A 3.5 × 3.5-inch square post in a 12-inch-diameter cylindrical concrete zone, 30 inches high, needs about 1.751 cubic feet of concrete before allowance. At the cited 0.375 cu ft yield that rounds up to 5 bags of 50-lb mix; at 0.60 cu ft it rounds up to 3 bags of 80-lb mix.1 The 30 inches is concrete height around the post, not automatically the total excavation depth.
Bag count depends on actual post shape, cylinder diameter, concrete height and the exact bag yield. The tables below are quantity scenarios, not recommended footing dimensions. Use the fence post concrete calculator for identical square or round posts. A 4×6 rectangular post needs the separate width × thickness formula shown here.
Calculate the concrete zone, not every inch of the hole
Net concrete (cu ft) = [π × (hole diameter / 2)² – actual post width × thickness] × concrete fill height / 1,728.
Bags = net concrete × post count × (1 + allowance percent / 100) / mixed yield per bag, rounded up. All dimensions in the volume formula are inches. For a round post, replace width × thickness with π × (post diameter / 2)². Keep full precision until the final whole-bag rounding.
The formula assumes the post occupies the whole concrete height, and the concrete zone is cylindrical. Soil cover and gravel bedding are not concrete. A footing pad below the post does not have post displacement and must be calculated separately. Flared, bell-shaped or uneven holes need their actual geometry; the table is not a field measurement.
Example bag yields
| Product example | Bag weight | Approximate mixed yield |
|---|---|---|
| QUIKRETE Concrete Mix | 50 lb | 0.375 cu ft |
| QUIKRETE Concrete Mix | 60 lb | 0.45 cu ft |
| QUIKRETE Concrete Mix | 80 lb | 0.60 cu ft |
These are yields from the cited Concrete Mix sheet, not a guarantee for every product of the same weight.1 The cited Fast-Setting sheet also lists 0.375 cu ft for 50 lb and 0.45 for 60 lb.2 Check the actual bag, follow that product’s water and mixing directions, and rescale the volume if yield differs.
4×4, 4×6 and 6×6 bag table
Our calculated table uses the labeled actual post dimensions, a 12-inch cylinder diameter and no quantity allowance. The columns give concrete fill height, not total hole depth. Each cell shows net volume and bags rounded for one post. The diameter is a quantity example, not approved concrete clearance or a structural detail.
| Post model | 24 in concrete | 30 in concrete | 36 in concrete |
|---|---|---|---|
| 4×4 (3.5 × 3.5 in actual) | 1.401 cu ft 4 × 50 lb; 3 × 80 lb | 1.751 cu ft 5 × 50 lb; 3 × 80 lb | 2.101 cu ft 6 × 50 lb; 4 × 80 lb |
| 4×6 (3.5 × 5.5 in actual) | 1.303 cu ft 4 × 50 lb; 3 × 80 lb | 1.629 cu ft 5 × 50 lb; 3 × 80 lb | 1.955 cu ft 6 × 50 lb; 4 × 80 lb |
| 6×6 (5.5 × 5.5 in actual) | 1.151 cu ft 4 × 50 lb; 2 × 80 lb | 1.438 cu ft 4 × 50 lb; 3 × 80 lb | 1.726 cu ft 5 × 50 lb; 3 × 80 lb |
Diameter comparison for a 3.5-inch square post
Calculated at a 30-inch concrete fill height, no allowance and one post. Do not select an excavation diameter from bag count; design and clearance decide the diameter first.
| Concrete-zone diameter | Net volume | 50 lb at 0.375 | 80 lb at 0.60 |
|---|---|---|---|
| 10 in | 1.150865 cu ft | 4 | 2 |
| 12 in | 1.750822 cu ft | 5 | 3 |
| 14 in | 2.459862 cu ft | 7 | 5 |
| 16 in | 3.277985 cu ft | 9 | 6 |
Three checked examples
1. 1 post, 12-inch cylinder and 30-inch concrete height
Actual rectangular post: 3.5 × 3.5 inches. Net per post: [π × (12/2)² – 3.5 × 3.5] × 30 / 1,728 = 1.750822 cu ft. Across 1 posts with 0% example allowance: 1.750822 cu ft. At 0.375 cu ft per bag: 4.668858, rounded up to 5 bags. These dimensions are arithmetic examples, not an approved footing.
2. 10 posts, 12-inch cylinder and 30-inch concrete height
Actual rectangular post: 3.5 × 5.5 inches. Net per post: [π × (12/2)² – 3.5 × 5.5] × 30 / 1,728 = 1.629294 cu ft. Across 10 posts with 10% example allowance: 17.922234 cu ft. At 0.6 cu ft per bag: 29.870390, rounded up to 30 bags. These dimensions are arithmetic examples, not an approved footing.
3. 1 post, 12-inch cylinder and 26-inch concrete height
Actual rectangular post: 5.5 × 5.5 inches. Net per post: [π × (12/2)² – 5.5 × 5.5] × 26 / 1,728 = 1.246546 cu ft. Across 1 posts with 0% example allowance: 1.246546 cu ft. At 0.375 cu ft per bag: 3.324121, rounded up to 4 bags. These dimensions are arithmetic examples, not an approved footing.
Cover and bedding change the concrete height
The Fast-Setting sheet describes stopping the mix approximately 3 to 4 inches below the top and filling the remainder with soil or sod.2 The regular Concrete Mix sheet describes placing 6 inches of dry mix at the bottom before positioning the post, then premixing the rest.1 Those are different product instructions. Do not combine them into a universal “6 inches of gravel, then fill to grade” detail.
If a checked section has a 36-inch excavation, 6 inches of gravel bedding and 3 inches of soil cover, the concrete zone around the post would be 27 inches: 36 – 6 – 3. That is an illustration only. A concrete pad beneath the post must be added separately without subtracting the post from its volume. The calculator assumes displacement throughout its entered concrete height, so it will not automatically include that pad.
Round the batch, not every post, when sharing material is practical
For identical posts, add the unrounded concrete volumes, apply any separately chosen quantity allowance, then divide by bag yield and round up. Per-post rounding can produce a different result. Plan mixing batches and working time under the actual product instructions; do not mix all material at once just because the calculation combines it.
Example: ten 3.5-inch square posts in 12-inch cylinders with 30-inch concrete height use 17.508218 cu ft before allowance. At 0.60 cu ft/bag, the total is 29.180363, or 30 bags. Rounding each post to 3 bags first would give 30. With an example 10% allowance, batch rounding gives 33 bags. These are distinct planning models.
Hole size and depth need a separate check
QUIKRETE sheets give product-scoped post-setting proportions, while the regular Concrete Mix sheet says load-bearing applications must follow local footing requirements.1,2 This guide does not choose frost depth, soil bearing, wind/gate loads or concrete clearance. Verify the fence system, actual post size, property boundary and local requirements. Gate and corner posts can need different checked details, so estimate them as separate groups.
A square or rectangular post must fit by its diagonal, not just its side width: diagonal = sqrt(width² + thickness²). That is only a geometric fit test, not adequate concrete clearance. A round post uses its diameter. The calculator checks this basic fit for its listed options but does not approve the footing.
Before digging and mixing
- Contact 811 and follow your state’s locate, waiting, response and safe-digging requirements; private lines may need separate locating.
- Confirm the actual concrete-zone dimensions and any below-post pad, bedding or cover.
- Use the exact product’s water, placement, curing and loading instructions. A set-time claim is not automatic permission to hang or load the fence.
- Choose a quantity allowance for the real hole irregularity and handling plan; no universal extra-bag rule is given here.
FAQ
How many bags for a 4×4 post?
For the specific 3.5-inch square-post, 12-inch cylinder and 30-inch concrete-height example, 1.751 cu ft net gives 5 bags at 0.375 yield or 3 at 0.60, before allowance. A nominal post label alone does not determine bag count.
Does a 36-inch hole mean 36 inches of concrete?
Not necessarily. The chosen section may include soil cover, bedding or a pad below the post. Calculate the concrete zone and any pad separately; do not assume the table’s fill height is total excavation.
Can I use the calculator for a 4×6?
Its options are square posts and a 3-inch round post, not a 4×6 rectangle. Use actual width × thickness in the displacement formula above. Do not select a square option as a substitute.
How soon can I hang the fence?
Follow the exact mix and fence-system instructions for curing and loading. The Fast-Setting and regular Concrete Mix sheets give different directions; this page does not turn initial set time into a blanket load approval.
Related tools
Fence post concrete calculator | Concrete calculator | Fence picket calculator
Sources
- QUIKRETE: Concrete Mix sheet (bag yields, premixed post-setting detail, below-post mix and local footing caveat)
- QUIKRETE: Fast-Setting Concrete Mix sheet (bag yields, product-specific placement and soil-cover detail)