Find the concrete needed for an individual post hole or a group of holes. This page focuses on the hole geometry and post displacement, with clear alternatives for entering concrete height or deriving it from total hole depth.
Describe the hole and the concrete zone
Enter the hole shape and inside dimensions, then enter the height that will contain concrete. A circular hole needs its diameter and fill height; a rectangular hole needs inside length, width, and fill height. Set the number of identical holes, which defaults to one. If each hole differs, calculate each group separately rather than multiplying one set of dimensions across the whole site.
The default field is concrete height, not total excavation depth. It assumes the entered height is already the portion filled with concrete. An alternate mode accepts total hole depth and a distinct bottom-layer thickness, then subtracts that layer once. Do not use both modes together. If you are setting a post above gravel and enter the concrete height directly, the gravel is already excluded.
Account for the post inside the hole
The post displaces some of the hole’s gross volume. For a round hole, calculate the cylindrical hole volume. For a round post, calculate its cylinder over only the submerged length. For a rectangular post, calculate its actual cross-section area times submerged length. Then subtract: net concrete = hole volume − submerged post volume. If the post does not reach the full concrete height, enter its shorter immersed segment; exposed post does not displace concrete below the fill.
Geometry must be possible before a volume can be trusted. A square post needs its diagonal to fit inside a round hole, not merely an area comparison. A round post must have a smaller diameter than the hole. The submerged post segment cannot exceed the concrete zone. When a shape cannot fit or the subtraction is zero or negative, show an error next to the relevant dimensions instead of converting the invalid result to zero bags.
Worked example: an 8-inch hole and a round post
Consider one round hole with an 8 in diameter and 30 in of concrete. Its radius is 4 in, so the hole volume is π × 4² × 30 = about 1,508 in³, or 0.87266 ft³. A 4 in diameter round post submerged for 24 in displaces π × 2² × 24 = about 302 in³, or 0.17453 ft³. Net concrete is about 0.69813 ft³.
Applying the default 10% material allowance gives about 0.76794 ft³. QUIKRETE Concrete Mix No. 1101 lists approximate yield of 0.375 ft³ for a 50 lb bag, so this single hole needs three bags after rounding upward. The amount is small enough that purchase rounding matters: the geometric need is less than two full bag yields after allowance, but the third bag is required because the product is sold in whole bags. Actual filling and product instructions still apply.
Direct fill height or depth minus base
Use direct concrete height when you already know how high the concrete will extend. Use total depth minus a bottom layer only if both measurements are available and the layer occupies the bottom of the hole. For example, a 36 in hole with a 6 in layer has 30 in of concrete zone. That should match a direct 30 in fill-height entry and should produce the same hole volume before the post is subtracted.
A bottom layer may be rock or another specified material, but the calculator does not prescribe its type, compaction, or thickness. The two input modes are a measurement convenience, not a construction detail recommendation. If the layer is irregular or not level, a single subtraction may be a rough simplification; measure the actual concrete zone or use a more detailed takeoff. Do not deduct post length from the hole depth as though the post occupied empty space above the concrete.
Choosing bags and checking a group total
After calculating net volume for all holes, add the chosen allowance once to the combined total and convert that result using a named product yield. The product name and bag size should remain visible beside the count. Do not infer yield from the bag’s weight alone. If holes are purchased as one pooled project quantity, the calculator rounds the combined volume up to a whole number of bags. If you prefer to allocate unopened bags to each hole, it can also round each hole separately; that usually produces a larger total.
For repeated holes, the group summary helps stage materials, but it does not guarantee identical field conditions. Actual hole width, sloughing sides, over-excavation, and uneven bottoms can change consumption. A long row of fence posts may be easier to calculate with the fence-post page, which adds a straight-run count helper and highlights the distinction between project-level and per-hole bag rounding. This post-hole page remains useful for one or several holes with known individual geometry.
Limits and practical next steps
This is a fill-quantity estimate, not a recommendation for embedment, post spacing, structural support, drainage, or concrete mix selection. Local frost depth, soil, loads, wind, gate forces, and project plans may affect the details. Do not infer that a post is adequately supported because a volume calculation succeeds. Obtain dimensions from the intended design and check site requirements before excavation.
Before ordering, verify whether the dimensions are inside hole dimensions, whether the fill height excludes any base layer, how much of the post is submerged, and whether a bag yield matches the product you will buy. If the post or hole has a flared or bell-shaped region, calculate the extra volume separately. A known cylinder or rectangle is a useful model, but a calculator cannot see the actual hole. Use the final bag count as a purchasing estimate and follow the package instructions.
Frequently asked questions
Does post-hole volume subtract the post?
Yes. It subtracts only the cross-section of the post segment entered as submerged in the concrete. The above-ground length does not affect concrete displacement.
What is the difference between total hole depth and concrete height?
Concrete height is the fill zone only. Total depth includes any bottom layer; the optional mode subtracts a separately entered layer once.
How much concrete for an 8-inch hole?
Depth, post size, and submerged length are also needed. In the example on this page, an 8-inch round hole, 30-inch fill, and 4-inch round post submerged 24 inches use three 50 lb bags with 10% allowance.
Can a square post go into a round hole of the same width?
Not necessarily. Check the square post’s diagonal against the hole diameter; the corners must fit inside the circular boundary.
Should I round bags for each hole separately?
That depends on how you will buy and allocate them. The combined project count pools material, while per-hole rounding shows the quantity if each hole gets complete unopened bags.
Does this page recommend how deep to bury a post?
No. It calculates material for dimensions you provide and does not establish a suitable embedment depth or structural performance.