Post Hole Concrete Calculator
The Post Hole Concrete Calculator estimates how many bags of concrete you need to set posts, accounting for the post's own volume displacing space in the hole. Works in U.S. or metric units. The formula treats both the hole and the post as perfect cylinders (π × radius² × depth), so a square post's width is used as if it were a circular diameter — a circle of that diameter has about 21% less cross-section than the square it fits inside, so this slightly understates how much volume a square post actually displaces, which makes the concrete estimate marginally conservative rather than short. If you set a post width equal to or greater than the hole diameter, the calculator floors the concrete volume at zero instead of returning a negative number, since a post that fills the whole hole needs no concrete around it. Because bag yield jumps in fixed steps (about 0.3 cu ft for a 40 lb bag, 0.45 for 60 lb, 0.6 for 80 lb), switching bag sizes changes your bag count by more than a simple linear ratio — always round the final total up, since suppliers don't sell partial bags.
Switch between U.S. Customary and Metric units.
How This Tool Works
Pick your units, then enter your hole's diameter and depth, your post's width, and the number of posts to calculate total bags of concrete needed.
Formula & Method
Concrete volume = hole volume (π × radius² × depth) minus post volume (π × radius² × depth), then divided by bag yield. Enter hole and post dimensions in inches (lb bags) or centimeters (kg bags).
Example Calculation
A 12 inch diameter hole, 24 inches deep, with a 4 inch (4×4) post, for 2 posts using 60 lb bags: hole volume per hole = π × 6² × 24 = 2,714.34 cubic inches. Post volume per hole = π × 2² × 24 = 301.59 cubic inches. Concrete volume per hole = 2,714.34 − 301.59 = 2,412.74 cubic inches ÷ 1,728 = 1.396 cubic feet. For 2 posts: 1.396 × 2 = 2.79 cubic feet total. At 0.45 cubic feet per 60 lb bag: 2.79 ÷ 0.45 = 6.2, rounded up to 7 bags. In metric, a 30 cm diameter hole, 60 cm deep, with a 10 cm post, for 2 posts using 25 kg bags: concrete volume per hole ≈ 1.331 cubic feet (37.7 L), total for 2 posts ≈ 2.66 cubic feet (75.4 L, 0.075 m³) — at 0.415 cubic feet per 25 kg bag, that's also 7 bags.
About the Post Hole Concrete Calculator
Setting a post in concrete looks simple until you stand in a hardware store aisle and try to guess how many bags a whole fence line will take. The volume of a round hole grows with the square of its diameter, so a hole that is only a few inches wider than planned swallows far more concrete than people expect. This calculator removes the guessing. It treats the hole and the post as cylinders, subtracts the volume the post occupies, multiplies by the number of posts, and divides by the yield of the bag size you choose.
The answer is a count of premixed concrete bags, rounded up to the next whole bag. The yield per bag is the amount of wet concrete a bag makes once water is added: about 0.3 cubic feet for a 40 pound bag, 0.45 for a 60 pound bag, and 0.6 for an 80 pound bag. Metric mode uses bags of 20, 25, and 30 kilograms and reports volume in liters and cubic meters, which is how many stores outside the United States label their products.
Fence builders, deck and pergola builders, mailbox installers, and homeowners putting in a basketball hoop or flagpole all use this kind of estimate. It is especially useful for jobs with many posts, where a small error per hole multiplies into dozens of bags. The tool covers the concrete around the post only. It does not account for a gravel drainage layer, extra concrete for a bell-shaped base, or concrete poured above ground level for a collar.
When to Use It
Building a privacy fence
A homeowner is installing twelve 4x4 posts for a backyard fence. They enter the planned hole diameter and depth, the actual post width, and the number of posts, then compare the bag counts for 40, 60, and 80 pound bags to see which size makes the load easier to carry and the price per bag worthwhile.
Setting deck or pergola posts
A builder framing a pergola needs four large posts set deep enough for stability. They enter bigger holes and a thicker post, read the total volume and bag count, and add the figure to the materials list alongside lumber, brackets, and fasteners so the supplier delivers everything in one trip.
Installing a mailbox or flagpole
A single post still needs the right amount of concrete. Someone installing a mailbox post enters one post and a modest hole, gets a bag count of one or two, and avoids the common mistake of buying a pile of bags that will go unused and harden in the garage.
Comparing bag sizes before shopping
A landscaper wants to decide between buying many small bags or fewer large ones. They run the same job with each bag size, compare the counts and the lifting involved, and choose the size that balances price, effort, and how much concrete they can mix at once.
How to Use It, Step by Step
- Select the U.S. or Metric option first. U.S. mode takes inches and offers 40, 60, and 80 pound bags, while Metric mode takes centimeters and offers 20, 25, and 30 kilogram bags.
- Enter the Hole diameter, which is the width of the dug hole. A common planning guide is a diameter about three times the width of the post, but your fence plan, soil, or code may call for something different.
- Enter the Hole depth, measured from the ground surface to the bottom of the hole. If you will put a layer of gravel in the bottom for drainage, enter only the depth that the concrete will actually fill.
- Enter the Post width using the actual measured size. A nominal 4x4 post is about 3.5 inches wide in reality, and a nominal 6x6 post is about 5.5 inches, so measure instead of trusting the label.
- Enter the Number of posts, counting every post in the run, including the end and corner posts, which are often set deeper or with larger holes in practice.
- Pick the Bag size that you plan to buy. The yield changes in fixed steps, so the same job can need quite different bag counts depending on this choice.
- Press the calculate button and read the concrete volume per hole, the total concrete volume, and the number of bags. Compare the volume per hole with a quick hand estimate to catch unit errors.
- Round your purchase up for waste, and check the bag label for the yield figure for the specific brand, since manufacturers publish slightly different yields.
Understanding Your Results
The first volume line tells you how much wet concrete one hole needs after the post's own volume has been subtracted. In U.S. mode it is in cubic feet, and in Metric mode it is in liters. A 10 inch hole that is 3 feet deep with a 4 inch post needs about 1.4 cubic feet, which is a handy number to remember when you eyeball other sizes.
The second line is the total for all posts, and the bag line converts that total into the number of bags of your chosen size, rounded up. A result that lands just above a whole number rounds up by a full bag, so if you see something like 36.6, you will need 37 and have a small surplus. That surplus is useful, because spillage and uneven holes will use it up.
Use the bag count as a minimum, then add a few extra bags for large jobs. Check the bag label for the water quantity and set time, and mix in small batches if the product is fast setting. If your plan includes a gravel base, a concrete collar, or deeper corner posts, add those separately, since they are not in the calculation.
More Worked Examples
A twelve-post fence line, three bag sizes
Scenario: Twelve posts in holes 10 inches across and 36 inches deep, with a 4 inch post width, compared for 40, 60, and 80 pound bags.
Hole volume = pi x 5 x 5 x 36 = 2,827 cubic inches. Post volume = pi x 2 x 2 x 36 = 452 cubic inches. Concrete per hole = 2,375 cubic inches = 1.374 cubic feet. For 12 posts the total is 16.49 cubic feet. With 40 pound bags at 0.3 cubic feet, that is 55 bags. With 60 pound bags at 0.45 it is 37 bags, and with 80 pound bags at 0.6 it is 28 bags.
Metric posts for a garden screen
Scenario: Eight posts, each in a hole 30 cm across and 90 cm deep, with a 10 cm post, compared for 20 kg and 30 kg bags.
The tool reports 56.5 liters of concrete per hole and a total of 452.4 liters (0.452 cubic meters) for the eight posts. With 30 kg bags, which yield 14.1 liters each, the answer is 33 bags. With 20 kg bags, yielding 9.4 liters each, the same job needs 49 bags. The bigger bag reduces the count by 16 bags, but each is heavier to carry and mix.
Four heavy pergola posts and the effect of the post
Scenario: Four posts in holes 16 inches wide and 42 inches deep, with 6 inch posts, using 80 pound bags, compared with entering a post width of 0.
With the 6 inch post subtracted, the tool reports 4.2 cubic feet per hole and 16.8 cubic feet in total, which needs 28 bags at 0.6 cubic feet each. If you enter a post width of 0, as if the post took no space, the volume per hole becomes 4.887 cubic feet, the total 19.55, and the bag count 33. The post's displacement therefore saves five bags in this job.
Tips for Better Results
- Measure the hole you actually dug. Auger bits and posthole diggers often leave holes slightly wider than their nominal size, and the volume rises with the square of the diameter.
- Put a few inches of gravel at the bottom of each hole so water drains away from the base of the post, and enter the depth that the concrete will actually fill.
- Slope the top of the concrete away from the post so rain runs off instead of pooling around the wood, which is where rot often begins.
- Brace posts plumb with stakes and scrap lumber while the concrete sets. Check them with a level on two adjacent faces, because they are easiest to correct within the first few minutes.
- Set the depth with the frost line and the post's height in mind. In cold climates, holes often need to extend below the frost depth to avoid movement, which your local building office can confirm.
- Buy a few more bags than the tool says, and keep the bags dry. Opened bags harden quickly if they absorb moisture, so a surplus is only useful if you can store it properly.
Common Mistakes to Avoid
- Entering the nominal post size instead of the actual one. A 4x4 post is closer to 3.5 inches wide, and using 4 slightly underestimates the concrete you need.
- Forgetting that the bag size changes the count in steps. A job that needs a fraction of a bag still needs the whole bag, and switching sizes can reduce the total unexpectedly.
- Counting only the line posts and forgetting the end, corner, and gate posts. Those usually get bigger holes, so add them separately with their own dimensions.
- Mixing up units, for example entering centimeters in U.S. mode. The volume line is the quickest check, since a hole that needs hundreds of cubic feet signals a unit error.
- Ignoring gravel and collars. If you place gravel at the bottom or pour a concrete collar above ground, the calculation will not include them, so adjust the depth or add bags.
Assumptions and Limits
- The hole and the post are both treated as cylinders. A square post such as a 4x4 occupies more volume than a circle of the same width, so using the width as a diameter slightly understates the post's displacement and leaves the bag count marginally high.
- Bag yields are fixed values built into the tool: 0.3, 0.45, and 0.6 cubic feet for the U.S. sizes, and 9.4, 11.75, and 14.1 liters for the metric sizes. Brands can differ a little, so check your bag label.
- The calculation assumes the concrete fills the hole from the bottom to the top over the full depth entered. It does not model gravel layers, bell-shaped bases, collars, or holes that are wider at the bottom.
- Footing depth, soil conditions, frost, and wind loads can all affect how a post should be set. For structural posts, such as decks, guard rails, and tall poles, follow your local building rules and consult a qualified professional.
Key Terms
- Yield
- The volume of wet concrete a bag produces after mixing, usually printed on the bag in cubic feet or liters and used to convert volume into bags.
- Frost line
- The depth to which soil freezes in winter. Footings and post holes in cold climates often extend below it so the ground cannot heave them upward.
- Auger
- A rotating drill used to dig round holes for posts. The auger's diameter largely determines the hole size and therefore the amount of concrete needed.
- Nominal size
- The name of a lumber size, such as 4x4, which differs from the actual dimensions. A 4x4 post measures about 3.5 by 3.5 inches.
- Plumb
- Exactly vertical. A plumb post stands straight up and down, which you check with a level on adjacent faces before the concrete sets.
- Fast-setting concrete
- A premixed product designed to harden within minutes to hours after water is added, often used for posts so they can be braced and left.
Frequently Asked Questions
How deep should a fence post hole be?+
A common rule is to bury one-third to one-half of the post's above-ground height, with a minimum depth of 24 inches (about 60 cm) for most residential fences.
Can I use metric hole sizes and kg bags?+
Yes — switch the unit toggle to Metric and enter your hole and post dimensions in centimeters. Bags are sized in kilograms and the volume is returned in liters and cubic meters.
Do I need to subtract the post's volume?+
Yes — the post itself displaces concrete, so subtracting its volume gives a more accurate bag count instead of over-ordering.
Should I add gravel at the bottom of the hole?+
Many installers add a few inches of gravel at the bottom for drainage before pouring concrete — factor that into your hole depth if you plan to do this.
What if my post is round instead of square?+
Enter the post's actual diameter in the post width field — the calculator already treats the post as a circle in its math, so a round post plugs in directly with no conversion needed. For a square post like a 4×4, entering its face width is a reasonable approximation since the formula runs it through the same circular-volume calculation as the hole.
How much does bag size change my total?+
Bigger bags aren't just cheaper per pound — they yield more mixed concrete per bag: about 0.3 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag, and 0.6 for an 80 lb bag. Since the 80 lb yield is exactly double the 40 lb yield, switching from 40 lb to 80 lb bags for the same job roughly halves your bag count, even though the total weight of concrete you need stays the same.
How wide should a post hole be?+
A common guideline is a hole about three times the width of the post, which leaves roughly one post-width of concrete on every side. For a 4x4 that means around 10 to 12 inches. Larger or taller posts, loose soil, and gate posts may call for wider holes, so follow the fence or deck plan and your local requirements.
How long should I wait before attaching rails or panels?+
It depends on the product. Fast-setting concrete can be firm enough within an hour or so, while standard mixes need a day or more before you load the post, and full strength takes weeks. Check the bag instructions, and avoid putting side loads on a post until the concrete has set enough to hold it.
Can I just pour dry concrete into the hole and add water?+
Some products are designed for this, and the manufacturer's instructions describe how to do it. The method relies on groundwater and added water reaching the dry mix, which can leave pockets that are not fully hydrated. Mixing in a wheelbarrow and pouring is more predictable. Follow the bag directions for the product you choose.
Should the concrete go all the way to the top of the hole?+
Usually the concrete is brought to a little below or about level with the surface and then shaped to slope away from the post. Some builders stop a few inches short and fill the rest with soil or gravel. Whichever method you use, avoid leaving a dish around the post that collects water.
Do I need concrete for every post?+
Not always. Some fences and lighter structures use compacted gravel or tamped soil around posts, which allows drainage and easier replacement. Concrete gives stronger resistance to leaning, which is why it is common for gates, corners, taller privacy fences, and structural posts. Check the fence or deck plan and local code for the method required.
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