Post Hole Concrete Calculator
The Post Hole Concrete Calculator estimates how many bags of concrete you need to set posts, accounting for the post's own volume displacing space in the hole. Works in U.S. or metric units. The formula treats both the hole and the post as perfect cylinders (π × radius² × depth), so a square post's width is used as if it were a circular diameter — this slightly overstates how much volume the post actually displaces, since a square post's corners never reach the assumed circular edge, making the estimate marginally conservative rather than short. If you set a post width equal to or greater than the hole diameter, the calculator floors the concrete volume at zero instead of returning a negative number, since a post that fills the whole hole needs no concrete around it. Because bag yield jumps in fixed steps (about 0.3 cu ft for a 40 lb bag, 0.45 for 60 lb, 0.6 for 80 lb), switching bag sizes changes your bag count by more than a simple linear ratio — always round the final total up, since suppliers don't sell partial bags.
Switch between U.S. Customary and Metric units.
How This Tool Works
Pick your units, then enter your hole's diameter and depth, your post's width, and the number of posts to calculate total bags of concrete needed.
Formula & Method
Concrete volume = hole volume (π × radius² × depth) minus post volume (π × radius² × depth), then divided by bag yield. Enter hole and post dimensions in inches (lb bags) or centimeters (kg bags).
Example Calculation
A 12 inch diameter hole, 24 inches deep, with a 4 inch (4×4) post, for 2 posts using 60 lb bags: hole volume per hole = π × 6² × 24 = 2,714.34 cubic inches. Post volume per hole = π × 2² × 24 = 301.59 cubic inches. Concrete volume per hole = 2,714.34 − 301.59 = 2,412.74 cubic inches ÷ 1,728 = 1.396 cubic feet. For 2 posts: 1.396 × 2 = 2.79 cubic feet total. At 0.45 cubic feet per 60 lb bag: 2.79 ÷ 0.45 = 6.2, rounded up to 7 bags. In metric, a 30 cm diameter hole, 60 cm deep, with a 10 cm post, for 2 posts using 25 kg bags: concrete volume per hole ≈ 1.331 cubic feet (37.7 L), total for 2 posts ≈ 2.66 cubic feet (75.4 L, 0.075 m³) — at 0.415 cubic feet per 25 kg bag, that's also 7 bags.
Frequently Asked Questions
How deep should a fence post hole be?+
A common rule is to bury one-third to one-half of the post's above-ground height, with a minimum depth of 24 inches (about 60 cm) for most residential fences.
Can I use metric hole sizes and kg bags?+
Yes — switch the unit toggle to Metric and enter your hole and post dimensions in centimeters. Bags are sized in kilograms and the volume is returned in liters and cubic meters.
Do I need to subtract the post's volume?+
Yes — the post itself displaces concrete, so subtracting its volume gives a more accurate bag count instead of over-ordering.
Should I add gravel at the bottom of the hole?+
Many installers add a few inches of gravel at the bottom for drainage before pouring concrete — factor that into your hole depth if you plan to do this.
What if my post is round instead of square?+
Enter the post's actual diameter in the post width field — the calculator already treats the post as a circle in its math, so a round post plugs in directly with no conversion needed. For a square post like a 4×4, entering its face width is a reasonable approximation since the formula runs it through the same circular-volume calculation as the hole.
How much does bag size change my total?+
Bigger bags aren't just cheaper per pound — they yield more mixed concrete per bag: about 0.3 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag, and 0.6 for an 80 lb bag. Since the 80 lb yield is exactly double the 40 lb yield, switching from 40 lb to 80 lb bags for the same job roughly halves your bag count, even though the total weight of concrete you need stays the same.
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<a href="https://everyfix.net/tools/post-hole-concrete-calculator/">Post Hole Concrete Calculator</a> by <a href="https://everyfix.net">EveryFix</a>Related Tools
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