Post Hole Concrete Calculator
Find how many bags of concrete you need to set a post, from the hole size and post size.
Reviewed by the ToolNestr Editorial Team — July 2026
How the post hole concrete calculator works
Setting a post starts with the volume of the hole itself: for a round, auger-dug hole, that's π times the radius squared times the depth. But the post sitting in that hole takes up real space too, and skipping that subtraction overestimates how much concrete you actually need. For a square post, subtract width times width times the depth it occupies; the calculator does that automatically once you enter the post's width.
A widely used rule of thumb sizes the hole diameter at roughly three times the post's width — a 4×4 post, whose actual dimension is 3.5×3.5 inches, typically gets a 10 to 12-inch hole, leaving enough space for concrete to fully surround and lock the post in place. Hole depth generally runs one-third to one-half of the post's above-ground height, and must reach below the local frost line wherever frost heave could push a shallow post out of the ground over winter.
Once the net concrete volume is known, converting it to bags is straightforward: a standard 50-pound bag of fast-setting post concrete yields about 0.375 cubic feet once mixed, so dividing the net volume by that figure and rounding up gives a safe bag count. Always check your local code for the required hole depth and diameter for fence, deck, and structural posts in your area — frost depth and post-hole requirements vary significantly by region.
Size the hole
Diameter and depth, below the frost line.
Enter the post
Its width, so its volume is subtracted.
Buy the bags
Rounded up per post, times the post count.
The formula explained
Hole volume
= π × (dia⁄24)² × depth. A 10 in hole, 2 ft deep: π × 0.417² × 2 = 1.09 ft³.
Post volume (subtract)
= (width⁄12)² × depth. A 3.5 in post, 2 ft deep: 0.292² × 2 = 0.17 ft³.
Net concrete & bags
1.09 − 0.17 = 0.92 ft³ ÷ 0.375 ft³/bag ≈ 3 bags.
Worked example
Six fence posts, 10 in holes, 2.5 ft deep, 3.5 in posts.
Where post-set concrete is used
Fence posts
The most common use — setting posts for privacy, picket, or split-rail fencing along a property line or yard boundary.
Deck and pergola posts
Structural posts supporting a deck or pergola generally need a deeper hole and a wider post, following local code for footing depth.
Mailbox and sign posts
Small single-post jobs that rarely need more than a bag or two, though breakaway mailbox post rules apply near roadways in some areas.
Round pen and corral posts
Larger agricultural posts set in a ring or straight run, where the same per-post math scales up across many identical holes.
Real-world context
A single mailbox post is the smallest common case: an 8-inch hole, 2 ft deep, around a standard 3.5-inch (4×4) post nets 0.70 − 0.17 = 0.53 cubic feet — just 2 bags of 50 lb fast-setting mix, small enough to mix by hand in a bucket with no equipment beyond a shovel and a post level.
A structural deck post footing shows a bigger, deeper case: a 12-inch hole, 3.5 ft deep (to clear a colder-climate frost line), around a 5.5-inch (6×6) post nets about 2.01 cubic feet — 6 bags per post, which adds up fast across a deck with 6 or 8 footings and is often the point where a homeowner starts comparing bagged mix against a bulk delivery of pre-mixed concrete.
A full fence run multiplies a single-post calculation across many posts: 12 fence posts at 10-inch holes, 2 ft deep, 3.5-inch posts each need 3 bags, for 36 bags total — a case where a small per-post rounding error (say, forgetting to subtract the post volume) compounds into a meaningfully wrong total order across the whole run.
Common misconceptions
"The post takes up so little space in the hole that it's not worth subtracting." It adds up more than it looks. For a typical 4×4 post in a 10-inch hole, the post itself displaces around 15% of the hole's volume — skip that subtraction across a dozen posts and you'll overbuy a meaningful number of bags.
"Deeper is always safer for a fence or deck post." Depth should match the frost line, not exceed it arbitrarily. The point of digging below frost depth is to reach soil that doesn't freeze and heave — digging much deeper than that adds cost and labor without adding meaningful holding power, since the post's resistance to movement comes mostly from the surrounding soil and concrete collar, not sheer depth.
Related calculators
Frequently asked questions
How much concrete do I need per fence post?
Find the hole volume (π × radius² × depth for a round hole), subtract the volume the post itself displaces, then convert the remaining volume to bags. A 10-inch diameter, 2-foot-deep hole with a 4×4 post needs about 0.65 cubic feet of concrete, roughly one 50 lb bag.
Do I need to subtract the post from the hole volume?
Yes. The post occupies real space inside the hole, so ignoring it overestimates the concrete needed. For a square post, subtract width × depth of post in the hole; for a round post, subtract π × radius² × that depth.
How deep should a post hole be?
A common rule is one-third to one-half of the post's above-ground height, and always below your local frost line for a post that must resist frost heave. Deck and fence post depths are set by local code — verify with your building department.
What size hole should I dig for a 4×4 post?
A widely used rule is a hole diameter three times the post width, so a 4×4 post (actually 3.5×3.5 in) typically gets a 10 to 12-inch diameter hole, giving enough room for concrete to fully surround the post.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.