Concrete Post Hole Calculator
A 12 inch hole with 24 inches of concrete and a 3.5 inch post needs 3 bags of 80 lb mix. Ten holes ordered separately are 30 bags.
Last updated . Weight starts at 2,400 kg/m³. An 80 lb bag yields 0.60 ft³. The post is subtracted. No prices.
Count the concrete in a fence or deck post hole. The hole is a cylinder. The post, square or round, is taken out for the same depth. Bags are shown per hole and for the whole row.
Gravel under the post is the post hole gravel calculator. How far apart the posts stand is the fence post spacing calculator. A pier with no post in it is the column calculator.
Post hole concrete
This is the concrete, not gravel under the post. Shorten the depth if the post sits on stone.
A dressed 4×4 is 3.5 inches square. Enter 4 only if the wood is a full 4 inches. A larger post subtracts too much concrete.
Same yields as the bag calculator. Weight uses 2400 kg/m³. No price.
Results
How the hole is counted
The hole volume is π × radius × radius × concrete depth. The post volume is the post’s cross section times that same depth. Concrete is the difference. A 12 inch hole, 24 inches deep, minus a 3.5 inch square post, is 1.40 cubic feet. The post is about 11 percent of the hole.
A square post fits only when its diagonal is less than the hole. The diagonal is the side times 1.414. A dressed 4×4 is 3.5 inches, not 4. Using 4 inches subtracts wood that is not there and the concrete order comes out short.
Worked example
Leave the hole at 12 in by 24 in, the post on square 3.5 in, the count at 10, and the bag on 80 lb. One hole reads 0.05 yd³ · 1.40 ft³ · 0.04 m³. Bags per hole read 3. The order, per hole times the count, reads 30. All holes read 0.52 yd³ · 14.01 ft³ · 0.40 m³. Bags for one pour read 24. Weight reads 0.95 tonnes · 1.05 short tons.
Frequently asked questions
How much concrete is in one post hole?
Leave the hole at 12 inches across and 24 inches of concrete, and the post at 3.5 inches square. One hole reads 0.05 yd³, 1.40 ft³, and 0.04 m³. The post is subtracted for that full depth. Bags per hole read 3 of the 80 lb mix.
How many bags for 10 post holes?
The count opens at 10. Ordering each hole on its own is 3 × 10 = 30 bags. The order line reads 30. All ten holes together are 0.52 yd³, 14.01 ft³, and 0.40 m³, which is 0.95 tonnes and 1.05 short tons at 2,400 kg/m³.
Why is one pour 24 bags instead of 30?
Each hole is 1.40 ft³. Divided by 0.60 ft³ that is 2.33, which rounds up to 3 bags. Ten separate roundups are 30 bags. The combined 14.01 ft³ ÷ 0.60 is 23.35, which rounds up to 24 bags. Rounding every hole up costs more than rounding the whole pour once. Buy 30 if you mix each hole from its own bags.
Should I enter 3.5 inches or 4 inches for a 4×4?
A dressed 4×4 is 3.5 inches square. Enter 3.5. Entering 4 subtracts a larger post than the wood, so the concrete comes out low and the bag count can run short. Use 4 only when the timber is a full 4 inches.
What if the post does not fit?
A square post has to clear the hole on the diagonal, the side times 1.414. A round post has to be smaller than the hole. When it does not fit, the volume reads 0.00 and the note says the post does not fit. Widen the hole or use the real post size.
Does the depth include gravel under the post?
No. The depth is the concrete only. If the post sits on gravel, shorten this depth to the concrete and count the stone on the post hole gravel calculator. This page does not add that gravel, and the gravel page does not subtract a post from concrete.
Is this a footing or a sonotube?
No. A rectangular footing is the footing calculator. A round pier with no post subtracted is the column calculator. This page is the concrete left after the post is taken out of a round hole. Spacing along a fence is the fence post spacing calculator.