Post Hole Concrete Calculator

The formula

Coverage and yield assumptions

Yields follow Quikrete published data: 50 lb Fast-Setting ≈ 0.375 cu ft and 80 lb standard ≈ 0.60 cu ft. A 4×4 post is 3.5 in square and a 6×6 is 5.5 in square. For a sono tube pier with no post, the full hole volume is used.

Hole size rules of thumb

A common guide is a hole diameter about three times the post width (10–12 in for a 4×4) and a depth of one third to one half of the post's height above ground, with a minimum of 24 in. In cold climates, go below the frost line to prevent heave. Many fence installers put 3–4 in of gravel in the bottom of the hole for drainage, which slightly reduces concrete.

Fast-setting vs standard mix

Fast-setting mixes are poured dry into the hole and then water is added; they set in 20–40 minutes and suit fence posts that carry little load. Standard mix is pre-mixed with water and takes longer to set but is stronger and better for deck footings and structural posts.

Deck footings

Decks need footings sized for the load and soil bearing capacity, often 12–24 in diameter, and must follow local code and the American Wood Council's DCA 6 guide. Posts usually sit on a bracket on top of the footing rather than embedded in concrete, so enter "no post" for the pier volume.

Setting posts straight

• Call 811 to locate utilities before digging.
• Brace posts in two directions and check plumb with a level.
• Crown the top of the concrete so water runs away from the post.
• Use pressure-treated posts rated for ground contact.

Common mistakes to avoid

• Digging holes too shallow for the post height and frost depth.
• Using a hole only slightly wider than the post.
• Letting concrete dome around the post below grade, trapping water.
• Not bracing posts while the concrete sets.
• Digging without calling 811.

Waste factor

10% covers holes that end up wider or deeper than planned, which is common with augers in rocky soil. Use 20% for hand-dug holes with bell-shaped bottoms.

Imperial and metric

Switch the toggle above the inputs to enter in ↔ cm. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Ten fence posts, 4×4

Inputs: number of posts 10; hole diameter 10 in; hole depth 30 in; post size: 4×4 (3.5 in actual); waste factor 10%.

Result: You need 34 bags of 50 lb fast-setting concrete.

• Units to buy (rounded up): 34 50 lb bags
• Concrete per hole: 1.15 cu ft
• Total volume: 12.66 cu ft (0.47 cu yd)
• Or 80 lb standard bags: 22

Privacy fence, 6×6 posts

Inputs: number of posts 16; hole diameter 12 in; hole depth 36 in; post size: 6×6 (5.5 in actual); waste factor 10%; cost per unit $7.5.

Result: You need 82 bags of 50 lb fast-setting concrete.

• Units to buy (rounded up): 82 50 lb bags
• Concrete per hole: 1.73 cu ft
• Total volume: 30.38 cu ft (1.13 cu yd)
• Or 80 lb standard bags: 51
• Estimated material cost: $615.00

Deck piers, 12 in tubes

Inputs: number of posts 6; hole diameter 12 in; hole depth 42 in; post size: No post / sono tube footing; waste factor 10%.

Result: You need 49 bags of 50 lb fast-setting concrete.

• Units to buy (rounded up): 49 50 lb bags
• Concrete per hole: 2.75 cu ft
• Total volume: 18.14 cu ft (0.67 cu yd)
• Or 80 lb standard bags: 31

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the post hole concrete calculator result scales.

Limitations

Assumes straight cylindrical holes and square posts. Round posts and irregular holes change the volume. Structural footings must be designed to code.