Post Hole Concrete Calculator — Bags per Hole
Enter hole diameter, depth and post dimensions to find the exact concrete volume per hole and total bags needed for your project. Supports 20 kg, 25 kg and 40 kg pre-mix bags, and subtracts the post volume automatically.
m
m
Post cross-section
m
Pre-mix bag size
Volume of concrete needed around the post in one hole
- 1
Hole volume (cylinder)
π × 0.15² × 0.6 = 0.0424 - 2
Post volume (square)
0.1² × 0.6 = 0.006 - 3
Concrete per hole (litres)
(0.0424 − 0.006) × 1000 = 36.4
How does this calculator work?
Concrete per hole = π × (holeDia/2)² × depth − post volume. Divide by bag yield (0.010–0.019 m³ per bag) and round up for bags per hole; multiply by post count for the project total. The post is subtracted because it displaces concrete inside the hole.
Formula
How this is calculated
A post hole is a cylinder: its volume is π × (hole radius)² × depth. To find the concrete you actually need to pour, the volume of the post itself is subtracted — the post already occupies that space inside the hole. For a square or rectangular post, post volume is width² × depth; for a round post or pipe it is π × (post radius)² × depth.
The remaining net concrete volume is divided by the yield of one pre-mix bag to give bags per hole, rounded up. Bag yields differ by size: a 20 kg bag yields roughly 0.010 m³ (10 L), a 25 kg bag roughly 0.012 m³ (12 L) and a 40 kg bag roughly 0.019 m³ (19 L) — values consistent with manufacturers such as QUIKRETE and Boral. Multiply the per-hole bag count by the total number of posts for a project total. Add 5–10% to the bag count for spillage, uneven holes and mixing waste.
For hole sizing, the standard guideline is a hole diameter about three times the post width. Hole depth follows the 1/3 rule (fence height ÷ 3) plus any frost-line requirement — see the fence-post-depth-calculator for depth guidance.
Frequently asked questions
Yes. The post occupies space inside the hole, reducing how much concrete you pour. For a 100 mm square post in a 300 mm hole, the post displaces roughly 11% of the hole volume. This calculator handles the subtraction automatically once you enter post dimensions.
Both work for fence posts. Fast-setting (rapid-set) concrete can be poured dry into the hole and activated by pouring water on top — no mixing required. Regular pre-mix is cheaper per kilogram. Both types yield approximately the same volume per kilogram, so the bag count is the same.
The calculator uses metric (metres and litres). To convert: 1 inch = 0.0254 m, 1 foot = 0.3048 m. A 12-inch-diameter, 3-foot-deep hole is about 0.305 m diameter × 0.914 m deep. A 50 lb bag of QUIKRETE yields about 0.014 m³ (14 L).
Also known as
TG we-Calculate Editorial Team. (2026). Post Hole Concrete Calculator — Bags per Hole [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/post-hole-concrete-calculator
TG we-Calculate Editorial Team. "Post Hole Concrete Calculator — Bags per Hole." TG we-Calculate. 2026. https://we-calculate.com/calculator/post-hole-concrete-calculator.
TG we-Calculate Editorial Team, "Post Hole Concrete Calculator — Bags per Hole," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/post-hole-concrete-calculator
@misc{wecalculate_post_hole_concrete_calculator, title = {Post Hole Concrete Calculator — Bags per Hole}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/post-hole-concrete-calculator}}, year = {2026}, note = {TG we-Calculate} }
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