Post Hole Calculator
Concrete per hole with the post subtracted, and bags for the whole fence.
Post and hole
= 3 1/2"
= 10"
= 30" (2' 6")
Results update as you typeLive
Post Hole · Result
CalculatedBAGS FOR THE RUN
24 bags for the whole run
- Per hole
- 1.15 ft³
- Holes
- 12
- Total
- 13.81 ft³
- Post fills
- 16% of hole
Fill allowance defaults to 0; manufacturers differ (Sakrete 4-6 in gravel, Quikrete 3-4 in sod)
SUGGESTED DEFAULTWaste defaults to 0% unless supplied
SUGGESTED DEFAULTOutput unit defaulted to cubic feet because at least one dimension was entered in imperial units
SUGGESTED DEFAULTBag yields are the manufacturer's printed approximate figures, never fitted to printed bags-per-hole claims
MFR SPECCode references & assumptions · 7
How this result is supported
- ASSUMPTIONPost dimensions are actual dressed sizes; the 3x hole rule uses the nominal size
- ASSUMPTION
- SUGGESTED DEFAULTFill allowance defaults to 0; manufacturers differ (Sakrete 4-6 in gravel, Quikrete 3-4 in sod)
- ASSUMPTION
- ASSUMPTIONThe post's own volume is subtracted from the hole
- INTERPRETED
- SUGGESTED DEFAULTWaste defaults to 0% unless supplied
- ASSUMPTION
- SUGGESTED DEFAULTOutput unit defaulted to cubic feet because at least one dimension was entered in imperial units
- ASSUMPTION
- ASSUMPTIONBags are pooled across the job total, not rounded up per hole
- ASSUMPTION
- MFR SPECBag yields are the manufacturer's printed approximate figures, never fitted to printed bags-per-hole claims
- PRIMARY-VERIFIED
What do these evidence labels mean?
- PRIMARY-VERIFIED
- directly supported by one authoritative primary source, quoted with section/page/figure
- CROSS-VERIFIED
- independently supported by two or more appropriate authoritative sources
- DERIVED
- mathematically derived from PRIMARY-VERIFIED values; the derivation is preserved in data and in code
- INTERPRETED
- a source exists but translating it into calculator behaviour required interpretation
- CONVENTION
- recognised trade practice, not a mandatory provision
- ASSUMPTION
- product/design assumption or useful default with no authoritative basis
- UNVERIFIED
- insufficient authoritative evidence
Code references and product data are quoted in the units their source uses.
Reference checks are informational, not compliance determinations.
How post hole is calculated
The post takes up room in the hole, and a 4×4 in a 10 in hole displaces roughly a sixth of it. Subtracting it is the difference between ordering the right number of bags and finding out at the last post that you are short.
The gravel base and the sod cap are one allowance because they cancel at opposite ends of the hole: one manufacturer's method wants gravel underneath, another fills the top few inches with soil. It defaults to zero, since neither convention can be assumed for you.
Hole depth is a suggestion here, not a rule. The frost line governs it and varies by location, so the calculator always says so alongside the depth rather than presenting a number as settled.
hole = π × diameter² ÷ 4 × depth
post = section area × depth
Round or rectangular.
concrete per hole = hole − post − allowance