Calculate concrete needed for fence post holes. Enter hole diameter, depth, post size, and count to get bags or cubic yards required.
Setting posts in concrete is a fundamental task for fences, decks, mailboxes, signs, and pergolas. The concrete anchors the post firmly in the ground, preventing movement from wind, soil pressure, and freeze-thaw cycling. Calculating the exact amount prevents wasted material and unnecessary store trips.
This calculator computes the concrete needed for each post hole by subtracting the post volume from the cylindrical hole volume, then multiplies by your total number of holes. It outputs the total in cubic feet, cubic yards, and number of bags for the three standard bag sizes.
The most common mistake is underestimating — post holes look small but the concrete adds up quickly when you have 20–50+ holes for a fence line. A typical 10-inch hole 36 inches deep around a 4×4 post requires about 0.65 bags of 80-lb concrete per hole.
By quantifying this parameter precisely, construction teams can optimize material orders, reduce on-site waste, and ensure structural requirements are met safely and efficiently.
Fence and deck projects often involve dozens of post holes. Estimating "about two bags per hole" can lead to 20–30% error across the project. This calculator gives a precise per-hole volume and total bags needed, saving you from making extra trips during the crucial drying window when posts need to be aligned.
Hole volume = π × (hole diameter/2)² × depth (all in inches, convert to ft³) Post volume = post width² × depth Concrete per hole = (Hole volume − Post volume) ÷ 1728 Total = Per hole × Number of holes
Result: 17 bags (80-lb)
Each 10" diameter hole, 36" deep, with a 3.5" actual 4×4 post: hole volume = π × 5² × 36 = 2,827 in³. Post volume = 3.5² × 36 = 441 in³. Net = 2,386 in³ = 1.38 ft³. Per hole concrete = 0.41 ft³. With 25 holes: 34.6 ft³ total. That's about 58 bags of 80-lb mix.
The hole diameter should be at least 3 times the post width. A 4×4 post (3.5" actual) needs at least a 10" hole. A 6×6 post (5.5" actual) needs at least a 14" hole. Larger holes provide more concrete anchorage and lateral resistance.
Quick-setting concrete reaches walk-away strength in 20–40 minutes, making it ideal for fence posts where you want to set many posts in one day. Standard concrete is cheaper but requires bracing for 24 hours. For critical structural posts, pre-mixed concrete offers better consolidation.
Always crown the concrete 1–2 inches above grade and slope it away from the post. This prevents water from pooling where the post meets the concrete, which is the most common point of wood post rot.
For a 4×4 post in a 10-inch hole, 30 inches deep: about 1 bag of 80-lb mix per hole. For a 6×6 post in a 12-inch hole, 36 inches deep: about 2 bags per hole. These are approximate — use the calculator for precision.
Not necessarily. Many brands (like Quikrete) can be poured dry into the hole around the post, then soaked with water. The concrete sets from the moisture in the soil and added water. For best results, pre-mixing ensures complete hydration.
The general rule is 1/3 of the total post length should be in the ground. For a 6-foot fence, set posts 2 feet deep. In cold climates, the hole should also extend below the frost line to prevent heaving.
Yes, place 4–6 inches of compacted gravel at the bottom for drainage. This prevents water from pooling at the base of the post, which accelerates rot in wood posts.
In firm, well-drained soil, fence posts can be set in compacted gravel alone. However, concrete provides much better lateral resistance for gates, corner posts, and end posts that experience the most stress.
With standard concrete, wait at least 24–48 hours. With quick-setting concrete, you can usually attach rails after 4 hours. Check the specific product instructions for load-bearing time.