Cylinder-volume math for fence, deck, and mailbox post holes.
Per hole: 0.042 m³ (1.50 ft³)
Total: 0.042 m³ (1.50 ft³)
20 kg bag: 5 bags
Alternative bag sizes:
Round-hole volume uses π × r² × depth, not the rectangular prism approximation. Switch to "Square" for clamshell-dug holes or sleeve pours.
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Two rules of thumb cover most domestic posts:
In freezing climates, post holes must extend below the local frost line to avoid heave. Check the local code — it usually overrides the depth rule of thumb.
This post hole concrete calculator works out how much concrete goes into round or square post holes for fences, decks, pergolas, and mailbox posts. It uses true cylinder math — π × radius² × depth — rather than the rectangular prism approximation, so the per-hole volume matches what an actual auger-dug hole holds. It is part of our concrete calculators set.
For a round hole dug with a post-hole auger or earth drill:
Volume per hole (m³) = π × r² × depth
where r is the hole radius in metres (half the diameter) and depth is the hole depth in metres. A typical 300 mm diameter, 600 mm deep fence hole gives π × 0.15² × 0.6 ≈ 0.042 m³.
For a clamshell-dug or formed square hole, use prism math:
Volume per hole (m³) = side² × depth
A round hole holds about π/4 ≈ 79% of an equivalent-size square hole's concrete. The calculator above toggles between the two formulas.
Two simple guides cover most domestic post installations:
| Post type | Post height | Hole diameter | Hole depth |
|---|---|---|---|
| Mailbox post | 1.2 m / 4 ft | 250 mm / 10 in | 500 mm / 20 in |
| Fence post | 1.8 m / 6 ft | 300 mm / 12 in | 600 mm / 24 in |
| Deck post | 2.4 m / 8 ft | 400 mm / 16 in | 900 mm / 36 in |
| Pergola post | 3.0 m / 10 ft | 450 mm / 18 in | 1,000 mm / 40 in |
These figures assume firm, stable ground. In soft soil, clay, or frost-prone ground, increase depth and check the local building code.
1. Single fence post — 300 mm × 600 mm round
2. Small fence / mailbox post — 250 mm × 500 mm round
3. Deck post — 400 mm × 900 mm round
Quick-setting post-mix concrete (Postcrete / Quikrete Fast-Setting) is dropped dry into the hole, has water added on top, and sets within 5–10 minutes. It is ideal for fence posts because it lets the next post go in immediately and the panels go up the same day. Standard ready-mix or site-mixed concrete is cheaper per cubic metre and stronger long-term — use it for deck posts, pergolas, and anything carrying significant structural load. Bag yield is similar (≈ 0.010–0.018 m³ per bag depending on size), so the bag-count estimates from this calculator hold for both product types.
For a round hole, volume = π × radius² × depth. A 300 mm diameter × 600 mm deep hole is π × 0.15² × 0.6 ≈ 0.042 m³ per hole. Multiply by the number of holes for the project total.
A 300 mm × 600 mm round hole holds ~0.042 m³, which is about 4 × 20 kg bags or 3 × 25 kg bags. A larger 400 mm × 900 mm deck post hole takes about 11 × 20 kg bags. Use the calculator above for your specific dimensions.
A common rule of thumb is depth ≈ ⅓ of the visible post height, with a minimum of 600 mm in stable ground. Soft, clay, or frost-affected ground needs deeper holes — go below the frost line in freezing climates.
Hole diameter ≈ 3× the post width is the usual guide. A 100 mm (4 in) fence post takes a ~300 mm (12 in) hole. Deck and pergola posts are wider — see the sizing table on this page.
Quick-setting concrete (Postcrete-style) is poured dry into the hole, water is added on top, and it sets in 5–10 minutes. It is the fastest option for fence posts. Standard ready-mix or site-mixed concrete takes longer to set but is cheaper per cubic metre and is preferred for structural deck posts and pergolas.
For a thin metal or 100 mm timber fence post in a 300 mm hole the post takes a small fraction of the hole, but enough to round up rather than down on the bag count. The calculator above accepts an optional post diameter or side that gets subtracted from the concrete volume.
Yes — toggle the hole shape to square. The calculator switches from cylinder math (π × r² × depth) to prism math (side² × depth or length × width × depth). Round holes are slightly more concrete-efficient than square holes of the same nominal size.
A 50–100 mm layer of compacted gravel at the bottom of the hole helps drainage and reduces rot for timber posts. Subtract that depth from the concrete volume — the calculator's depth input should be the concrete depth, not the total hole depth.
Up to the hub: Concrete Calculator. Sideways: Concrete Slab Calculator and Concrete Footing Calculator. Supporting converters: Length and Volume.