What it is
This Quikrete bags for fence post calculator estimates how many bags of fast-setting or standard concrete mix you need to set fence posts. Enter your hole size, post dimensions, post count, and bag size, and it returns the total bags (rounded up), exact bags, and concrete volume.
How to use it
Measure your hole diameter and depth in inches, enter the nominal post width and thickness (a "4x4" is actually 3.5 in × 3.5 in), choose how many posts you are setting, and pick a bag size by its concrete yield. The calculator subtracts the volume the post occupies from the hole volume so you do not over-order.
The formula
Concrete per post = \(\pi\cdot(d/2)^{2}\cdot\text{depth} - (\text{post width} \times \text{post thickness} \times \text{depth})\), with all measurements converted from inches to feet. Bags = total concrete volume ÷ yield per bag. An 80 lb bag yields ≈ 0.60 cu ft, a 60 lb bag ≈ 0.45 cu ft, and a 50 lb bag ≈ 0.375 cu ft.
$$\begin{gathered} \text{Bags} = \left\lceil \dfrac{V \cdot \text{Posts}}{\text{Bag Yield}} \right\rceil \\[1.5em] \text{where}\quad \left\{ \begin{aligned} V &= \pi\left(\frac{\text{Dia}}{24}\right)^{2}\frac{\text{Depth}}{12} \\ &\quad - \frac{\text{Width}}{12}\cdot\frac{\text{Thick}}{12}\cdot\frac{\text{Depth}}{12} \end{aligned} \right. \end{gathered}$$
Worked example
Three posts, 12 in diameter × 36 in deep holes, 4×4 posts (4 in nominal), 60 lb bags (0.45 cu ft). Hole volume per post = \(\pi\cdot(0.5\ \text{ft})^{2}\cdot 3\ \text{ft} = 2.35619\ \text{cu ft}\). Post volume = \((0.3333\ \text{ft})^{2}\cdot 3\ \text{ft} = 0.33333\ \text{cu ft}\). Concrete per post = \(2.02286\ \text{cu ft} \rightarrow 4.4953\ \text{bags}\). Total for three posts = \(6.06858\ \text{cu ft} \rightarrow 13.49\ \text{bags}\), rounded up to 14.
FAQ
Should I use nominal or actual post size? Use actual dimensions for accuracy (a 4×4 is 3.5×3.5). Using the nominal 4×4 slightly under-counts the displaced volume.
Why round up? You cannot buy a fraction of a bag, so always round up and consider buying one extra.
Does this work for round posts? Enter an equivalent square (width = thickness ≈ post diameter) for a close estimate.
