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A post hole has two dimensions that both need to be right, diameter and depth, and getting either one wrong undermines the footing even if the other is correct. This calculator uses the standard rule of sizing hole diameter at roughly three times the nominal post width, which works out to about a 10 to 12 inch hole for a 4x4 post and 12 to 14 inches for a 6x6, and combines that with a depth built from the one-third embedment rule plus a gravel drainage cushion at the bottom. From diameter and depth together it computes the cylinder volume, subtracts the space the post itself displaces, and converts the remainder into 60 and 80 pound bag counts.

Hole shape matters as much as hole size, and it is one of the easier mistakes to make with a hand auger or post hole digger. A straight-sided or slightly bell-bottomed hole, wider at the bottom than the top, resists frost heave because freezing soil has no angled surface to grab and lift. A funnel-shaped hole, wider at the top and narrower at the bottom, does the opposite: it gives frost-heaving soil exactly the wedge shape it needs to work a post upward over repeated freeze-thaw cycles. Gravel at the bottom of the hole, typically a few inches, is there for drainage rather than structure, keeping the post base out of standing water that would otherwise sit against the end grain.

Concrete volume and bag count follow directly from the hole dimensions this calculator establishes, which is why the two numbers are produced together rather than as separate tools you have to reconcile by hand. The concrete bags calculator runs the same volume math across a full set of holes at once if your run has more than a handful of posts, and the bags per post guide gives typical bag counts for common hole and post combinations as a sanity check. Fast-setting concrete is a reasonable substitute for standard mix in this context, since it lets you pour dry mix and water directly in the hole without separate mixing, and it carries a similar published yield per bag, so the bag counts here hold either way.

Diameter and depth are only useful once you know the post length and spacing they need to support in the first place. The fence post length calculator determines the stock length a given hole and above-ground height require, and the fence post calculator determines how many of these holes a full run actually needs once corners and ends are counted. Where soil conditions favor drainage over a rigid concrete collar, the gravel calculator prices the same hole in tons of stone instead of bags of mix. The full sequence, from depth and setting rules through to finished installation, is covered in the post depth and setting guide and the wood fence installation guide, with the rest of the footing tools gathered under the foundation calculators category.

Digging method affects how closely a real hole matches the clean cylinder this calculator assumes, and that gap is worth accounting for when buying concrete. A power auger typically cuts a more consistent, truly cylindrical hole close to its rated diameter, while a hand-held post hole digger or digging bar tends to produce a slightly irregular, sometimes wider hole, especially in rocky or root-filled soil where the tool has to work around obstructions rather than cut a clean circle. That irregularity usually means a hand-dug hole needs a bit more concrete than the exact calculated volume for a perfect cylinder of the stated diameter, which is one reason buying one extra bag beyond the calculated total per hole is common practice for hand-dug projects. Rocky soil and tree roots are also the main reasons hole dimensions sometimes have to be adjusted mid-project: a hole that cannot reach the calculated depth because of bedrock, or that has to be widened to work around a root, changes the actual volume from what was planned, and it is worth recalculating the concrete needed for that specific hole rather than assuming the original estimate still applies. For a large project with ten or more holes, renting a two-person gas or hydraulic auger is usually faster and more consistent than hand digging, and the consistent diameter a power auger produces makes the cylinder-based volume math in this calculator a closer match to the real hole than hand digging typically achieves. Marking each hole location accurately before digging, rather than adjusting positions as digging proceeds, also keeps the actual spacing between finished holes consistent with whatever spacing plan the rest of the project is built around, since a hole moved a few inches to avoid an obstruction can throw off a panel or rail length calculated for the original spacing.