Post Hole Concrete Calculator — ft, 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 volume to determine the exact concrete needed. Choose your preferred bag size (40, 60, or 80 lb) for accurate bag counts.
Post size — nominal sizes use actual lumber dimensions (4×4 = 3.5"×3.5", 6×6 = 5.5"×5.5").
Hole diameter — should be 3× the post width minimum per IRC.
Waste — add 10% for spillage and uneven holes.
Saved Calculations
| Time | Posts | Hole | Bags/Hole | Total Bags | Cost |
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How to Calculate Concrete for Post Holes
This calculator determines the exact amount of concrete needed for fence posts, deck posts, mailbox 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
40 lb bag ≈ 0.30 ft³, 60 lb bag ≈ 0.45 ft³, 80 lb bag ≈ 0.60 ft³. These are approximate yields for standard premixed concrete (e.g., Quikrete, Sakrete).
FAQ
How deep should a post hole be? General rule: 1/3 of total post length underground, minimum 24" for fence posts, 42" below frost line for structural posts per IRC R403.1.
How wide should the hole be? IRC recommends 3× the post width. For a 4×4 post (3.5" actual), use a 10–12" diameter hole.
