For a 4x4 post in a 10-inch hole 24 inches deep, you need about 0.56 cubic feet of concrete: two 50 lb bags of fast-setting mix, or one 80 lb bag of standard mix. Go up to a 12-inch hole 30 inches deep and it is four 50 lb bags or two 80 lb bags. The chart below covers the common post and hole sizes, and the math is at the bottom if your job is not on it.
Bags per post: the chart
These figures fill the hole the way the manufacturer's instructions describe it: 6 inches of gravel at the bottom for drainage, concrete stopping 3 to 4 inches below grade, soil backfilled on top. Yields are the published ones — 0.375 cu ft for a 50 lb bag, 0.60 cu ft for an 80 lb bag.
| Post | Hole diameter | Hole depth | Concrete needed | 50 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| 2 3/8 in round (chain link line post) | 8 in | 24 in | 0.39 cu ft | 2 | 1 |
| 2 3/8 in round (chain link line post) | 8 in | 30 in | 0.54 cu ft | 2 | 1 |
| 4x4 wood (3.5 in actual) | 10 in | 24 in | 0.56 cu ft | 2 | 1 |
| 4x4 wood (3.5 in actual) | 10 in | 30 in | 0.79 cu ft | 3 | 2 |
| 4x4 wood (3.5 in actual) | 12 in | 30 in | 1.20 cu ft | 4 | 2 |
| 4x4 wood (3.5 in actual) | 12 in | 36 in | 1.55 cu ft | 5 | 3 |
| 4 in round (terminal or gate post) | 10 in | 30 in | 0.78 cu ft | 3 | 2 |
| 6x6 wood (5.5 in actual) | 12 in | 30 in | 0.98 cu ft | 3 | 2 |
| 6x6 wood (5.5 in actual) | 16 in | 36 in | 2.62 cu ft | 7 | 5 |
| 6x6 wood (5.5 in actual) | 16 in | 42 in | 3.21 cu ft | 9 | 6 |
Buying for a whole fence? Multiply by the number of posts, then add a bag or two: corner, gate and terminal posts usually get a bigger hole than the line posts, and one hole always comes out wider than planned. For a mixed order, the concrete bag calculator converts any volume into bags of any size with a waste allowance.
How big should the hole be?
The bag manufacturers publish a rule that is worth following, because it is what their yield examples assume:
- Diameter: 3 times the post diameter. A 4x4 gets a 10 to 12 inch hole, a 6x6 gets 16 to 18 inches.
- Depth: one third of the overall post length, plus 6 inches for the gravel base. An 8-foot post for a 6-foot fence lands around 32 inches.
- Structural posts — decks, carports, anything holding weight — go at least 6 inches below the frost line, and follow the equipment manufacturer's footing dimensions for things like basketball backboards.
- Loose or sandy soil: dig a larger, dish-shaped hole — wider at the top than at the bottom is the wrong shape for frost, but the right shape when the soil will not hold a wall.
Frost depth is local and it is not optional: a post set above the frost line will heave, and no amount of concrete stops it. Check your local building department's frost depth before you dig, especially for anything structural.
The math, if your post is not on the chart
D = hole diameter in feet · A = post cross-section in square feet · h = height of the concrete column in feet
Two details do most of the work:
- h is not the hole depth. Subtract the 6-inch gravel base and the 3 to 4 inches of soil on top: a 30-inch hole holds about 20.5 inches — 1.71 ft — of concrete.
- Subtract the post. A 4x4 is 3.5 in × 3.5 in actual, which is 0.085 sq ft. On a 6x6 (5.5 in actual, 0.21 sq ft) it matters a lot more.
Worked example — 4x4 post, 12-inch hole, 30 inches deep:
- Concrete height h = 30 − 6 − 3.5 = 20.5 in = 1.71 ft
- Hole area = π ÷ 4 × 1 ft² = 0.785 sq ft → hole volume = 0.785 × 1.71 = 1.34 cu ft
- Post volume = 0.085 × 1.71 = 0.15 cu ft
- Concrete = 1.34 − 0.15 = 1.20 cu ft → 1.20 ÷ 0.375 = 3.2 → 4 bags of 50 lb, or 1.20 ÷ 0.60 = 2 → 2 bags of 80 lb
For a slab, a footing or anything bigger than a post hole, the concrete slab calculator does the same arithmetic in volume, bags and cost.
Fast-setting mix: pour it dry
For posts, fast-setting concrete is not mixed in a wheelbarrow. The manufacturer's instructions are to pour the dry mix straight from the bag into the hole until it is 3 to 4 inches from the top, re-check the post for plumb, brace it, then pour water on the dry mix and let it soak in — about 1 gallon per 50 lb bag placed, more in loose or sandy soil.
- Set time: 20 to 40 minutes. Heavy loads — a basketball backboard, a gate — after 4 hours, longer below 72 °F.
- Holes deeper than 2 ft 6 in get filled in lifts: mix and water up to 2.5 ft, wait for the water to soak all the way through, then the rest.
- Standard concrete mix works too — same yield per pound — but it has to be mixed with water first and it will not hold a post plumb in half an hour.
Bottom line
- A 4x4 in a 10-inch hole 24 inches deep: 2 bags of 50 lb, or 1 bag of 80 lb.
- A 4x4 in a 12-inch hole 30 inches deep: 4 bags of 50 lb, or 2 bags of 80 lb.
- Hole diameter = 3 × the post; depth = 1/3 of the post length plus 6 inches of gravel; deeper than the frost line for anything structural.
- Yields are printed on the data sheet: 0.30, 0.375, 0.45, 0.60 and 0.675 cu ft for 40, 50, 60, 80 and 90 lb bags.
- Buy one spare bag per five posts. Nobody ever regretted the spare bag; everybody regrets the second trip.