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.
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.