Post Hole Concrete Calculator for Fence & Deck Posts

Worked post-hole examples

These quantity examples use round cylindrical holes, a square post unless stated, and representative bag yields. Load one, then edit every dimension to match the job.

Calculation method and formulas

The estimator models each hole as a round cylinder, subtracts the optional gravel base from excavated depth, then subtracts the part of the post occupying the concrete. The results panel substitutes the current measurements into every step.

• Concrete fill depth: total excavated depth - gravel depth
• Hole volume: pi × (diameter / 2)² × concrete fill depth
• Round post displacement: pi × (post diameter / 2)² × length inside concrete
• Square post displacement: post side² × length inside concrete
• Net concrete per hole: max(hole volume - post displacement, 0)
• Purchase volume: net per hole × hole count × (1 + waste% / 100)
• Whole bags: ceil(purchase volume / product yield)

Quantity-only warning: this calculator does not design a footing, set post depth, check frost protection, or establish code compliance.

Practical measurement guidance

• Use actual post width. Nominal lumber names are not finished dimensions; a nominal 4x4 is commonly about 3.5 inches, while material and finishing can vary.
• Measure widened holes. If an augered hole bells out, sloughs, or becomes irregular, use representative widest measurements, increase waste, or split it into measurable shapes rather than relying only on the auger diameter.
• Separate the depths. Total excavated depth includes the gravel base. Concrete fill depth is the excavated depth minus gravel and any other portion that will not receive concrete.
• Group matching holes. Run separate estimates for corner, line, gate, and structural posts when their sizes differ, then add their purchase quantities.
• Treat ranges as examples. The presets span roughly 10–18 inch hole diameters and 24–36 inch excavated depths to demonstrate typical calculator inputs, not to recommend a footing size.
• Check frost and structural requirements. Required depth can depend on local frost line, soil, uplift, wind, and code. Gate and deck posts can carry concentrated or lateral loads and may need project-specific drawings or professional sizing.

For structural work: use the dimensions in approved project drawings, check local code and utility-clearance requirements, and consult a qualified professional when the footing or post supports a deck, roof, gate, retaining element, or other significant load.

Bag handling and ready-mix comparison

The comparison table reports whole bags and total dry weight so you can consider lifting, vehicle payload, storage, mixing rate, and cost. One cubic yard equals exactly 27 cubic feet. At an approximate 0.60-cubic-foot yield, that volume is about 45 eighty-pound bags before waste.

This is a purchasing comparison, not a fixed delivery threshold. For larger orders, compare local ready-mix minimums, short-load fees, site access, placement time, cleanup, labor, and the risk of cold joints with the price and handling of bags.

Review and source notes

Sourced product facts: the representative 40, 50, 60, and 80 lb yields come from the QUIKRETE Concrete Mix No. 1101 data sheet. A second manufacturer publishes comparable standard yields in its Sakrete High-Strength Concrete Mix product data. These yields are approximate; specialty and regional mixes can differ.

Metric presets: 20 kg is shown at approximately 8.5 L and 25 kg at approximately 11.6 L as representative planning values. Equal-weight products can yield different volumes, so the label on the exact bag being purchased overrides every preset.

General guidance: measurement, waste, frost, and logistics notes explain how to use the estimator; they are not manufacturer sizing instructions or structural standards.