Running out of concrete halfway through a fence line costs a trip to the store and a crew standing around. A post hole is a cylinder, so the volume is simple geometry: pi times the radius squared times the depth. This recipe converts that to cubic feet, multiplies by the number of posts and divides by the yield of one bag.
20 posts in 10-inch by 30-inch holes with a 0.6 cubic-foot bag need 46 bags.
The example
Three jobs with different hole sizes. Wider and deeper holes grow the bag count quickly because diameter is squared.
| A | B | C | D | E | F | G | |
|---|---|---|---|---|---|---|---|
| 1 | Case | Posts | Dia (in) | Depth (in) | Yield cu ft | Bags | Per post |
| 2 | Backyard | 20 | 10 | 30 | 0.60 | 46 | 2.3 |
| 3 | Pool fence | 12 | 12 | 36 | 0.60 | 48 | 4.0 |
| 4 | Gate run | 8 | 8 | 24 | 0.60 | 10 | 1.3 |
The formula
The volume of one round hole, scaled by the number of posts and the bag yield, rounded up:
How it works
Here is the logic, one piece at a time:
PI()*(C2/2)^2is the area of the circular hole in square inches; the radius is half the diameter.- Multiplying by depth
D2gives cubic inches per hole, and dividing by 1728 converts to cubic feet. - Multiplying by the post count in
B2and dividing by the bag yield inE2gives the bags needed. ROUNDUP(...,0)rounds up because a partial bag still has to be bought.
Concrete bag yields are printed on the bag: roughly 0.6 cu ft for an 80 lb bag and 0.45 cu ft for a 60 lb bag.
Try it: interactive demo
Enter the post count, hole size and bag yield.
Variations
Subtract the post itself
A 4x4 post displaces concrete. Subtract its footprint from the hole area for a tighter count.
Add a spill allowance
Multiply by 1.05 inside ROUNDUP to cover waste and uneven holes.
Pitfalls & errors
Diameter is squared, so a hole 2 inches wider than planned adds far more concrete than 2 inches of extra depth would.
Do not mix inches and feet. If depth is in inches the divisor is 1728; if depth is in feet, drop the 1728 and convert the diameter.
Gravel in the bottom of the hole displaces concrete too. Subtract the gravel depth from the depth cell if you use it.
Practice workbook
Frequently asked questions
Why 1728?
Does this work for square post holes?
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