Concrete Post Hole Calculator

Post Hole Sizing Rules of Thumb

Make the hole about 3 times the post width: a 4×4 post (3.5 in actual) takes a 10-12 in hole, and a 6×6 post (5.5 in actual) takes a 14-18 in hole. For depth, Quikrete's setting guide says 1/3 to 1/2 of the above-ground post height, plus 6 in for a gravel base. A 6 ft fence therefore wants holes 30-42 in deep, and deeper where your local frost depth is greater. Gate, corner and pergola posts should go deeper and wider than a line post; size deck footings with the Deck Footing Calculator and your local code.

Concrete and Bags Per Hole

A post hole is a cylinder: π × (diameter ÷ 2)² × depth. Take the standard 10 in × 36 in fence hole: π × (0.417 ft)² × 3 ft = 1.64 cu ft. The 4×4 post sitting in it fills 0.085 sq ft of cross-section, or 0.26 cu ft over 3 ft, leaving 1.38 cu ft of concrete. Add 10% waste and you have 1.52 cu ft, which is 2.5 bags at 0.60 cu ft per 80-lb bag. You buy 3.

With 6 in of gravel in the bottom, the concrete is only 30 in deep: enter 2.5 ft. That gives 1.36 cu ft gross, still 3 bags of 80-lb mix with a 4×4 in place.

Bag counts include 10% waste and round up, at 0.60 cu ft per 80-lb bag and 0.45 cu ft per 60-lb bag. The calculator above takes Diameter and Height/Depth in feet (enter a 10 in hole as 0.83 ft) and does not subtract the post, so for eight 10 in × 36 in holes it gives 0.53 cubic yards with waste instead of 0.45. For the full bag-count method, see how many bags of concrete you need.

Bags Per Hole: 4×4 vs. 6×6 Posts

A 6×6 displaces about 0.21 sq ft of cross-section, two and a half times what a 4×4 does, so it can cut a full bag off the count at the same hole size. One 18 in × 60 in deck hole still takes 15 or 16 bags, so a row of them is a mixer job, not a wheelbarrow job.

Total Bags by Number of Posts

This table pools the concrete for 10 in × 36 in holes with 4×4 posts, net of the posts, plus 10% waste. If you open fresh bags for each hole, buy 3 per hole instead (24 for 8 posts).

Past about 1 cubic yard (20 standard holes), ready-mix delivery or a short-load trailer can beat stacking 50-plus bags. Before you buy a pallet of bags, compare bagged vs ready-mix.

Expansive Clay: Where Posts Heave and Lean

The SlabCalc Soil Risk Index rates 180 US cities from USDA NRCS SSURGO soil survey data over an 8 km area around each city (generated Oct 15, 2026). The median city's soil has a linear extensibility of 2.4%. In the cities below, the swell is three to seven times that.

Clay like Houston Black swells when wet and shrinks in drought, so it grips a concrete plug, lifts it, then leaves it loose. Post-setting advice rarely changes with the soil. If you are setting posts in Garland or Dallas, we would change three things:

• Bell the bottom of the hole wider than the top, so the wider foot anchors the plug when swelling clay grips the sides and pushes up.
• Keep the 6 in of compacted gravel under the plug, but slope the finished grade away from the post so surface water does not collect in that gravel; in tight clay it has nowhere to drain.
• Take the depth from the local building department instead of a rule of thumb.

The index screens an area, not your lot; for deck posts in these bands, ask for a geotechnical check.

Fast-Setting vs. Standard Mix

For fence and mailbox posts, fast-setting concrete is the usual pick: you pour the dry mix into the hole around a braced post, add about 1 gallon of water per 50-lb bag, and it sets hard in 20-40 minutes. Bag size changes the count, though. Fast-setting mix comes in 50-lb bags (0.375 cu ft) and 60-lb bags (0.45 cu ft), not 80 lb, so a 10 in × 36 in hole takes about 5 bags of 50-lb mix rather than 3.

Standard mix (Quikrete Concrete Mix, 4,000 PSI at 28 days) takes 24-48 hours to reach working strength and must be mixed first (see how to mix concrete).

Wet-Set vs. Dry-Pour

Wet-set (mix, then place) is how standard mix goes in, since it must be mixed first. Dry-pour (dry mix in the hole, water on top) is faster and fine for light fence posts, but water has to reach the bottom. Quikrete's fast-setting data sheet caps each stage at 30 in, so fill a 36 in hole about halfway, water it, then fill and water the rest (details in the dry-pour concrete guide).

Frost Depth and Setting the Post

Building codes rarely set fence-post depth; the IRC exempts fences up to 7 ft tall from permits (R105.2). IRC R403.1.4.1 requires deck and other structural post footings to extend below the frost line your jurisdiction lists in Table R301.2. Minnesota's rule 1303.1600, for example, sets 42 in for Zone II and 60 in for Zone I. Ask your building department for yours.

To set a post: put about 6 in of compacted gravel in the base, plumb and brace the post in two directions, fill the concrete to 3-4 in below grade, and finish the top with soil or sod. Hold the braces 4 hours for fast-setting mix or 24 hours for standard mix before you hang rails or gates (steps in the concrete fence post guide).

Sources

• Quikrete, Concrete Mix No. 1101 data sheet, PDF dated Oct 2022.
• Quikrete, Fast-Setting Concrete Mix No. 1004 data sheet, PDF dated Nov 2025.
• Quikrete, Setting Posts in Concrete, no date, accessed Oct 2026.
• IRC 2021, Ch. 1 and Ch. 4, up.codes, accessed Oct 2026.
• Minnesota Rules, 1303.1600 Footing Depth for Frost Protection, published Jan 2015.