◆ Calculators ◆ Fence Post Concrete Calculator: How Many Bags per Post
Fence Post Concrete Calculator: How Many Bags per Post
Work out how many bags of fast-setting concrete mix your fence posts need from post and hole size and bag size, with the manufacturer method sourced.
Both area formulas are basic geometry, not a manufacturer figure — the data sheet names no post shape or cross-section.
QUIKRETE data sheet No. 1004, "Method for Setting Posts": "Start by digging the post hole about 3 times the diameter of the post." The default here (9 in) is 3 times the default post size (3 in).
The same data sheet section gives this as orientation: "Hole depth should be 1/3 the overall post height." Post height is not a field here — work it out yourself if you want to apply the rule.
Yields are the manufacturer’s approximate figures. The number printed on the bag you buy takes precedence.
The data sheet names no waste figure for setting posts, so this one is yours to set. 0 % is a starting point, not a claim that no loss occurs.
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How the number is worked out
The steps from input to result
Concrete for setting fence posts is bought and carried in bags, not in cubic feet, so the number that matters at the counter is the bag count — this calculator works out how many bags a given number of post holes needs from their dimensions, not a raw volume.
QUIKRETE Fast-Setting Concrete Mix, data sheet No. 1004 (Revised November 2025), section "Method for Setting Posts", gives three rules of thumb: "Start by digging the post hole about 3 times the diameter of the post." · "Hole depth should be 1/3 the overall post height." · "Fill the upper 3 in to 4 in (75 mm to 100 mm) of the hole with sod or with the soil that was removed."
This calculator always assumes the conservative end of that soil-fill range — the upper 3 in of every hole is treated as soil, not concrete — because a smaller soil zone means more concrete, and for a shopping list an extra bag on hand is cheaper than a second trip to the store. 3 in is this calculator’s own choice within the data sheet’s "3 in to 4 in" range, not the only figure the manufacturer gives.
The data sheet itself states: "Two 50 lb (22.6 kg) bags of QUIKRETE® Fast-Setting Concrete Mix will set a 3 in (75 mm) square post in a 9 in (225 mm) diameter hole 30 in (0.75 m) deep." Run through this calculator’s own formula, the same inputs come to 2.2757 bags before rounding up, which rounds up to 3 bags — not the two the data sheet names for its own worked example. The data sheet gives no reason for the remaining difference, and none is invented here: the gap is shown, not smoothed over.
The formula
Written out, so you can check it by hand
Bags = round up( ( (π/4 × hole diameter² − post area) × (hole depth − 3 in) ÷ 1728 ) × number of posts × (1 + waste ÷ 100) ÷ yield per bag (ft³) ); post area = side² (square) or π/4 × post size² (round)
Worked example
Worked with the preset values
A 3 in square post in a 9 in diameter hole, dug 30 in deep, needs a square post area of 3² = 9 in² and a hole area of π/4 × 9² ≈ 63.6173 in². Excluding the top 3 in of the hole as soil leaves a 27 in concrete zone, so the net volume is (63.6173 − 9) × 27 ÷ 1728 ≈ 0.8534 ft³. With one post and no waste allowance, that is also the total volume; at 0.375 ft³ per 50 lb bag, 0.8534 ÷ 0.375 ≈ 2.2757 bags, which rounds up to 3 bags — and needs 3 × 1 = 3 gal of mixing water. The data sheet itself names "two bags" for this exact case; see Limits for why this page’s own formula comes to three.
What this calculator does not do
Named, not left out
- 01Only QUIKRETE Fast-Setting Concrete Mix, data sheet No. 1004, is covered — other fast-setting or standard concrete mixes are not.
- 02The hole is always treated as a cylinder, regardless of the post’s own cross-section.
- 03The top 3 in of every hole is treated as soil, not concrete — this calculator’s own choice at the conservative end of the data sheet’s "3 in to 4 in" range, not the only figure the manufacturer gives.
- 04The 1/3-of-post-height rule for hole depth is guidance from the data sheet, not a calculation this page performs — post height is not a required input here.
- 05No gravel base, frost depth, local building code or structural post-embedment requirement is accounted for. This is a materials calculation, not construction planning.
- 06One post size and one hole size per run — projects mixing multiple post or hole sizes need to be run separately for each size.
- 07The data sheet’s own worked example — a 3 in square post in a 9 in diameter hole, 30 in deep, with 50 lb bags — states "two bags"; this calculator’s own formula comes to 3 bags (2.2757 before rounding up) for the identical inputs. The data sheet gives no reason for the remaining difference, and none is invented here.
- 08The 1 gal per 50 lb bag mixing-water figure is named by the data sheet only for the 50 lb bag; for the other bag sizes, the figure printed on the bag itself applies.
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Updated: 9/5/2026