Post Hole Concrete Calculator

Measurement system:

Metric is the default; choose imperial for inch and foot jobsite measurements.

Post-hole preset:

Use a preset for a fast estimate, then adjust the hole and post dimensions below.

Number of holes:

Keep corner, gate, or deck-post holes separate when they are wider or deeper.

holes

Hole diameter:

Measure the actual hole width, not the nominal post size.

Hole depth:

Depth controls volume and the depth check against the above-grade post height.

Post height above grade:

Use the finished exposed post height after the post is set.

Post shape:

Choose no post for empty piers or when you already measured net fill volume.

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Concrete bag:

Bag yields vary by product; use the bag label or data sheet for purchase-critical jobs.

Custom bag yield:

Use the yield printed on your bag, not the dry bag volume.

Waste and irregularity:

Most post-hole estimates use a small buffer because real holes are rarely perfect cylinders.

Gravel base depth:

Leave at 0 when the whole hole depth is filled with concrete.

Above-grade concrete collar:

Use this for deliberate raised collars; water-shedding shaping is still a field detail.

Bag price:

Use 0 to omit cost from purchase decisions.

Concrete density:

Normal concrete is often near 2200-2400 kg/m3, but bag mixes vary.

kg/m3

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Bag option	Yield	Exact bags	Buy	Surplus	Copy
{{ row.bag }}	{{ row.yield }}	{{ row.exact }}	{{ row.buy }}	{{ row.surplus }}

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A post hole looks like a simple cylinder until the post itself is included. Concrete fills the space around the post, not the whole drilled hole. A square fence post, a round mailbox pipe, or a deck support removes part of the volume, while gravel at the bottom and a raised collar at grade change the concrete-filled height.

Small measurement changes matter because round-hole volume grows with radius squared. A hole that is 300 mm wide is not just a little larger than a 250 mm hole; it has much more cross-sectional area before depth is even considered. For a long fence line, one wider or rougher hole can add several bags across the job.

Bagged concrete estimates also depend on mixed yield, not just bag weight. Two products with similar weight can publish different mixed volumes, and the usable amount can change with mixing, compaction, spills, over-digging, and crowned tops. That is why a waste margin and whole-bag rounding belong in the estimate rather than being left until checkout.

Post-hole concrete estimating terms
Term	Plain meaning	Estimating effect
Gross hole volume	The full round cylinder from hole diameter and concrete height.	Sets the upper volume before subtracting anything inside the hole.
Post displacement	The space occupied by the post inside the concrete area.	Reduces concrete volume, but the post still must physically fit the round hole.
Gravel base	Stone placed below the concrete for drainage or bearing preparation.	Subtracts from the depth filled with concrete.
Waste margin	A buffer for rough holes, cave-ins, spills, and mounded tops.	Raises purchase volume before dividing by bag yield.

Post hole cross-section showing hole diameter, concrete depth, gravel subtraction, post displacement, and rounded bag planning.

Many post-setting guides start with a hole about three times the post width and a depth near one-third to one-half of the exposed post height. Those are planning checks, not universal design rules. A gate hinge post, deck support, pergola corner, mailbox, sign post, and line fence post can all use the same volume arithmetic while needing different structural choices.

Local conditions can control the real hole before the calculator does. Frost depth, soil bearing, slope, wind exposure, post hardware, inspection requirements, and utility locations may force a deeper or larger footing. The material takeoff is useful after the geometry is chosen; it does not prove the geometry is strong enough.

How to Use This Tool:

Estimate one repeated hole layout at a time. If gate, corner, deck, or pergola posts have different hole dimensions, calculate those as separate passes.

Set Measurement system, then choose a Post-hole preset or keep custom dimensions.
Enter Number of holes, Hole diameter, Hole depth, and Post height above grade.
Choose Post shape. Use square face width for square posts, outside diameter for round posts, or No post displacement when the concrete fills an empty pier.
Select a Concrete bag. If the product is not listed, choose Custom bag yield and enter the mixed volume printed on the bag or data sheet.
Set Waste and irregularity for rough digging, cave-ins, spills, or deliberate mounding.
Open Advanced when you need to subtract a Gravel base, add an Above-grade concrete collar, enter Bag price, or adjust Concrete density for estimated mixed weight.
Read Bag Takeoff first, then compare Bag Size Comparison, Hole Checks, and Waste Ladder before buying material.

If the summary says Enter hole dimensions, a required dimension or bag yield is missing or zero. If it says Review post clearance, increase hole diameter or reduce post size before trusting the concrete quantity.

Interpreting Results:

Bags to buy is the purchase number because it rounds exact bags up to whole bags. Exact bags explains where the rounding happened, and Rounded-bag surplus shows the leftover mixed volume after buying whole bags. A small surplus is normal and often useful on rough holes.

Post-hole concrete result interpretation
Result area	What it means	What to check
Bag Takeoff	Net concrete per hole, total volume, purchase volume, whole bags, surplus, weight, and optional cost.	Confirm the dimensions match the actual holes, especially if some posts are wider or deeper.
Bag Size Comparison	Whole-bag counts for common metric and imperial bag sizes using the same purchase volume.	Use the local product yield, not only the bag weight or a store listing.
Hole Checks	Rule-of-thumb flags for hole-to-post width, clearance, depth ratio, gravel, waste, rounding, and finish.	Treat flags as review prompts, not as pass-fail structural approval.
Waste Ladder	How 0%, 5%, 10%, 15%, 20%, and 25% waste change exact bags and whole bags.	Look for the point where one extra bag appears if transport, cost, or lifting load is tight.

The clearance warning is a practical safeguard. A square post needs corner-to-corner room inside a round hole, so face width alone can miss a tight fit. A quantity can be mathematically positive even when the chosen post shape does not fit the hole.

The mixed concrete weight, dry bag load, water amount, and cost rows are planning aids. Follow the bag instructions for water, mixing, placement, set time, and loading, and follow local requirements for structural posts.

Technical Details:

The calculation is based on annular volume: round-hole area minus post area, multiplied by the concrete-filled height. Gravel reduces that height because it occupies the bottom of the hole. A same-diameter collar above grade increases the height because it adds concrete above the original depth.

After volume per hole is known, the estimate multiplies by the number of matching holes, applies the waste percentage, divides by selected bag yield, and rounds up. Length inputs are converted to meters and bag yields to cubic meters before the calculation, which lets metric and imperial field units mix without changing the formula.

Formula Core:

The per-hole volume subtracts post displacement from the round hole before multiplying by fill height.

V_{hole} = (π {(\frac{d}{2})}^{2} - A_{post}) (h_{hole} - h_{gravel} + h_{collar})

d is hole diameter. A_post is zero for no post displacement, post width squared for a square post, or circular area for a round post. Gravel depth is capped at the hole depth, and net area is not allowed to become negative.

V_{purchase} = V_{hole} n (1 + \frac{w}{100}), B_{buy} = ⌈ \frac{V_{purchase}}{Y_{bag}} ⌉

n is the number of matching holes, w is the waste percent, and Y_bag is the selected mixed yield per bag. The upward rounding means the purchase count increases as soon as exact bags pass a whole number.

Post-hole concrete calculation stages
Stage	Rule	Practical consequence
Hole area	Diameter is halved to radius, squared, then multiplied by pi.	Diameter errors grow quickly because radius is squared.
Post displacement	Square posts subtract face width squared; round posts subtract circular area.	Large posts reduce volume but can also trigger a clearance warning.
Fill height	Hole depth minus gravel base plus above-grade collar height.	Drainage gravel lowers concrete volume, while collars add concrete above grade.
Waste and rounding	Waste is added before exact bags are rounded up to whole bags.	A small margin change can be the difference between one trip and another.

With the default fence-line preset, eight 300 mm by 900 mm holes around 90 mm square posts produce about 56.3 L of net concrete per hole. With 10% waste and 25 kg bags that yield 12 L each, the purchase volume is about 495.7 L, or 41.31 exact bags, so the buy count rounds to 42 bags.

The geometry checks use simple thresholds. Hole-to-post width is marked strong near 3 times post width, usable near 2.5 times, and tight below that. Depth is compared with exposed post height, and values below about one-third are flagged for review. These checks are intentionally cautious because soil, wind, frost, loads, and code can require more than a rule of thumb.

Accuracy and Safety Notes:

This is a material estimate, not a footing design or permit decision. Confirm frost depth, soil bearing, utility locations, post hardware, bracing, and inspection requirements before digging or placing concrete. Bag yields and water amounts should come from the actual product label or data sheet, especially when custom mixes, rapid-set products, or local metric bag sizes are used.

Worked Examples:

Fence-line preset

Eight 300 mm holes, each 900 mm deep around a 90 mm square post, use about 56.3 L per hole before waste. With 10% waste and a 25 kg bag yield of 12 L, Bag Takeoff rounds 41.31 exact bags to 42 bags and leaves about 8.3 L of rounded-bag surplus.

Mailbox round post

One 250 mm by 600 mm hole around a 60 mm round post, with 8% waste and 20 kg bags that yield 9.5 L, needs about 30.0 L of purchase volume. That is 3.16 exact bags, so the buy count is 4 bags. The surplus is useful if the hole walls are rough or the top is mounded.

Post clearance warning

A 90 mm square post in a 120 mm round hole can produce a positive volume number, but the square-post diagonal is about 127 mm. The summary warns to review post clearance because the corners do not fit inside the round hole. Increase the hole diameter before using the bag count.

Advanced Tips:

Group holes by geometry. Do not average small line-post holes with larger gate, corner, or deck holes if the bag count matters.
Use the mixed yield from the actual bag. The listed common bag sizes are starting references, and custom yield should override them for purchase-critical work.
Set Gravel base only for stone that actually replaces concrete at the bottom of the hole. Do not subtract gravel that sits outside the concrete-filled depth.
Use Waste Ladder to choose a margin before buying. Rough auger holes, wet soil, slumping sides, and deliberate crowns usually need more buffer than neat formed tubes.
Use Concrete density only for estimated mixed weight. It does not change bag count because bag count depends on mixed volume yield.

FAQ:

Does the calculator include the post volume?

Yes. Square and round post choices subtract post displacement from the hole volume. Choose No post displacement only for empty piers or when you already measured net fill volume.

Why did the bag count round up?

Concrete is bought in whole bags. The calculator divides purchase volume by selected bag yield, then rounds up so the buy count is not short by a partial bag.

Should hole depth be one-third of the post height?

The Hole Checks tab flags values below about one-third of exposed height, but local frost depth, soil, loads, and code can require a different depth or footing design.

Why does post clearance fail for a square post?

A square post must fit corner to corner inside a round hole. The calculator checks the square diagonal, not only the face width, before treating the geometry as usable.

What should I do if the selected bag yield is not listed?

Choose Custom bag yield and enter the mixed volume from the product label or data sheet. Bag weight alone is not enough because products can yield different volumes.

Glossary:

Annular volume: The concrete space around the post after subtracting the post area from the round hole area.
Bag yield: The mixed concrete volume produced by one bag.
Post displacement: The space occupied by the post inside the concrete-filled part of the hole.
Purchase volume: Total concrete volume after multiplying by hole count and adding waste margin.
Rounded-bag surplus: The extra mixed volume available after exact bags are rounded up to whole bags.

References:

Setting Posts in Concrete, QUIKRETE.
Fast-Setting Concrete Mix Technical Data Sheet, QUIKRETE.
Prescriptive Residential Wood Deck Construction Guide, American Wood Council, 2018.
ACI Concrete Terminology, American Concrete Institute, 2025.