically 1–2 bags of 80 lb concrete per post for a standard 4×4 wood post in a 10" × 24–30" hole.
Yes — pour dry concrete in around the post, then add water on top. Works fine for residential fences but takes longer to cure.
One and a half to two 80 lb bags for a 10 inch wide, 30 inch deep hole. In 60 lb bags that is two to three per post. Run your exact hole size above rather than guessing.
A 6x6 wants an 18 inch hole, which roughly doubles the volume — plan on three to four 80 lb bags per post at 30 inches deep.
Yes, and it is what most fence crews use. You pour the dry mix around a braced post, add water on top, and it sets in 20 to 40 minutes. It reaches full strength in a few days, which is plenty for a fence.
Fast set and quick set are the same idea: no mixing, sets in under an hour, costs a little more per bag. Standard mix costs less and cures stronger, but you must mix it and brace the post overnight. For a typical residential fence, fast set wins on time.
Any 4,000 psi fast-setting post mix does the job. What matters far more than the brand is hole depth, a gravel base for drainage, and crowning the concrete above grade so water runs away from the post.
About 23 cubic feet, or roughly 39 bags of 80 lb mix, using 10 inch by 30 inch holes with 4x4 posts. Add one spare bag per ten holes for irregular hole shapes.
Yes. Four to six inches of gravel lets water drain away from the post end grain instead of sitting in a concrete bathtub. This single step adds years to a wood post.
Quikrete Fast-Setting mix yields about 0.375 cu ft per 50 lb bag and 0.6 cu ft per 80 lb bag. A 10 inch by 30 inch hole with a 4x4 post takes roughly 1.15 cu ft, so that is two 80 lb bags or three 50 lb bags per post.
Sakrete Fast Setting yields the same as Quikrete — around 0.6 cu ft per 80 lb bag and 0.45 per 60 lb bag. Run your hole size above and use those yields; the brands are interchangeable for fence work.
Quikrete's own rule is to use a hole three times the post width and one third the post length, then allow one to two 50 lb bags per hole. This calculator does the same math from your exact hole dimensions instead of a rule of thumb.
Yes. Pour the dry mix in around a braced post to about three inches below grade, add the water listed on the bag, and let it set. Do not stir it — the mix wets through on its own in a few minutes.
At 8 ft post spacing that is about 14 posts, or roughly 27 bags of 80 lb mix for 10 inch by 30 inch holes. Add a couple of spares for the gate posts, which usually get wider, deeper holes.
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