Concrete Post Calculator
Calculate how much concrete you need for fence posts, deck posts, or mailbox posts.
Concrete post footings provide the foundation that keeps posts stable and upright. Whether you are installing fence posts, deck support posts, or mailbox posts, correctly calculating the amount of concrete required saves money and ensures structural integrity.
How the calculation works: Each post hole is essentially a cylinder. The concrete fills the space around the post within the hole. The volume is calculated using the cylinder formula minus the volume occupied by the post itself:
Hole Volume = π × (hole radius)² × hole depth
Post Volume = post width × post width × hole depth (for square posts)
Concrete Volume = Hole Volume - Post Volume
Standard post hole dimensions: The general rule for post hole depth is one-third of the total post length plus 6 inches for a gravel base. For a 6-foot fence, the post should be 8 feet long with approximately 28-30 inches buried.
Hole diameter depends on post size:
- 4×4 posts (3.5" actual): 10-12 inch diameter hole
- 6×6 posts (5.5" actual): 12-14 inch diameter hole
- Round posts (4" diameter): 10-12 inch diameter hole
Concrete bag coverage: A standard 50-pound (23 kg) bag of premixed concrete yields approximately 0.375 cubic feet when mixed. An 80-pound (36 kg) bag yields approximately 0.6 cubic feet. Bags of 60 pounds yield approximately 0.45 cubic feet.
Important installation tips: Always place 4-6 inches of gravel at the bottom of the hole before adding concrete. This provides drainage to prevent water from pooling around the base of the post, which causes rot in wooden posts.
The post should be plumb (perfectly vertical) when the concrete sets. Use temporary braces to hold the post in position. Most premixed concrete sets enough to remove braces in 4-6 hours, but full cure takes 24-48 hours.
Climate considerations: In regions with freeze-thaw cycles, post footings should extend below the frost line to prevent heaving. Frost line depths vary from 12 inches in mild climates to 48 inches or more in northern regions. Check your local building codes for the required depth in your area.
Quick-set vs. standard concrete: Quick-setting concrete can be poured dry into the hole and then wetted, making it easier for DIY projects. Standard concrete should be mixed before pouring for a more consistent result and stronger footing.