Method and guidance
How to use the post-hole concrete calculator
- Count only holes that share the same diameter and concrete-filled depth. Run separate calculations for different groups.
- Enter the diameter of the round hole and the depth that will actually contain concrete.
- If a post occupies the hole, select round or square and enter its actual diameter or width. Leave displacement off for anchors, sleeves, or shapes this version does not model.
- Review the waste allowance. Ten percent is a sourced planning starting point, not a rule for every site.
- Check the mixed-volume yield printed on the exact concrete product, then edit or clear the bag-yield field.
- Add a price per bag only when a whole-bag material-cost estimate is useful.
Formula and calculation order
Calculate My Project converts every dimension to meters, calculates the round-hole cylinder, subtracts the selected post prism or cylinder, and only then applies waste and whole-bag purchasing.
gross hole volume = π × (hole diameter ÷ 2)² × depth
round post displacement = π × (post diameter ÷ 2)² × depth
square post displacement = post width² × depth
exact concrete per hole = gross hole volume − post displacement
exact project concrete = exact concrete per hole × number of holes
concrete including waste = exact project concrete × (1 + waste percent ÷ 100)
exact bags = concrete including waste ÷ mixed yield per bag
purchase bags = round exact bags up to the next whole bag
estimated cost = purchase bags × price per bagThe implementation uses BigDecimal and a documented 50-digit π value. Display rounding never feeds bag count, leftover, or cost.
Independently verified worked example
Ten round holes, each 12 in in diameter and 36 in of concrete-filled depth, have a gross volume of 23.5619449… ft³. Ten centered 4 in round posts displace 2.6179939… ft³, leaving 20.9439510… ft³ of exact concrete before waste.
A 10% allowance adds 2.0943951… ft³, producing 23.0383461… ft³, or 0.8532721… yd³. At an entered yield of 0.375 ft³ per bag, the exact requirement is 61.4355897… bags. Purchase quantity rounds once to 62 bags, with an estimated 0.2116539… ft³ left after whole-bag rounding.
At $7.25 per bag, the material estimate is $449.50 before tax or delivery. These expected values are hard-coded independently and are not generated by the calculator function under test.
Material assumptions and sources
The 0.375 ft³ preset is the approximate mixed yield published for a specific 50 lb QUIKRETE Fast-Setting Concrete Mix used for post setting. Product yields vary, so this value is editable and the technical sheet or label for the product you buy controls. Read the QUIKRETE Fast-Setting Concrete data sheet (manufacturer source).
The editable 10% starting allowance follows Sakrete’s post-setting calculator guidance. Review Sakrete’s post-setting calculator (manufacturer guidance).
QUIKRETE publishes general post-setting guidance, including checking hole diameter and embedment depth. Calculate My Project does not convert that guidance into a structural recommendation or hidden dimension default. Review QUIKRETE’s setting-post instructions (manufacturer guidance).
None of these sources sponsors Calculate My Project, and no product or commercial relationship can alter the formula or result.
Common estimating mistakes
- Using total excavation depth even though gravel, soil backfill, or an unfilled space reduces the concrete-filled depth.
- Treating an irregular hand-dug hole as a perfect cylinder without a suitable allowance.
- Entering nominal post size when the actual width or diameter differs.
- Forgetting to subtract the post, or subtracting a post that does not occupy the full modeled concrete depth.
- Applying waste before displacement or rounding intermediate cubic feet before calculating bags.
- Treating bag weight as mixed-volume yield instead of checking the product label.
Limitations
This version models identical round, straight-sided holes and optional centered round or square posts. It does not model tapered or bell-shaped footings, sleeves, brackets, anchors, gravel, soil backfill, voids, multiple hole groups, or posts that occupy only part of the concrete depth.
The result is a material estimate, not footing or fence design. It does not select safe hole dimensions, embedment, concrete strength, post material, drainage, frost depth, wind resistance, utility clearance, or code requirements. Confirm those decisions with applicable instructions, local authorities, utilities, suppliers, or a qualified professional.
Frequently asked questions
Why subtract the post from the hole?
A post already occupies part of the cylindrical hole. When the selected post extends through the full entered concrete depth, its volume is not concrete and is subtracted before waste is applied.
What if my post is a nominal 4×4?
Enter the actual measured width when using square displacement. A nominal product name may not equal its finished dimension, and the calculator intentionally does not assume one.
Should hole depth include the gravel base?
No. Enter only the depth that will contain concrete. Gravel, soil backfill, and any space intentionally left below grade are outside the concrete cylinder.
Can I calculate holes without posts?
Yes. Select “No post displacement” and leave post size blank. The calculator then treats each entered cylinder as fully concrete-filled.
Why is the purchase count higher than the exact bag count?
The exact decimal requirement is preserved first. Because bags are purchased as whole packages, the calculator then rounds up once and shows the estimated leftover created by that purchase step.
Does this tell me how wide or deep to dig?
No. Manufacturer instructions, post loading, soil, frost, wind, local code, and project design affect safe dimensions. This calculator estimates quantity from dimensions you have already established.