Post Hole Calculator
Calculate how much concrete you need to fill any number of post holes. Works for round auger holes and square dug holes. Get volume, bag count and cost instantly.
🔨 Post Hole Calculator
📐 How It Works
Concrete volume for a post hole is the hole volume minus the post volume (the post displaces some concrete). Round holes use π×r²×depth. Square holes use width²×depth. A 20kg bag of premix concrete yields approximately 0.009 m³.
📋 Worked Example
12 fence posts, 300mm auger holes × 800mm deep, 100mm square posts: Hole = π×0.15²×0.8 = 0.0565 m³. Post = 0.1²×0.8 = 0.008 m³. Concrete/hole = 0.0485 m³ = 6 bags. Total = 72 × 20kg bags.
❓ Frequently Asked Questions
General rule: 1/3 of total post length plus 100mm. For a 2.4m post: 800mm deep minimum. In soft or sandy soil go deeper. In frost-prone areas (alpine Australia, NZ, US): below frost line. Pool fencing posts: minimum 600mm. Always check local council requirements.
300mm hole, 800mm deep: 5–6 × 20kg bags. 250mm hole, 600mm deep: 3–4 bags. 400mm hole, 1000mm deep: 10–12 bags. This calculator gives you the exact quantity — always buy a few extra bags for spillage and variation in hole size.
Dry-pour method (pour dry concrete then add water) works for non-structural applications like fence posts. Mix properly for structural posts, retaining walls and any load-bearing application. Properly mixed concrete is always stronger and more consistent than dry-pour.
Don't load fresh post footings for at least 24–48 hours. For full fence panel installation: wait 3–7 days. For structural applications carrying significant load: wait the full 28-day cure period. Keep fresh concrete damp in hot or windy weather to prevent cracking.