Concrete bags per post calculator
Use this quick calculator when you already know the hole diameter and concrete depth for one fence post.
Conservative full-hole estimate using planning yields of 0.30, 0.45 and 0.60 ft³ per 40, 60 and 80 lb bag. Confirm the yield printed on your mix.
Concrete bags per fence post chart
| Hole diameter × depth | Concrete volume | 40 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 8 in × 24 in | 0.70 ft³ | 3 | 2 | 2 |
| 8 in × 36 in | 1.05 ft³ | 4 | 3 | 2 |
| 10 in × 24 in | 1.09 ft³ | 4 | 3 | 2 |
| 10 in × 36 in | 1.64 ft³ | 6 | 4 | 3 |
| 10 in × 48 in | 2.18 ft³ | 8 | 5 | 4 |
| 12 in × 24 in | 1.57 ft³ | 6 | 4 | 3 |
| 12 in × 36 in | 2.36 ft³ | 8 | 6 | 4 |
| 12 in × 48 in | 3.14 ft³ | 11 | 7 | 6 |
| 14 in × 36 in | 3.21 ft³ | 11 | 8 | 6 |
| 14 in × 48 in | 4.28 ft³ | 15 | 10 | 8 |
| 16 in × 36 in | 4.19 ft³ | 14 | 10 | 7 |
| 16 in × 48 in | 5.59 ft³ | 19 | 13 | 10 |
Planning assumptions: approximately 0.30 ft³ per 40 lb bag, 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag. Product yields vary; use the yield printed on the mix you purchase.
Fence-post concrete formula
Treat a round hole as a cylinder: volume = π × radius² × concrete depth. Keep all dimensions in the same unit, convert the result to cubic feet, divide by the concrete mix yield in cubic feet per bag, and round up to a whole bag.
How much concrete for a 4×4 or 6×6 fence post?
The post size does not determine the bag count by itself. A nominal 4×4 is about 3.5 inches square and a nominal 6×6 is about 5.5 inches square, but the concrete quantity is driven mainly by the hole diameter and concrete depth. The post displaces some concrete, while over-digging and irregular soil can add volume, which is why the chart uses the conservative full-hole volume.
Example: 12-inch hole, 36 inches deep
A cylindrical hole 12 inches in diameter and 36 inches deep has about 2.36 cubic feet of volume before accounting for the volume occupied by the post itself. Using the conservative full-hole method shown here, that rounds up to about 4 × 80 lb bags, 6 × 60 lb bags or 8 × 40 lb bags.
How many bags of Quikrete per fence post?
Quikrete bag sizes and product yields vary, so use the cubic-foot yield printed on the specific bag. Divide the hole volume by that yield and round up. If an 80 lb mix yields about 0.60 ft³, the 12 in × 36 in example above requires about 4 bags with the full-hole planning method.
How much Postcrete or post mix per post?
Fast-setting post mixes such as Postcrete are sold in package sizes with their own stated coverage or yield. Do not convert by package weight alone. Calculate the hole volume, check the product's stated yield, and round up to enough whole bags for each post.
How many posts can one bag of concrete set?
Reverse the same calculation: posts per bag = bag yield ÷ concrete volume per post. If a hole needs more concrete than one bag yields, the answer is less than one post per bag. Small mailbox or sign-post holes may use less than one bag; typical fence-post holes often use multiple bags.
How much concrete for an entire fence?
Bag count per post is only part of the takeoff. The Fence Post Concrete Calculator estimates post count from fence length and spacing, then calculates the concrete for all holes. For posts, rails, pickets, gates, concrete and fasteners together, use the PlanDIYBuild Fence Calculator.
Why your real quantity can differ
The post itself displaces some concrete, while irregular, bell-shaped or over-dug holes can use more. Hole dimensions should be selected for the fence, soil, frost conditions, loading and local requirements—not from a bag-count chart alone.
Frequently asked questions
How many bags of concrete do I need per fence post?
The bag count depends on the hole diameter, concrete depth and yield of the mix. As a conservative full-hole example, a 12-inch-diameter hole filled 36 inches deep is about 2.36 cubic feet, or about 4 80 lb bags when each bag yields approximately 0.60 cubic foot.
How many bags of Quikrete do I need for a fence post?
Use the yield printed on the Quikrete bag and divide the post-hole volume by that yield. For planning, an 80 lb bag is often estimated around 0.60 cubic foot, so a 12-inch by 36-inch full hole is about 4 bags before accounting for post displacement.
How much Postcrete or post mix do I need per post?
Post-setting products vary in package size and yield. Calculate the hole volume first, then divide by the stated yield on the Postcrete or post-mix package and round up.
How many fence posts can one 80 lb bag of concrete fill?
It depends on the post-hole dimensions. One 80 lb bag with an approximate 0.60 cubic foot yield can fill one small hole requiring no more than that volume, but larger or deeper holes require multiple bags.
Should I subtract the fence post volume from the concrete calculation?
The post displaces concrete, but irregular or over-dug holes can add volume. This chart intentionally uses the conservative full-hole volume for planning. Use the yield printed on your mix and adjust for your actual installation.