Buildculus
Concrete Post-Hole Calculator
Calculate bag count, cubic feet, and cost for any cylindrical post hole — fence, deck, mailbox, or pergola.
Most pours need 5–10% extra to cover spillage, uneven subgrade, and formwork. Skipping this often means a costly second trip.
Enter dimensions to calculate.
| Hole size | Post | Volume | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| Light fence: 8″ × 24″ | 3.5″ | 0.58 cu ft | 2 | 1 |
| Standard fence: 8″ × 36″ | 3.5″ | 0.87 cu ft | 2 | 2 |
| 10″ × 36″ | 3.5″ | 1.52 cu ft | 4 | 3 |
| Frost-line fence: 10″ × 48″ | 3.5″ | 2.03 cu ft | 5 | 4 |
| 12″ × 36″ | 5.5″ | 1.90 cu ft | 5 | 4 |
| Deck footing: 12″ × 48″ | 5.5″ | 2.53 cu ft | 6 | 5 |
| Empty footing: 12″ × 48″ | — | 3.46 cu ft | 8 | 6 |
Multiply by your hole count for the full project — or use the calculator above for exact numbers.
How to measure
Measure the auger's actual diameter — a "12-inch" auger usually digs closer to 13″, and hand-dug holes flare wider at the top. For depth, measure from grade to the bottom of the hole; structural footings need to extend below the local frost line. Count every hole in the project, and for the post itself enter the actual cross-section, not the nominal — a 4×4 is 3.5″ across.
The math
A post hole is a cylinder, so volume is π × r² × depth × number of holes — where r is half the hole diameter. When a post sits in the concrete, subtract the post's cross-section × depth × count to account for the volume it displaces. Convert every measurement into the same units before multiplying, then convert the result into cubic yards or cubic meters.
Why add a waste percentage?
Forms flex, the subgrade settles, and a small amount of mix is always lost in the pour. A 5–10% waste allowance prevents you from coming up short before the truck pulls away.
Bagged mix vs. ready-mix
Bagged concrete is convenient for small pours under roughly half a cubic yard. Above that, ready-mix delivered by truck is almost always cheaper per cubic yard and avoids cold-joint problems from mixing one bag at a time.
Frequently asked questions
- How much concrete does one post hole take?
- For a standard 10″ diameter × 36″ deep hole, plan on about 1.6 cubic feet of concrete per hole — roughly 6.5 cubic feet, or 11 × 80-pound bags of fast-setting mix, for a typical 4-post run. With a 4×4 post set in each hole, that drops to about 5.5 cubic feet total and 10 bags, since the post itself displaces concrete. A larger deck footing (12″ × 48″ around a 6×6) pushes a 4-hole run to about 9.2 cubic feet, or 16 bags. Run your exact dimensions in the calculator above — diameter and depth both matter more than you'd think, since volume scales with the square of the radius.
- What's the math behind the post-hole concrete calculation?
- A post hole is a cylinder, so volume = π × r² × depth. Convert everything to feet first: a 10″ diameter hole has a radius of 5″, which is 5/12 ≈ 0.417 ft. For 36″ of depth (3 ft): π × 0.417² × 3 ≈ 1.64 cubic feet of hole volume. If the post itself extends into the concrete, subtract the post's submerged volume — a 4×4 has an actual cross-section of 3.5″ × 3.5″, so 36″ of post displaces about 0.25 cubic feet, leaving roughly 1.39 cubic feet net per hole. Divide the net volume by the bag yield (typically 0.6 cubic feet per 80-pound bag of fast-setting mix) and round up.
- When should I switch from bagged concrete to a ready-mix truck for post holes?
- For post holes specifically, bagged mix stays practical longer than it does for slabs because pouring small volumes from a ready-mix chute into narrow holes is awkward and wasteful. Stick with bagged mix for up to roughly 15–20 holes, depending on size. For a long fence run or a deck with 25+ footings, get a ready-mix quote — but plan how you'll move the concrete from the chute to each hole (wheelbarrows, buggies, extra hands). Below about 1 cubic yard of total volume, bagged is almost always the right call.
- Why does this calculator add 10–15% for waste on post holes?
- Augers and post-hole diggers rarely produce a perfectly cylindrical hole. Sidewalls slough, the diameter widens at the top, and soft soils give unpredictably under the bit. You also crown the top of each footing slightly so water sheds away from the post, which takes a little extra material. The default 10% covers clean augered holes in firm soil; bump it to 15% for hand-dug holes, sandy or loose soil, or cold-weather pours where partial bags of fast-setting mix kick before you can use them.
- What are the most common mistakes when estimating concrete for post holes?
- Three come up over and over. First, forgetting to subtract the post's volume when the post sits inside the concrete — a 4×4 in a 36″ hole displaces roughly 0.25 cubic feet, which adds up over 20 posts. Second, using nominal hole diameter instead of actual auger diameter — a "12-inch" auger usually digs closer to 13″, and hand-dug holes flare wider at the top. Third, buying exactly the calculated number of bags with no buffer — running out mid-pour on fast-setting mix means the post in the ground sets up crooked while you drive to the store. Always add one or two spare bags on site.
- How much does it cost to set a post in concrete?
- As of 2026, an 80-pound bag of fast-setting concrete mix runs roughly $7–$10 at U.S. big-box retailers. A standard 10″ × 36″ post hole with a 4×4 post takes about 2.5 bags, putting materials at roughly $18–$25 per post. The all-in cost adds the post itself, gravel for the base, any rebar or anchor hardware, and equipment rental if you're using an auger or mixer. Prices vary by region and supplier — call your local yard for an accurate quote on larger jobs. The calculator estimates materials only.
- How deep should a post hole be, and what does the building code require?
- For structural posts — deck footings, porch columns, anything carrying a roof — the bottom of the footing must extend below the local frost line per IRC R403.1.4, with deck footing requirements specifically addressed in IRC R507.3. Frost depths range from zero in the Gulf South and parts of southern California to 48″ or more across the northern Midwest, and the adopted depth varies by jurisdiction, so confirm with your local building department before you dig. For non-structural fence and mailbox posts, frost depth isn't usually code-mandated but is still good practice: a footing above the frost line will heave each winter. A common rule of thumb for fence posts is to set one-third of the post's above-grade height in the ground, with a minimum of 24″.