Enter the hole diameter, depth, and number of posts to instantly calculate concrete volume in cubic yards, exact bag counts, and material cost for your fence, deck, or structural post project.
Free to use
·
No sign-up required
·
Post displacement accounted for
·
Imperial & metric supported
✓ Bag counts (40 lb, 60 lb & 80 lb)✓ Multi-hole projects supported✓ Post size displacement included✓ Last verified May 2026
Reviewed by the AllConcreteCalculator.com editorial team — cylindrical volume formula verified against ACI 318 and standard post-hole setting practice, May 2026.
Enter Your Post Hole Dimensions
Measure the inside diameter of the augered hole. Most fence posts: 6–8 in. Deck posts: 10–12 in.Please enter a valid hole diameter greater than 0.
Below-grade depth of the hole. Rule of thumb: bury ⅓ of post length, minimum below frost line.Please enter a valid hole depth greater than 0.
Total count of post holes for the project. Results show per-hole and total volume.
Nominal post size (e.g. 4×4 = 3.5 in actual). Calculator subtracts post volume from concrete needed.
Add 10% for standard jobs. Increase to 15% if soil walls are loose or holes are irregular.
$
Leave blank to skip cost estimate. 80 lb bags: ~$6–$8 at hardware stores. Results show bag count by size.
Results appear instantly. No sign-up required.
Your Concrete Estimate
Total Concrete Volume (all holes, with waste)
—
Cubic Yards (yd³)
—
Cubic Feet (ft³)
—
Cubic Meters (m³)
Bags Required — All Holes (includes waste)
—
40 lb bags
—
60 lb bags
—
80 lb bags
—Per Hole (ft³)
—80 lb Bags / Hole
—Holes
—Waste Factor
Estimated Bag Material Cost
—
Based on 80 lb bag price entered × bags required. Excludes labor, delivery, tools, and post materials. For full project cost see our Full Project Estimator.
Step 1: Hole volume (ft³) = π × (diameter/2)² × depth [all in feet]
Step 2: Post volume (ft³) = post_width² × depth [square post] or π × (post_dia/2)² × depth [round post]
Step 3: Net volume per hole = Hole volume − Post volume
Step 4: Total net volume = Net volume × number of holes
Step 5: Final volume = Total × (1 + waste% ÷ 100)
Step 6: Bags = CEIL(Final ft³ ÷ bag yield) — never round down
Bag yields: 40 lb = 0.30 ft³ | 60 lb = 0.45 ft³ | 80 lb = 0.60 ft³
Note: Post size input uses square cross-section for dimensional lumber (4×4, 6×6) and circular for round posts — the calculator assumes square.
How to Use This Post Hole Concrete Calculator
Measure the hole diameter and depth before you start mixing.
Use a tape measure at the top of the freshly augered hole for diameter — most power augers cut a consistent hole, but hand-dug holes can vary. Measure depth from ground level to the bottom of the hole. If your holes vary in size, calculate the largest size and use it as your reference, or run separate calculations and add them together.
Enter the total number of holes for the project.
The calculator gives you a total bag count for the entire job, plus a per-hole breakdown so you know exactly how many bags to carry to each hole. For a fence with 20 posts, enter 20. If some posts need a different size hole (e.g. gate posts), calculate those separately.
Enter the post size to get a more accurate concrete estimate.
The post itself displaces concrete volume. A 4×4 post (3.5 in actual) at 36 inches deep displaces about 0.25 ft³ — roughly half a 60 lb bag per hole. Over 20 posts that's 10 bags you'd otherwise overbuy. Enter the post's nominal dimension (e.g. 4 for a 4×4) and let the calculator subtract it automatically.
Use the bag count to buy materials before the pour day.
The 80 lb bag count is your most economical option per cubic foot — buy those unless you're working alone or on difficult access terrain, in which case 60 lb bags are easier to handle. Always add 10% waste — loose soil walls and irregular hole bottoms absorb more concrete than a perfect cylinder.
⚠ Pro Tip: Mix post-hole concrete drier than you think. Bags poured directly into a wet hole should use the dry-pack method — no mixing water added. For structural deck posts or any load-bearing application, mix wet and consolidate with a rod or stick. Sloppy wet concrete in a post hole has lower compressive strength and can wash away from the post face before it sets.
Post Hole Concrete Volume Formula
A post hole is a cylinder. The concrete volume is the volume of the cylinder minus the volume of the post embedded in it. All dimensions must be converted to feet before calculating.
Step
Formula
Example (10 in dia, 36 in deep, 4×4 post)
1. Convert all to feet
inches ÷ 12
diameter = 0.833 ft, depth = 3.0 ft, post = 0.292 ft (3.5 in actual)
2. Hole volume
π × r² × depth
3.14159 × (0.417)² × 3.0 = 1.636 ft³
3. Post volume (square)
side² × depth
(0.292)² × 3.0 = 0.256 ft³
4. Net concrete per hole
Hole − Post
1.636 − 0.256 = 1.380 ft³
5. Add 10% waste
× 1.10
1.380 × 1.10 = 1.518 ft³ per hole
6. 80 lb bags per hole
CEIL(ft³ ÷ 0.60)
CEIL(1.518 ÷ 0.60) = 3 bags
Common Post Hole Reference Table
Net concrete volumes and 80 lb bag counts per hole — no waste factor applied. Add 10% for real-world ordering. Post displacement not deducted.
Hole Diameter
Depth
ft³ per Hole
60 lb Bags
80 lb Bags
Typical Use
6 in
24 in (2 ft)
0.39
1
1
Light fence, mailbox
6 in
36 in (3 ft)
0.59
2
1
Privacy fence 4×4 post
8 in
36 in (3 ft)
1.05
3
2
Privacy fence 4×4 or 6×6
8 in
48 in (4 ft)
1.40
4
3
Tall fence, gate post
10 in
36 in (3 ft)
1.64
4
3
Deck post (4×4)
10 in
48 in (4 ft)
2.18
5
4
Deck post (6×6)
12 in
48 in (4 ft)
3.14
7
6
Structural deck post
12 in
60 in (5 ft)
3.93
9
7
Deep frost zone post
16 in
48 in (4 ft)
5.59
13
10
Large structural column
18 in
60 in (5 ft)
8.84
20
15
Heavy structural pier
Values are per hole, no waste added. Add 10% for real-world ordering. Subtract post displacement for precise estimates (use calculator above).
How Deep Should a Post Hole Be?
Hole depth is the single most critical variable for post longevity. Too shallow and the post will lean or heave in freeze-thaw cycles. Too deep wastes concrete and labor without structural benefit. The table below reflects standard building practice and most residential building codes.
Recommended post hole depth by application and climate zone.
Application
Post Height (above grade)
Recommended Depth
Notes
Mailbox / Light Signage
Up to 4 ft
18–24 in
Stable soil required
Privacy Fence (6 ft)
6 ft
30–36 in
Below frost line in cold climates
Privacy Fence (8 ft)
8 ft
36–42 in
36 in min, check local code
Gate Post (6 ft fence)
6 ft
42–48 in
Use larger diameter hole for gates
Deck Post (up to 8 ft)
Up to 8 ft
42–48 in
Engineer may specify depth
Deck Post (8–12 ft)
8–12 ft
48–60 in
Must be below frost line; engineer recommended
Pergola / Shade Structure
8–10 ft
36–48 in
Larger diameter for wind load
Structural Column / Pier
Any
Below frost line + 12 in
Engineer approval required
In freeze-thaw climates (most of the northern US, Canada, northern Europe), post holes must extend below the local frost depth — typically 36–60 inches depending on region. A post set above the frost line will heave every spring, often irreversibly. Check your local frost depth with your county building department or on the NOAA frost depth map before digging.
Common Mistakes When Setting Posts in Concrete
⚠️
Setting posts above the frost line.
This is the number one reason fence and deck posts fail. If your hole doesn't extend below the local frost depth, the soil will freeze and expand in winter, pushing the post upward. After a few cycles the post leans, the concrete collar cracks, and water infiltrates. Dig to frost depth — full stop. Don't guess; call your building department.
📏
Using the hole diameter as the post diameter.
The hole is larger than the post — that gap is filled with concrete. If you estimate bags based on hole diameter alone and forget to subtract the post, you're over-ordering. On a 20-post project with 10-inch holes and 4×4 posts at 36 inches, the uncorrected overcount is about 5 bags of 80 lb. Not catastrophic, but avoidable.
💧
Adding too much water to the bag mix.
Overly wet concrete is easy to pour but dramatically weaker. For post holes, the ideal mix is stiff — about the consistency of thick oatmeal. Runny concrete can have 20–30% less compressive strength than the bag's rated PSI. Use a margin trowel or stick to consolidate, not excess water to make it fluid.
🔄
Not bracing posts while concrete cures.
Freshly poured post-hole concrete takes 24–48 hours to reach initial set and 7 days to reach usable strength. A post that isn't braced plumb will lean as you attach fence panels or framing, and once the concrete hardens, it's very difficult to correct. Use 2×4 diagonal braces from the post to stakes driven in the ground. Check plumb from two perpendicular directions before leaving the site.
🏚️
Burying wood posts directly in concrete without flashing or a standoff.
Concrete traps moisture against wood. Even pressure-treated lumber will rot faster when in direct concrete contact, especially at grade level where the wood transitions from concrete to open air. Best practice: use a metal standoff base set in concrete, or apply a bituminous coating to the buried section of wood posts.
Frequently Asked Questions
For a standard 4×4 fence post (3.5 inches actual) in a 6-inch diameter hole at 36 inches deep, you need about 1 bag of 80 lb concrete per hole after accounting for the post volume and a 10% waste factor. Use an 8-inch hole for the same post and depth and you'll need about 2 bags. For gate posts — which should use a larger hole, typically 10 inches, at 42–48 inches deep — plan on 3–4 bags per post. Use the calculator above to get an exact count for your specific dimensions.
The standard rule of thumb is to bury one-third of the total post length underground. So a 9-foot post (6 feet above grade) needs a 3-foot hole. In freeze-thaw climates, this isn't enough — the hole must extend below the local frost line, which ranges from 12 inches in the deep South to 60+ inches in Minnesota or Canada. Always check your local frost depth. Most residential building codes also specify a minimum depth of 24 to 36 inches regardless of frost. When in doubt, dig deeper — the extra concrete cost is trivial compared to a leaning fence.
Fast-setting concrete (such as Quikrete Fast-Setting or Sakrete Fast-Setting) is an excellent choice for post holes because it sets in 20–40 minutes, meaning you can brace and release posts the same day. The technique is different from standard mix: you pour the dry powder directly into the hole around the set post, then add water on top — no mixing required. Fast-setting products typically yield the same or slightly higher compressive strength as standard mix. The main trade-off is cost: fast-setting bags are typically 15–25% more expensive per bag than standard 80 lb concrete. For most fence projects, the labor savings easily justify the price difference.
Yes — two common alternatives are gravel backfill and expanding foam post anchors. Gravel backfill (crushed stone tamped in layers) drains better than concrete and is often recommended for wood posts because it doesn't trap moisture. However, gravel-set posts have significantly less lateral resistance and are not suitable for tall privacy fences, gate posts, or any structural application. Expanding foam products (Sika PostFix, Quikrete PostFix) are fast and provide good lateral resistance, but cost more per hole than bagged concrete. For any load-bearing post or fence taller than 4 feet, concrete remains the most reliable option.
The standard rule is to make the hole 3 times the post width. For a 4×4 post (3.5 inches actual), that means a minimum 10.5-inch hole, so a 10 or 12-inch auger bit is appropriate. For a 6×6 post (5.5 inches actual), use a 12-inch auger. The extra space around the post ensures the concrete can flow around all sides and form a solid footing with no voids. Holes that are just barely larger than the post don't leave enough room for concrete to fill properly, especially at depth.
Standard bagged concrete reaches initial set in 1–2 hours and is stable enough to release bracing in 24–48 hours. Full strength (28-day cure) is required before attaching heavy fence panels, framing loads, or deck structures — for residential fences, most contractors wait at least 3 days before attaching panels. Fast-setting products like Quikrete Fast-Setting reach working strength in 20–40 minutes, allowing same-day brace removal for light applications. Regardless of product, avoid heavy loads on newly set posts for at least 24 hours and full structural loading for 7 days.
Slightly mounded and sloped away from the post is best practice — not flush or depressed. A small crown around the post base (about 1–2 inches above grade, sloped outward) prevents water from pooling against the post. Standing water at the post-concrete junction is the primary cause of post rot and is entirely preventable with a proper crown. Do not fill the hole completely flush with grade — a flat surface will collect rainwater and direct it toward the post. This is a two-minute step that adds years of post life.
Run separate calculations for each unique size and add the results together. For a fence project with 16 standard posts at 8-inch holes × 36-inch depth and 4 gate posts at 10-inch holes × 48-inch depth, calculate the 16 standard posts first, then the 4 gate posts separately, and sum the bag counts. The calculator above handles one size at a time — use it twice and add the totals. This takes about 60 seconds and gives you the precise number of bags to buy for each size hole.
Standard 3,000 PSI bagged concrete is adequate for most fence and light deck post applications. The concrete in a post hole is primarily in compression and lateral bearing — it doesn't need the high compressive strength required for structural slabs or footings under heavy loads. Standard 60 lb and 80 lb bags of concrete (Quikrete, Sakrete) are rated at 4,000 PSI when properly mixed, which exceeds what most post hole applications require. For structural deck posts, pergolas, or any application with significant vertical load, specify at least 3,500 PSI and consult your local building code.
It depends on the hole size. One 80 lb bag (0.60 ft³) is enough for a small 6-inch diameter hole at 24 inches deep with a 4×4 post — that hole holds about 0.39 cubic feet, which one bag easily covers. For a larger 8-inch hole at 36 inches deep, you need approximately 2 bags of 80 lb. One bag is never enough for a 10-inch or larger hole at standard depth. Don't guess — enter your actual dimensions in the calculator above to get an exact bag count. Underestimating and running short mid-project wastes a trip to the store and may leave some posts with undersized footings.
