Post Hole Concrete Calculator — m, kg bags, cost
How to use this calculator
Select your post size, hole diameter, hole depth, and number of posts. The calculator subtracts the post volume from the hole to determine exact concrete needed. Choose your preferred bag size (20, 25, or 50 kg) for accurate bag counts.
Post size — common metric post sizes are 90×90 mm and 140×140 mm.
Hole diameter — should be approximately 3× the post width.
Waste — add 10% for spillage and uneven holes.
Saved Calculations
| Time | Posts | Hole | Bags/Hole | Total Bags | Cost |
|---|
How to Calculate Concrete for Post Holes (Metric)
This calculator determines the exact amount of concrete needed for fence posts, deck posts, or any post set in concrete. It calculates the cylindrical hole volume, subtracts the square or round post volume, and converts to bags of premixed concrete.
Formulas Used
Hole volume: V = π × (diameter/2)² × depth
Square post volume: V = side² × depth
Round post volume: V = π × (diameter/2)² × depth
Concrete needed: hole volume − post volume + waste
Bag Yields
20 kg bag ≈ 0.010 m³ (10 liters), 25 kg bag ≈ 0.012 m³ (12 liters), 50 kg bag ≈ 0.024 m³ (24 liters). Based on standard premixed concrete at 2,100 kg/m³ yield density.
FAQ
How deep should a post hole be? General rule: 1/3 of total post length underground, minimum 60 cm for fence posts. Below frost line for structural posts per local building codes.
How wide should the hole be? Approximately 3× the post width. For a 90×90 mm post, use a 250–300 mm diameter hole.
