Fence Post Concrete Calculator
Get an exact bag count for setting fence, deck, mailbox, or pergola posts. The calculator computes each hole's volume as a cylinder, subtracts the volume the post itself displaces, adds a waste allowance, and converts to 80, 60, and 40 lb bags - per post and for the whole fence line. Sizing rules and the formula are documented below.
Posts & Holes
Results
80 lb bags - total
– bags
Assumes the post is set the full hole depth. Real holes are never drilled clean - the 10% default waste covers ragged sides.
The formula this calculator uses
Net per post = (Hole − Post volume) × (1 + Waste%)
Bags: 80 lb = 0.60 ft³ · 60 lb = 0.45 ft³ · 40 lb = 0.30 ft³
The post displaces a surprising share of the hole - a 4×4 takes up about 12% of a 12-inch hole - which is why bag-per-post rules of thumb scatter so widely. Computing the net volume gives you a number you can defend at the checkout.
Post-hole sizing rules
| Fence height | Post length | Hole depth | Hole diameter (4×4) | 80 lb bags/post |
|---|---|---|---|---|
| 4 ft | 6–7 ft | 24 in | 10–12 in | 4–5 |
| 6 ft | 9 ft | 30–36 in | 12 in | 6–8 |
| 8 ft | 12 ft | 36–42 in | 12 in (6×6: 18 in) | 8–10 |
| Gate post | +1 size | +6 in deeper | +2–4 in wider | +2–3 |
Two rules generate the table: bury one-third of total post height (24-in minimum, below frost line in cold climates), and make the hole three times the post width. Gate and corner posts take more load - size them up.
Worked example: 60 ft of 6-ft privacy fence
- Posts at 8 ft spacing: 9 posts (including both ends)
- Holes: 12 in diameter, 32 in deep → π × 0.5² × 2.67 = 2.09 ft³ each
- Minus 4×4 post (0.085 ft² × 2.67 ft = 0.23 ft³): 1.86 ft³ net × 1.10 = 2.05 ft³
- Per post: 2.05 ÷ 0.60 = 4 bags of 80 lb (3.4 rounded up)
- Total: 18.5 ft³ → 31 bags of 80 lb - about 2,480 lb, so plan two supply runs or delivery
Setting posts right - the details that outlast the math
Gravel base first: 3–4 in of crushed stone under the post lets end grain drain; a post standing in a concrete cup rots from the bottom up.
Crown the top: slope the concrete collar away from the post at the surface so water sheds instead of pooling against the wood.
Brace and check plumb twice: once when set, again 10 minutes later - wet concrete lets posts drift.
Cure before load: fast-setting mixes take fence rails in 4 hours; standard mix wants 24–48 before you hang anything.
When to call a professional
Fences over 6 ft (often permit territory), retaining-fence combinations, gate spans over 5 ft, or post rows on slopes and in expansive clay benefit from a fence contractor's auger and experience. And call 811 before digging any post line - utility strikes from fence augers are among the most common DIY dig accidents.
Assumptions used in this calculator
- Hole modeled as a cylinder; embedded post displacement subtracted (3.5 in / 5.5 in actual sizes)
- Default waste 10% for ragged hole sides
- Bag yields as in the concrete calculator
Change the adjustable inputs above where your project differs. Supplier figures and local codes govern; see our methodology.
Built for planning, not blind ordering. Verify dimensions, supplier conversions, local codes, and site conditions before construction.
Frequently Asked Questions
How many bags of concrete per fence post?
For the common setup - 4x4 post, 12-inch hole, 24 inches deep - about 2.7 cubic feet of concrete per hole after subtracting the post: roughly 5 bags of 80 lb, or 6 bags of 60 lb, including 10% waste.
How deep should a fence post be?
Bury one-third of the post's total height, with a 24-inch minimum - so a 6-ft fence uses 9-ft posts set 3 ft deep. In frost climates, go below the local frost line so heaving doesn't lift the posts.
How wide should the post hole be?
Three times the post width: a 4x4 (3.5 in actual) gets a 10-12 inch hole; a 6x6 gets a 16-18 inch hole. The concrete collar around the post is what resists wind and gate loads.
Can I use fast-setting concrete poured dry?
Yes - fast-setting mixes are designed to be poured dry into the hole around the post, then charged with water. It sets in 20-40 minutes. Yield per bag is about the same; use the same bag counts.
Should the concrete reach the bottom of the hole?
Put 3-4 inches of gravel at the bottom first so the post's end grain drains instead of sitting in a concrete cup collecting water, then fill concrete around the post and slope the top away for runoff.