Fence Post Hole Concrete Calculator: Bag Counts vs. the Alternatives
Reviewed by the OnlineFree.app team · Updated
Key points
- A 9 in hole 30 in deep around a 4x4 post holds about 0.89 cubic feet of concrete, or roughly 2.4 fifty-pound bags.
- The Fence Post Hole Concrete Calculator subtracts the post's own volume, so ignoring it overshoots the bag count by about 20%.
- Ten fence posts on the default settings need 27 bags of 50 lb concrete or 17 bags of 80 lb.
- Bag yield is fixed by package size: 40 lb gives 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³ and 80 lb 0.60 ft³.
- Use a waste margin near 10% for uneven hand-dug holes, then round up to whole bags before you leave for the store.
What this concrete estimator gives you
Enter a post count, a hole diameter and a hole depth, and the answer comes back as one usable figure: the whole number of bags to buy. The same result card also shows total volume in cubic feet and cubic yards, a per-post breakdown, and a comparison table of bag counts at 40, 50, 60 and 80 lb so you can grab whatever size is actually on the shelf.
With the defaults — 10 posts, 9 in diameter, 30 in deep, 4x4 square posts, 50 lb bags and a 10% waste margin — the result is 27 bags, about 9.8 cubic feet, or roughly 0.36 cubic yards. Every field updates the result as you type, and on a phone the bag count sits in a sticky bar above the inputs, because that is where it is useful when you are standing in an aisle re-checking a hole size.
The point of the tool is narrow on purpose: one trip to the hardware store, with the right number of bags. If you just want the count, the Fence Post Hole Concrete Calculator gets you there in under 30 seconds. The rest of this article explains where the number comes from and when a different approach makes more sense.
How many bags of concrete per fence post?
On the defaults — a 4x4 post in a 9 in hole, 30 in deep, with 10% waste — each hole takes about 0.89 cubic feet of concrete, which is a little under 2.4 bags of 50 lb mix. Buying per post, round that to 3 bags.
Bigger posts and wider holes scale fast. A 6x6 post in a 12 in hole 36 in deep needs about 1.9 cubic feet per hole once waste is included; ten of those is roughly 19 cubic feet, or 51 bags at 50 lb, 32 bags at 80 lb, or about 0.70 cubic yards.
The per-post number is the one to keep in your head while digging. Mix about 3 bags for the first post on a standard job, and if you finish with more than half a bag spare, your holes are running narrower or shallower than what you typed in.
Bagged concrete vs ready-mix and other alternatives
For most residential fence jobs, the delivery truck does not pencil out. Ready-mix is sold by the cubic yard, often with minimum loads and short-load fees, and the default ten-post job is only about 0.36 cubic yards — small enough that minimums usually swamp any per-yard saving. As of 2026, the practical split is: haul bags for anything under roughly a yard, and start calling suppliers once an estimate climbs toward two yards or more, asking what their minimum is.
Tamping posts into compacted soil, backfilling with crushed gravel, and using a foam setting product are all real alternatives that many builders use. Gravel drains well and is easier to dig out later, and foam sets quickly without water, but neither adds the same mass against frost heave and lateral racking as a concrete collar in a hole dug below the frost line. The estimator models concrete volumes only, so it cannot size a gravel backfill for you.
If you mix your own from portland cement, sand and gravel, the bag yields here do not apply. The tool assumes packaged concrete and uses the published yields printed for each bag size.
How the bag count is calculated
Each hole is treated as a cylinder: π × (diameter ÷ 2)² × depth. A 9 in hole that is 30 in deep holds 1,908 cubic inches of space, or about 1.10 cubic feet. From that, the tool subtracts the volume of the post itself, using actual dressed dimensions — a nominal 4x4 is counted as 3.5 in × 3.5 in — which is why the bag count is lower than a naive hole-only estimate.
On the default job the post occupies 0.21 cubic feet and leaves 0.89 cubic feet of concrete per hole. That figure is multiplied by the post count, the waste margin is added (10% by default, adjustable from 0 to 30%), and the total is divided by the bag yield: 0.30 ft³ for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. The final count is rounded up to whole bags.
Those yields come from packaged-concrete manufacturers' published tables. ASTM C387 is the standard covering packaged dry combined concrete materials, and any given brand prints its own yield on the bag, so check the pallet if you want the most exact figure. If you would rather model several fence layouts side by side, free spreadsheet templates such as those at Feuille De Calcul Gratuite work alongside the calculator.
Mistakes that throw the estimate off
The most common error is entering the auger's nominal size instead of the hole it actually cuts. A 9 in auger in soft ground can drill a 10 in hole, and because volume scales with the square of the diameter, that turns 27 bags into about 34 on the same ten-post job.
Depth is the second trap. The calculator warns when you enter less than 32 in, which is a common minimum, but frost depth varies widely — parts of the northern United States run 48 in or more. Your local building department, not a default value, is the authority on how deep to go; a frost line reference such as the Wikipedia entry on frost lines is a starting point for understanding why.
Two more quiet ones: the estimate assumes the post occupies the full depth and that you fill to a slight mound above grade, so a gravel base or a stone under the post means you need less concrete than calculated for that depth. And a waste margin under about 10% tends to leave you a bag short on the final hole.
What the calculator doesn't cover
It does not design the fence. Post spacing, wind load, beam sizing and how deep a 6 ft privacy fence needs to go are code and structural questions that belong with your local building department, not with a volume formula.
It also cannot see your ground. Rocky soil, voids, and holes that collapse and widen all push the real volume above the number on screen, and cost per bag varies by brand and region, so the tool will not tell you what the job costs. Treat the count as a floor, round up, and keep the receipt — unopened bags are far easier to return than to buy twice. If you need other everyday calculators, the OnlineFree.app free online tools collection runs in the browser with no account.
Frequently asked questions
How many 50 lb bags of concrete do I need for 10 fence posts?
About 27 bags of 50 lb concrete, based on 9 in diameter holes 30 in deep for 4x4 posts with a 10% waste margin. The Fence Post Hole Concrete Calculator subtracts the volume the post itself displaces, which lowers the count. Using 80 lb bags, the same job needs roughly 17 bags.
Is the Fence Post Hole Concrete Calculator free to use?
Yes. It is free, needs no account or email, and the calculation updates live in the page as you change post count, hole diameter, hole depth, post size, bag size or waste margin. That live feedback matters when you are at the store deciding between a 50 lb and an 80 lb pallet.
Should I use concrete or just tamp soil around fence posts?
Concrete is the more common choice for wood privacy fences because the collar adds mass against frost heave and lateral racking. Compacted soil, crushed gravel and foam setting products are legitimate alternatives for short or light fences. The Fence Post Hole Concrete Calculator only estimates concrete volumes, so it will not size a gravel backfill.
What hole diameter and depth should I enter?
Enter the hole your auger actually cuts, not the nominal size, and a depth that reaches below your local frost line. A 9 in hole and 30–32 in depth is a common starting point for a 4x4 post. The calculator flags depths under 32 in, but confirm your own frost depth locally.
Does the calculator subtract the volume of the post?
Yes, and it uses actual dressed dimensions, so a nominal 4x4 counts as 3.5 in square rather than 4 in. On a 9 in hole 30 in deep, the post displaces about 0.21 of the 1.10 cubic feet of hole volume — roughly a 20% reduction in the bag count compared with ignoring the post entirely.