Post Hole Concrete Calculator
Setting fence posts, deck footings, or mailbox posts? Enter the hole diameter and depth and the number of holes to see how much concrete you need and how many bags to buy. A common rule is a hole about three times the post width and one-third to one-half the post’s above-ground height — but always set footing depth below your local frost line.
Total volume required
0.18Cubic Yards
Cubic feet
4.80
Cubic meters
0.14
Bags (80 lb)
8
Bags by size
40 lb
16
50 lb
13
60 lb
11
80 lb
8
Estimate includes your waste factor. Figures are estimates, not a quote.
How to use this calculator
- 1Enter the hole diameter in inches and the depth in feet.
- 2Set Quantity to the number of holes you are filling.
- 3Add a waste factor for the soil that sloughs in and the post that displaces some concrete.
- 4Read the total concrete and bags. Fast-setting post mix is sold by the bag, so the bag count is what you take to the register.
Formula & rounding
Each round hole is a cylinder: π × radius² × depth, where radius = diameter ÷ 2. Total volume is multiplied by the number of holes, then by the waste factor. Bag counts round up. The post itself displaces a little concrete, which the waste factor helps absorb.
4 holes, 10 in diameter, 2 ft deep
- Radius = 10 in ÷ 2 = 5 in = 0.417 ft
- One hole = π × 0.417² × 2 = 1.09 cubic feet
- 4 holes with 10% waste = 1.09 × 4 × 1.10 = 4.8 cubic feet
- 50 lb bags = 4.8 ÷ 0.375, rounded up = 13 bags
= ≈ 4.8 cubic feet for 4 holes (≈ 13 × 50 lb bags)
6×6 deck post, 12 in diameter, 3 ft deep
- Radius = 12 in ÷ 2 = 6 in = 0.5 ft
- One hole = π × 0.5² × 3 = 2.36 cubic feet
- With 15% waste for rough soil: 2.36 × 1.15 = 2.71 cubic feet
- 80 lb bags = 2.71 ÷ 0.60, rounded up = 5 bags per hole
= ≈5 × 80 lb bags per 12 in × 3 ft hole
Frost-line depth reference
Post-hole depth is a code and site question, not just a volume question. The 2024 IRC minimum-depth rules require exterior footings to be below local frost depth where frost protection applies; your local building department sets the actual frost-line value.
| Reference depth | Typical situation | Calculator setting |
|---|---|---|
| 12 in minimum | Warm climates or frost-protected cases where local code allows shallow footings | Use 1 ft only when code allows it |
| 24 in | Mild freeze areas and many light fence-post checks | Enter 2 ft and keep waste at 10–15% |
| 36 in | Cold-winter areas and many deck/fence footing details | Enter 3 ft; verify with local frost map/code |
| 48 in or deeper | Severe frost areas and some deck/structural post footings | Enter local required depth, not a rule of thumb |
Code reference: 2024 IRC R403.1.4 / R403.1.4.1. Local adopted code, soil, post load, and frost depth override this planning table.
Frequently asked questions
How much concrete do I need per post hole?
For a 10 inch diameter hole 2 feet deep, about 1.1 cubic feet of concrete — roughly two to three 50 lb bags before waste. Multiply by the number of holes; the calculator does this for you.
How many bags of concrete per fence post?
It depends on hole size, but a typical fence post hole takes one to three bags of fast-setting concrete. Enter your diameter and depth to get the exact count by bag size.
How deep should a post hole be?
A common guideline is one-third to one-half of the post’s above-ground height, and at least below the local frost line so the footing does not heave. Always check your local building code for fence and deck footings.
What diameter hole for a 4×4 or 6×6 post?
A common planning rule is a hole about three times the post width: roughly 10–12 inches for a 4×4 post and 12–18 inches for a 6×6 post, depending on load and local code. Larger holes use much more concrete, so set diameter before counting bags.
Do I subtract the post from the hole volume?
This calculator estimates the full hole volume, which slightly over-orders to cover the post’s displacement and soil that falls in. That is usually the safe choice for setting posts.
How much extra concrete for loose or collapsing soil?
If the auger hole caves in, measure the actual widest diameter and raise the waste factor to 15–20%. A 10 inch hole that sloughs out to 12 inches uses about 44% more concrete per foot of depth, so guessing low can leave you short.
Assumptions & sources
- Volume formula
- π × radius² × depth per hole, × number of holes, with waste added.
- Bag yields
- ≈0.60 cu ft per 80 lb bag; smaller 40–50 lb bags (0.30–0.375 cu ft) suit narrow holes.
- Post-to-hole sizing
- Common planning rule: hole diameter about 3× post width, adjusted for deck/fence load, soil, and local code.
- Frost line
- 2024 IRC R403.1.4 / R403.1.4.1 point footing depth to local frost protection requirements — verify the adopted local frost depth before pouring.
- Soil waste
- Loose or collapsing soil over-excavates the hole; raise the waste factor or measure the actual bored diameter.
See the methodology & sources for how these values, formulas, and rounding are chosen.
Helpful concrete guides
- How to calculate concrete — explains the cylinder formula behind post holes and how waste factor changes bag counts.
- Concrete bag sizes & yields — helps pick 40, 50, 60, or 80 lb bags for fast-setting post mix.
This is an estimate, not a quote. Concrete quantities, bag yields, block coverage, and prices vary with product, brand, mix, region, supplier, tax, delivery, and on-site conditions. Always confirm with your supplier and round up for safety. For structural or code-related work, consult a qualified professional or your local building authority.
Frost line: Footing and post-hole depth is often governed by local code. Check your local frost line and building code — footing depth must meet local requirements. This tool does not determine code compliance.