Post hole concrete calculator

How to calculate concrete for a post hole

A cylindrical hole for a fence, deck or mailbox post. The post displaces concrete, so subtracting it gives a materially more accurate bag count. The volume formula is π × radius² × depth, minus the volume the post itself occupies, worked in feet and then converted. There are 27 cubic feet in a cubic yard, which is the unit every ready-mix plant quotes in. The default project on this page comes to 18.2 cubic feet, or 0.68 cubic yards.

The conversion that catches people on a round pour is the radius. Holes and forms are sold by diameter, but the formula squares the radius. A 10-inch hole has a 5-inch radius, or 0.417 feet, and squaring the diameter instead returns four times the concrete you actually need.

On top of the geometric volume the calculator adds a 10% waste allowance. That is not padding: subgrade is never perfectly level, forms bow outward under the weight of wet concrete, and some is always lost to the wheelbarrow. Ordering the exact calculated volume reliably leaves you short, and being short is the expensive direction. A partial second load carries the $75 short-load fee and leaves a cold joint where it meets concrete that has already begun to set.

How many bags of concrete for a post hole

Bag yields are the manufacturers' published figures: a 40 lb bag of QUIKRETE or Sakrete concrete mix yields 0.3 cubic feet, a 60 lb bag 0.45, and an 80 lb bag 0.6. That is 90, 60 or 45 bags to a cubic yard. For this project the concrete calculator returns 31 eighty-pound bags, 41 sixty-pound or 61 forty-pound.

All three counts are shown at once because the decision at the store is rarely about price. Larger bags are marginally cheaper per cubic foot and considerably heavier to move, 31 eighty-pound bags is 1.2 tons to load, carry and cut open, and 60 lb bags are what many people can actually handle repeatedly.

At 0.68 cubic yards this is comfortably inside DIY territory, a rented mixer and a helper will get through it in a morning.

Bags or ready-mix for a post hole?

Ready-mix is cheaper per yard almost every time: $155 delivered against $293 a cubic yard bought in 80 lb bags. What bags buy is scheduling, no delivery window, no minimum load, and the freedom to stop and start. That is worth real money below about 2 cubic yards and worth nothing above it.

For this project specifically, bags come to $202 and ready-mix about $180, including the $75 short-load fee that applies under 5 yards. There is no finishing labor line on this shape. The concrete is buried, so nobody is troweling a surface.

A truck also arrives uniformly mixed, which matters more than it sounds. Concrete mixed in batches at slightly different water ratios cures to different strengths, and a pour that stretches over several hours develops cold joints where each batch meets the last.

Post holes, frost depth and the post itself

Depth is set by whichever is deeper: a third of the post's above-ground height, or the local frost line. For a 6-foot fence that means about 30 inches in a mild climate and up to 60 inches in Minnesota or North Dakota. Each extra foot of depth in a 10-inch hole is another 0.55 cubic feet of concrete, or roughly one more 80 lb bag per post, which is how a northern fence ends up costing meaningfully more than an identical southern one.

This calculator subtracts the volume the post itself occupies, which most post hole calculators do not. On a 4x4 in a 10-inch hole that is about 15% of the cylinder, and across a fence with twenty posts it is the difference between the right number of bags and half a pallet too many.

Do not fill the hole to the top and leave it flat. The usual detail is 4 to 6 inches of compacted gravel at the bottom for drainage, then concrete up to just below grade, crowned so water sheds away from the post. A flat concrete collar holds water against the post, which is where wood posts almost always begin to rot.

How we calculated these estimates

Every figure on this page comes from the same open formula the calculator above uses. Volume is pure geometry, the shape's formula worked in feet, plus a 10% waste allowance. Prices start from 2026 national baselines: $155 per cubic yard delivered, a $75 short-load fee under 5 yards, retail bag prices against the yields QUIKRETE and Sakrete publish, and $3.50 per square foot for placing and finishing flatwork. Ready-mix and labor are then scaled by the construction earnings multiplier the Bureau of Labor Statistics publishes for your state; bagged concrete is not, because it is priced nationally. Nothing is hidden behind a lead form and no supplier pays to appear here. What the result is not is a quote: excavation, base preparation, forms, reinforcement, permits and demolition are excluded, and a plant will price your delivery distance and mix specification on top.