How this instrument works
A post hole filled with concrete isn't quite a full cylinder of concrete — the upright itself sits inside that cylinder, taking up space concrete would otherwise fill. This instrument computes the hole's full cylindrical volume from its diameter and depth, then subtracts that upright's own volume (its actual cross-section, not its nominal size, times the same depth) to get the true concrete volume needed — a detail some simpler fence-post calculators skip by treating it as negligible.
That concrete volume, in cubic feet, then gets divided by a bag's yield to return a bag count, rounded up. Yield depends entirely on the product and package size — an 80 lb bag of QUIKRETE Concrete Mix yields approximately 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³, per the manufacturer's own technical data sheet. Those figures are typical for that specific product line, not a universal constant, so check your own package's printed yield and adjust the Bag yield field if it differs.
Set Post width (in) to 0 for a bare footing or pier with no upright inserted — the post-volume subtraction simply becomes zero, and the full hole volume is used. For an actual upright, enter its real (actual, not nominal) width — a 'nominal' 4×4 post, for example, actually measures about 3.5 by 3.5 inches once dressed, and using the true dimension is what makes the subtraction meaningful rather than a rounding error.
- Enter Hole diameter (in) — the post hole's diameter, in inches.
- Enter Hole depth (ft) — how deep the hole is dug, in feet.
- Enter Post width (in) — the post's actual (not nominal) width; use 0 for a bare footing with no post.
- Enter Bag yield (ft³ per bag) — check your specific concrete product's printed yield rather than assuming a default; common values are 0.60 for 80 lb bags, 0.45 for 60 lb, and 0.30 for 40 lb.
- Read Bags needed beneath the inputs, rounded up to a whole bag.
Worked example — a 10 in hole for a nominal 4×4 fence post
Enter 10 into Hole diameter (in), 2 into Hole depth (ft), 3.5 into Post width (in) — the actual width of a nominal 4×4 post — and leave Bag yield (ft³ per bag) at its 0.6 default, the typical yield for an 80 lb bag. Hole volume works out to π × (10÷24)² × 2 ≈ 1.0908 ft³, and post volume to (3.5÷12)² × 2 ≈ 0.1701 ft³.
Subtracting gives 1.0908 − 0.1701 ≈ 0.9207 ft³ of concrete actually needed. Bags needed reads 2, since 0.9207 divided by 0.6 is about 1.53, rounded up to the next whole bag — buying exactly 1.53 bags isn't possible, so the instrument always rounds up rather than leaving the pour short.
Questions
Why subtract the post's volume from the hole?
Because the post physically occupies space inside the hole that concrete would otherwise fill — ignoring it overstates how much concrete is actually needed, sometimes by a meaningful margin on a wide upright in a narrow hole. This calculator subtracts its actual cross-sectional area times the hole depth from the hole's own volume before converting to bags, a detail that a simpler line-item fence-material estimate typically skips.
What if I'm pouring a bare footing with no post in it?
Set Post width (in) to 0. The post-volume subtraction becomes zero automatically, so the full hole volume is used as the concrete requirement — correct for a standalone concrete pier, footing, or any hole that isn't getting an upright inserted into the wet concrete.
Where do I find my concrete bag's actual yield?
Check the package's own label or the manufacturer's technical data sheet — yield varies by product and package size, not just weight. As one reference point, QUIKRETE's own published data sheet for its standard Concrete Mix lists roughly 0.60 ft³ for an 80 lb bag, 0.45 ft³ for a 60 lb bag, and 0.30 ft³ for a 40 lb bag; other brands and mix types can differ, so use your specific product's number in the Bag yield field for an accurate count.
Should I use nominal or actual post dimensions?
Actual dimensions — a nominal 4×4 post, for example, is dressed down to roughly 3.5 by 3.5 inches once milled, and using the nominal 4-inch figure instead would understate its real volume and slightly overstate the concrete needed. Measure it directly if you're unsure, since actual sizes can vary a little by product line.
Should I buy extra bags beyond the calculated number?
A small overage is common practice, since real holes are rarely a perfect cylinder — sides can be uneven, and a little concrete typically gets used tamping or working the mix into place. Buying one extra bag beyond the rounded-up figure is a reasonable cushion for a single post, especially since bags round up to whole units anyway and a slight surplus costs far less than a second trip to the store.