Free Post Hole Concrete Calculator – Bags & Cubic Yards

Post Hole Concrete Calculator Guide

The Post Hole Concrete Calculator estimates the pre-mix concrete needed to set fence, deck, mailbox, sign, or railing posts, returning cubic feet, 80-lb bag count, and an equivalent ready-mix order. Enter the hole diameter, hole depth, and number of posts; the tool computes the annular fill volume around the post and totals it across the run. Measure the hole, not the post — concrete fills the cylindrical hole minus the volume the post occupies, so a chunky 6×6 post in a 12-inch hole displaces noticeably more concrete than a 4×4. Set the hole geometry before estimating: a common rule sizes the diameter at about three times the post width and the embedment depth at one-third of the above-grade post height, but frost depth governs in cold climates. Add a few inches of compacted gravel at the bottom for drainage; that gravel reduces the concrete volume slightly, so measure the concrete portion of the depth.

Hole volume (cylinder) = π × (diameter ÷ 2)² × depth, in consistent units. Convert: cubic feet = π × (radius_in ÷ 12)² × (depth_in ÷ 12). Subtract the post's buried volume for the net concrete. An 80-lb pre-mix bag yields about 0.60 ft³ (60-lb ≈ 0.45 ft³, 40-lb ≈ 0.30 ft³). Worked example: a 6-ft-tall 4×4 fence post in a 10-inch-diameter hole, 24 inches deep. Gross hole = π × (5 ÷ 12)² × (24 ÷ 12) = 3.1416 × 0.1736 × 2 = 1.09 ft³. The 3.5-inch-square post over 24 inches buried = (3.5 ÷ 12)² × 2 = 0.17 ft³. Net concrete ≈ 0.92 ft³ → 0.92 ÷ 0.60 ≈ 1.5 → 2 bags per post. For 20 posts: about 32–34 bags, or 32 × 0.60 ÷ 27 ≈ 0.71 cubic yards of ready-mix.

Order a partial bag of surplus per hole — pre-mix is cheap, and a post that runs short mid-pour means a weak cold joint. Once you exceed roughly 15–20 bags, ready-mix in quarter-yard increments is usually cheaper and more consistent than mixing by hand. Set embedment by frost line, not habit: posts must extend below the local frost depth (often 24–48 inches in cold regions) so freeze-thaw heave cannot lift them, a principle reflected in IRC footing-depth provisions. For structural deck posts the load path matters — deck posts generally bear on a proper footing sized to soil capacity per IRC, not merely on a collar of concrete, so verify whether your post needs a bell footing or bearing pad. Crown the concrete slightly above grade and slope it away from the post so water sheds; a flat or dished top traps moisture and rots wood or rusts steel at the most vulnerable point. Brace every post plumb until the concrete sets.