Post Hole Calculator — Concrete Bags for Fence or Deck Posts
Post holes are cylindrical, so the volume calculation is different from a slab. This calculator works out the concrete volume for each hole, multiplies by the number of posts, and converts to bags — so you can order exactly what you need.
How we calculate
Volume per hole = π × (diameter ÷ 2)² × depth. Total = volume per hole × holes × (1 + waste %). Bags = ⌈total ÷ bag volume⌉.
Worked example
6 holes, 300 mm diameter, 600 mm deep. Per hole: π × 0.15² × 0.6 ≈ 0.042 m³. Total: 0.042 × 6 × 1.1 ≈ 0.28 m³. At 0.012 m³/bag: 24 bags.
Common mistakes
- Digging holes that are too narrow — concrete needs enough mass to resist lateral forces on the post.
- Not going below the frost line in cold climates — shallow posts heave in winter.
- Filling the hole flush to the surface instead of slightly below to allow a top cap.
- Adding water by eye when mixing — the water-to-cement ratio determines strength, follow the bag instructions.
Frequently asked questions
How deep should a fence post hole be?+
A common rule is to bury at least one-third of the total post length. For a 1.8 m fence with a 2.4 m post, dig at least 600 mm deep. In soft or sandy soils, or in frost-prone areas, go deeper — 750–900 mm is safer.
How wide should a post hole be?+
The hole diameter should be 2–3 times the post width. A 100 mm square post needs a 200–300 mm wide hole. A wider hole means more concrete around the post for better lateral stability.
How many bags of concrete per fence post?+
A typical 300 mm diameter × 600 mm deep hole needs about 2 bags (25 kg each) of premix concrete. Larger holes need more — use this calculator to get an accurate count based on your actual hole dimensions.
Should I use dry or mixed concrete for post holes?+
You can use dry-pack (pour dry mix into the hole and water it in place) or pre-mix properly before pouring. Pre-mixing gives a more consistent, stronger result. Dry-pack is faster but must be used with premix bags rated for dry application.