Fence Post Concrete Calculator
Enter the post size, the hole diameter and depth and the number of posts: you get the concrete in each hole, the bags per post and the total bags to buy, using the yields published on the bag. Both major manufacturers size the hole the same way — three times the post width, one third of the post length deep.
Bags to buy
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Results are estimates for planning purposes. Verify with your supplier, engineer, or a qualified professional before ordering material or performing work.
Checked against the manufacturer's own table
Sakrete prints a post-setting table on the data sheet for its Fast Setting Concrete Mix: bags needed for a 10 in hole, by depth. Running the same case through this calculator — a 3.5 in 4×4 post, no gravel base, 0.38 ft³ per 50 lb bag — reproduces it:
| Hole depth (10 in diameter) | Sakrete data sheet | This calculator |
|---|---|---|
| 12 in | 1.25 bags | 1.21 |
| 18 in | 2 bags | 1.82 |
| 24 in | 2.5 bags | 2.42 |
| 30 in | 3 bags | 3.03 |
QUIKRETE's example runs lower than the geometry: two 50 lb bags to set a 3 in post in a 9 in hole 30 in deep, where the volume works out at about 2.5 bags. That is the dry method at work — mix poured into the hole and then saturated is not consolidated, so it fills more space than the mixed yield printed on the bag. This calculator uses the mixed yield, which is the conservative answer: you finish with a spare bag rather than a trip to the store.
How the manufacturers say to set the post
- Size the hole first. Both QUIKRETE and Sakrete give the same two rules: the hole is three times the width of the post, and one third of the total post length deep. An 8 ft 4×4 wants roughly a 10 in hole, 32 in deep.
- Frost beats the rule of thumb. The one third rule is a starting point. If your building department publishes a frost depth, the bottom of the hole goes below it, whatever the arithmetic says.
- Gravel under the post. Sakrete's procedure places 4 to 6 in of gravel in the bottom and compacts it before the post goes in, so water drains away from the end grain instead of standing against it. Set the gravel field to match; it is concrete you are not buying.
- Plumb and brace before any mix goes in. Both procedures have you stand and support the post first, then fill.
- Dry method, two versions. QUIKRETE pours the dry mix in to within 3 to 4 in of the top, then adds about a gallon of water per 50 lb bag until the powder is saturated, filling that last 3 to 4 in with soil or sod. Sakrete fills the hole one third with water first, then adds the mix from the bag. Neither needs a mixer.
- Set time is not load time. QUIKRETE's fast-setting mix reaches final set in 20 to 40 minutes and its data sheet says to wait 4 hours before the post takes any strain. Gate posts are worth leaving overnight.
- Slope the top. Finish the concrete so water drains away from the post rather than pooling against it.
How it's calculated
- Depth of concrete
d concrete (in) = hole depth (in) − gravel base (in)The gravel under the post is not concrete. Sakrete's procedure places 4 to 6 in (102 to 152 mm) of gravel, compacted, before the post goes in.- Volume of the hole
V hole (ft³) = π / 4 × (hole diameter (in) / 12)² × d concrete (in) / 12- Volume the post takes up
V post = (post size (in) / 12)² × d concrete / 12 — round post: × π / 4- Concrete per post
V = V hole − V post- Bags
bags per post = ceiling(V / yield); total = bags per post × number of postsRounded up per hole, because a fast-setting post is a separate pour and an opened bag does not keep. Yields: 0.375 ft³ per 50 lb and 0.45 ft³ per 60 lb bag of QUIKRETE Fast-Setting Concrete Mix, 0.38 ft³ per 50 lb bag of Sakrete Fast Setting Concrete Mix, 0.60 ft³ per 80 lb bag of QUIKRETE Concrete Mix.- Hole size rule of thumb
hole diameter = 3 × post width; hole depth = 1/3 × total post lengthPublished in the same words by both manufacturers. Frost depth, soil and local code can all push the depth deeper — the rule of thumb is a starting point, not a specification.
Frequently asked questions
For a 4×4 post in a 10 in hole 30 in deep on a 4 in gravel base, the concrete works out to 1.0 cubic foot per post, which is 3 bags of 50 lb fast-setting mix. Drop the gravel and it is 1.15 cubic feet, which tips the same hole to 4 bags. A 6×6 in a 12 in hole at the same depth needs about 1.25 cubic feet, or 4 bags.
Both QUIKRETE and Sakrete publish the same rule: one third of the total post length. An 8 ft post gives a 32 in hole and 5 ft 4 in of fence above ground. Frost is the other constraint — the bottom of the hole should sit below the frost depth your building department uses, which in cold states is deeper than the one third rule.
Three times the width of the post, in both manufacturers' instructions. A 3.5 in 4×4 wants about a 10 in hole, a 5.5 in 6×6 about a 16 in hole. Wider is not better: it is more concrete for no extra holding power, and a 12 in auger on a 4×4 costs you about half again as much mix as a 10 in one.
Fast-setting mixes are designed for the dry method. QUIKRETE's instructions pour the dry mix into the hole to within 3 to 4 in of the top, then add about a gallon of water per 50 lb bag until the powder is saturated. Sakrete's fill the hole one third with water first, then add the mix from the bag. Either way the post is braced first, and neither product needs a mixer. A general-purpose concrete mix, by contrast, has to be mixed.
Because it is concrete you are not buying. Sakrete's post procedure puts 4 to 6 in of compacted gravel in the bottom of the hole so water drains away from the end grain of the post instead of standing on it. That gravel displaces the bottom 4 to 6 in of what would otherwise be concrete — around an eighth of the mix in a 30 in hole, and enough to change the bag count.
It matches Sakrete's published table exactly. Sakrete lists 1.25, 2, 2.5 and 3 bags for a 10 in hole at 12, 18, 24 and 30 in deep; the same geometry with a 4×4 post and no gravel gives 1.21, 1.82, 2.42 and 3.03. QUIKRETE's example comes out lower than the geometry — two 50 lb bags for a 3 in post in a 9 in hole 30 in deep, where the volume is nearer 2.5 bags — because dry mix poured into a hole is not consolidated and fills more space than the mixed yield on the data sheet.
QUIKRETE's fast-setting mix reaches final set in 20 to 40 minutes and its data sheet says to wait 4 hours before the post is subjected to any strain. That is the post standing, not the fence loaded — panels, gates and wind loads are a different matter, and a gate post is worth leaving overnight.
Per post, which is what this calculator does. Each hole is its own pour and a bag opened for the last post cannot be saved for tomorrow, especially a fast-setting one. Buying to the sum of the volumes and splitting bags between holes works only if you are batching in a mixer and placing continuously.
Yes, and plenty of fences are. Compacted gravel around the post drains better and can be reset years later, at the cost of holding less against overturning. Concrete holds a corner, gate or end post far better, which is why many crews concrete those and gravel the line posts in between.
Sources & references
- QUIKRETE Fast-Setting Concrete Mix No. 1004 — product data sheet: yields 0.375 ft³ (50 lb) and 0.45 ft³ (60 lb), method for setting posts, hole 3× the post diameter and 1/3 of the post height deep, final set in 20 to 40 minutes, 4 hours before load
- Sakrete Fast Setting Concrete Mix — technical data sheet: yield 0.38 ft³ per 50 lb bag, Posts and Poles procedure (4 to 6 in of compacted gravel, hole 3× the post width, depth 1/3 of the post length) and the published bags-per-post table for a 10 in hole
- QUIKRETE Concrete Mix No. 1101 — product data sheet (0.45 ft³ per 60 lb bag, 0.60 ft³ per 80 lb bag)
Content checked against these sources — last reviewed August 28, 2026.