Estimate How Much Concrete You Need For Fence Posts Before You Dig, Mix Or Buy Bags. Enter The Real Hole Size And Depth, Account For The Space Taken By The Post, Add An Optional Waste Allowance And Convert The Result Into Cubic Feet, Cubic Yards And Whole Concrete Bags.
Use A Single Round Hole, A Square Hole, A Full Fence Run Or Work Backwards From The Bags You Already Have. All Numeric Entries Start At 0.
Your result will appear after you enter measurements and press Calculate.
Bag yields are planning values only. Product yield, water demand and installation method vary by manufacturer and mix.
A post hole is not simply a solid cylinder of concrete because the post occupies part of the hole. This diagram keeps every label inside the figure so it stays readable on desktop, tablet and mobile.
The tool separates geometry from purchasing so you can see the actual hole volume first, then translate that volume into an order-friendly bag count and budget.
Calculate round or square post-hole volume from the dimensions you actually plan to dig.
Subtract the space occupied by a round or square post instead of treating the entire hole as concrete.
Enter a quantity directly or estimate posts for one straight fence run from length and spacing.
See cubic feet and cubic yards so you can compare bagged concrete with larger-volume alternatives.
Convert volume to whole bags using the selected or custom manufacturer yield.
Add your local bag price to build a simple concrete-only material estimate before shopping.
A concrete fence post calculator is most useful when it works from the dimensions that will actually be built. Fence posts may look simple from above, but the concrete requirement changes quickly when the hole becomes wider, deeper or more irregular. A 12-inch diameter hole does not use the same amount of concrete as an 8-inch hole, and a 36-inch-deep hole can require substantially more material than a 24-inch-deep hole. The calculator therefore starts with geometry rather than a fixed “bags per post” rule.
For a round post hole, enter the finished hole diameter, the depth that will actually be filled with concrete, the post width or diameter and the number of identical posts. If you place gravel at the bottom of the hole, enter only the depth that will contain concrete. For a square excavation, use the square-hole option. If you are planning one straight fence run, the fence-run mode estimates the number of posts from the fence length and your maximum spacing, then applies the same post-hole volume calculation.
The final purchasing quantity depends on the yield of the bagged concrete you buy. Bag weight alone does not define exact yield, because different mixes can be formulated differently. The calculator includes common planning yields for 50, 60 and 80 lb bags and also provides a custom-yield input. Check the actual product data sheet before purchasing a large quantity.
Hole Volume = π × (Hole Diameter ÷ 2)² × Concrete DepthAll dimensions must be in the same unit before the multiplication. This calculator converts inches to feet internally. The post displacement is then subtracted from the hole. For a square wood post, displacement is post width × post width × embedded concrete depth. For a round metal or wood post, the displacement uses the cylinder formula.
Concrete = (Hole Length × Hole Width × Depth) − (Post Area × Depth)Square holes are less common with augers but can occur when excavating by hand, cutting through an existing surface or preparing special gate and corner posts. Measure the finished excavation instead of relying on the nominal shovel or tool width.
For one uninterrupted straight fence run, a practical planning count is the number of spaces required plus one end post. The calculator uses ceil(fence length ÷ maximum spacing) + 1. For example, a run that is slightly longer than an exact multiple of the spacing needs another interval rather than stretching the final panel beyond the maximum spacing.
QUIKRETE's current post-setting instructions say to dig a post hole about three times the width of the post and describe depth as roughly one-third to one-half of the above-ground post length, with additional gravel at the bottom in its example method. Its set-post calculator also pairs 4-inch posts with 12-inch holes. These are useful manufacturer examples, but they are not a substitute for local code, engineered fence design, frost-depth requirements or conditions at your site.
| Post Width | Example Hole Diameter At 3× | 24 in Concrete Depth | 36 in Concrete Depth |
|---|---|---|---|
| 2 in post | 6 in | Use Calculator | Use Calculator |
| 3 in post | 9 in | Use Calculator | Use Calculator |
| 4 in post | 12 in | Use Calculator | Use Calculator |
| 6 in post | 18 in | Use Calculator | Use Calculator |
Do not treat the table as a universal specification. A privacy fence, open picket fence, agricultural fence, gate post and high-wind fence can impose very different forces. Soil also matters. Loose fill, saturated soil, expansive soil and frost-susceptible ground can require a different foundation approach.
There is no single answer without dimensions. The fastest accurate approach is to calculate the gross hole volume and subtract the post volume. The difference is the space concrete must fill. A small change in hole diameter matters because circular area grows with the square of the radius. Increasing a 10-inch hole to 12 inches does not increase volume by only 20%; the cross-sectional area increases more substantially.
This is why generic rules such as “one bag per post” or “two bags per post” can be misleading. Those rules may happen to fit a common combination of post size, hole diameter and depth, but they can understate or overstate a different fence. Use actual measurements and then round to whole bags.
Bag yield is the bridge between calculated volume and the number of bags to buy. As an example of manufacturer-published yields, Sakrete currently lists approximately 0.38 ft³ for a 50 lb bag, 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag on several concrete mix product pages. Its Fast-Setting Concrete Mix is sold in a 50 lb bag with an approximate 0.38 ft³ yield. These values are convenient defaults, but always use the yield for the exact product you plan to use.
| Bag Size | Planning Yield Used Here | Approx. Bags Per 1 ft³ | Best Practice |
|---|---|---|---|
| 50 lb | 0.38 ft³ | 2.63 | Verify Product Label |
| 60 lb | 0.45 ft³ | 2.22 | Verify Product Label |
| 80 lb | 0.60 ft³ | 1.67 | Verify Product Label |
| Custom | Your Entry | Calculated | Use Data Sheet Yield |
Post holes are rarely perfect cylinders. An auger can wander, loose soil can fall from the walls, roots can create irregular pockets and hand digging may widen part of the hole. A small planning allowance can help protect against coming up short. Sakrete's current calculator guidance encourages adding 10% to estimates as a safety allowance, while the right amount for your project still depends on how predictable the excavation is. This calculator leaves the waste field at 0 by default so you choose the allowance yourself.
If your holes are carefully formed and accurately measured, a smaller contingency may be appropriate. If the soil is loose, the holes are hand-dug or you have many posts, the total uncertainty can be greater. The safest approach is to inspect the actual holes before the final material purchase whenever possible.
These two dimensions are not always identical. Some post-setting methods use a drainage layer of compacted gravel or crushed stone below the concrete. If a 30-inch-deep hole contains 6 inches of gravel and 24 inches of concrete, enter 24 inches as the concrete depth. Otherwise the calculator will count the gravel zone as concrete and overstate the bags needed.
QUIKRETE's post-setting instructions show a 6-inch gravel layer in its example method and then concrete above it. Your installation may differ, particularly where frost depth, drainage, soil type or engineered requirements control the embedment. Follow the project specification that applies to your fence.
Cold climates introduce frost heave risk. The depth needed to resist frost movement is location specific and cannot be determined from a generic concrete volume calculator. Likewise, high-wind areas may require larger or deeper post foundations. Before digging, check local requirements and utility-marking procedures. For structural or unusually tall fencing, consult the appropriate building professional or local authority.
Not every post in a fence carries the same load. Gate posts must resist gate weight and repeated movement. Corner and terminal posts can receive greater lateral load from rails, panels or tensioned fencing. If your project documents specify larger holes for these posts, calculate them separately rather than averaging every post into one hole size.
A useful workflow is to calculate ordinary line posts first, record that result, then calculate gate and corner posts as separate groups. Add the concrete volumes and bag counts at the end. This avoids quietly underestimating the larger foundations.
The calculator can estimate concrete around square or round posts regardless of material, but installation details vary. Wood posts need attention to moisture management and compatible treatment. Metal posts may be round or square and can use smaller cross sections, which changes displacement. Vinyl fencing often uses posts or sleeves designed as part of a proprietary system. Always follow the fence manufacturer's installation details where they conflict with a generic example.
Bagged fast-setting concrete is convenient for small to medium post-setting projects because it can be carried to individual holes and, for certain products, used with a no-mix post-setting method. Large projects can involve dozens or hundreds of bags, at which point labor, lifting, water supply and mixing logistics become important. Compare your total volume with a Concrete Cost Calculator if you are deciding whether a larger-volume supply option is more practical.
If you already know a cubic-yard quantity, the Concrete Bags Per Cubic Yard Calculator can convert between yardage and bag quantities. For general slab, footing and volume work, use the site's Concrete Volume Calculator. These tools solve different parts of the same planning problem.
Example calculations are best treated as demonstrations of method, not universal specifications. Suppose a project uses a round hole and square post. The gross hole is calculated as a cylinder. The embedded post volume is a square prism. Subtracting the post gives net concrete volume. Multiply by the number of identical posts, add the chosen contingency, and divide by product yield to determine whole bags.
For a long fence run, post count comes first. Divide the run length by the maximum planned spacing, round the number of spaces up, then add one post for the far end. Add corner, terminal and gate posts separately where the layout requires them. Once the final number of each post type is known, calculate concrete by group.
Concrete bags are purchased as whole units. A calculation of 21.1 bags cannot be bought as 21.1 standard bags, so the purchasing estimate rounds up to 22. This is separate from the waste allowance. Waste adjusts the theoretical volume first; whole-bag rounding then converts that adjusted volume into a practical purchase quantity.
The Concrete Fence Post Calculator estimates material volume. It does not determine fence structural adequacy, allowable height, wind design, footing reinforcement, post strength, corrosion protection, rot resistance, frost depth, soil bearing capacity, property lines, permit requirements or utility clearance. Those items must come from the applicable fence system, plans, engineer, local authority and site conditions.
The number of bags is only as reliable as the dimensions and product yield behind it.
Verify actual diameter, width and concrete-filled depth.
Count line, corner, gate and terminal posts separately where needed.
Read the exact product label or technical data sheet before purchasing.
Confirm frost depth, permits, utilities and project-specific structural requirements.
These are the inputs most likely to make one fence-post estimate different from another.
Common questions about post-hole size, bag estimates, gravel, spacing and concrete volume.
Enter the actual hole diameter or width, concrete depth and post size. The required concrete is the hole volume minus the embedded post volume. Because hole sizes vary, there is no one bag count that fits every post.
The bag count depends on net concrete volume and the yield of the exact bag. Divide adjusted volume by the manufacturer's yield and round up to a whole bag.
QUIKRETE's post-setting instructions use about three times the post width as an example starting point. Project requirements, soil, frost, wind and local rules may require different dimensions.
Depth is design and location dependent. Manufacturer examples may reference a fraction of above-ground post height, while frost depth and local requirements can control in colder climates. Confirm the required depth for your specific fence.
Yes. Round-hole mode can subtract a square or round post, and square-hole mode subtracts a square post. You can also choose to ignore displacement in round-hole mode.
No. If part of the hole is filled with gravel, enter only the depth that will actually contain concrete.
The fence-run mode uses the run length and maximum spacing to estimate spaces, rounds up, then adds the second end post. Corners, gates and separate fence segments can add more posts.
Yes. Use the calculator again with the larger gate-post hole dimensions, then add that result to your line-post total.
Sakrete currently publishes approximately 0.38 cubic feet for certain 50 lb concrete mixes, including its Fast-Setting Concrete Mix. Always verify the exact product you buy.
A common published planning yield is about 0.45 cubic feet for certain 60 lb mixes. Product formulations vary, so use the manufacturer's stated yield when available.
A common published planning yield is about 0.60 cubic feet for certain 80 lb mixes. Confirm the exact bag before buying.
Some manufacturer calculators recommend a 10% safety allowance. This tool leaves the value at 0 so you can choose an allowance appropriate for your holes and site conditions.
Yes, products specifically intended for post setting are available. Follow the exact manufacturer's water, placement, set-time and safety instructions for the product you choose.
The geometry works for many round or square post holes, but structural requirements differ. Use the required hole dimensions and installation instructions for the actual application.
The result stays hidden until you press Calculate so the page does not imply a meaningful estimate before measurements and bag yield have been entered.
Use manufacturer instructions and local requirements alongside any online estimate.
Post-hole sizing, gravel and fast-setting concrete installation guidance.
Read QUIKRETE InstructionsManufacturer set-post calculator and example hole/post pairings.
Open QUIKRETE CalculatorPublished 50 lb product yield and post-setting product information.
View Sakrete Product DataManufacturer guidance about hole dimensions, gravel, placement and post-setting workflow.
Read Sakrete Guide