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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Post-hole concrete estimating starts with a simple shape: a round hole below grade. The usable concrete volume is not just the full cylinder if a post sits inside it. The post displaces some of the hole, and gravel at the bottom displaces more. A raised collar or mound above grade adds volume back.

Jobsite holes are rarely perfect cylinders. Augers wander, soil breaks out, the bottom bells wider than the top, water changes the sidewalls, and cleanup can spill material. That is why most practical estimates include a waste and irregularity margin, then round up to whole bags. The exact-bag number is useful for comparison, but the purchase count is the number that matters at the store.

Post hole cross-section showing diameter, depth minus gravel, post displacement, and concrete fill.

Hole size is also a structural clue, not only a volume input. Many fence and post-setting workflows start near three times the post width for hole diameter and compare depth against the exposed height, but local soil, frost depth, wind load, gates, decks, railings, and code requirements can override a rule of thumb. A bag estimate should not be used to shrink a footing that needs engineering or permitting.

Bag yield is another estimate boundary. Product data sheets publish approximate mixed volume per bag, and different brands, bag sizes, water amounts, and compaction can vary. For purchase-critical work, use the yield printed on the bag or product data sheet and keep a small buffer for irregular holes.

How to Use This Tool:

Enter one repeated post-hole layout at a time. Run a separate pass for corner, gate, deck, or pergola holes if their dimensions differ.

Choose Measurement system, then select a Post-hole preset or leave the layout custom.
Set Number of holes, Hole diameter, Hole depth, and Post height above grade.
Select Post shape and enter the post face width or post diameter. Choose No post displacement only when the full round hole is concrete.
Choose a Concrete bag size, or select Custom bag yield and enter the mixed volume printed on your product.
Set Waste and irregularity for rough digging, spills, over-excavation, and mounded tops.
Open Advanced if you need to subtract a gravel base, add an above-grade collar, include bag price, or change the concrete density used for estimated mixed weight.
Read Bag Takeoff for the purchase count, Bag Size Comparison for alternate bag sizes, Hole Checks for geometry cautions, and Waste Ladder for buffer sensitivity.

Interpreting Results:

The headline bag count is rounded up to whole bags. The exact-bag number explains the arithmetic, but it is not a buyable quantity. The surplus row shows how much mixed volume remains after rounding to whole bags.

Post hole concrete result interpretation
Output cue	Meaning	What to verify
Net concrete per hole	Round-hole volume minus the post displacement.	Confirm the post actually fits the hole and reaches the intended depth.
Purchase volume	Total net concrete after the waste and irregularity margin.	Increase the margin for rough holes, loose soil, or deliberate crowned tops.
Selected bag yield	The mixed volume assumed for one bag.	Check the bag label or product data sheet before buying.
Hole-to-post width	A practical comparison between hole diameter and post width.	Use it as a caution, not as a substitute for local code or engineering.

False confidence comes from a neat bag count with unrealistic hole dimensions. Verify the Hole Checks rows, especially post fit, depth versus exposed height, gravel subtraction, and waste buffer, before trusting the takeoff.

Technical Details:

The volume model treats each hole as a vertical cylinder and subtracts the post cross-section from the concrete-filled height. A square post uses square area, a round post uses circular area, and no-post mode fills the whole round hole. Gravel depth is subtracted from hole depth before concrete volume is calculated, while an above-grade collar adds height at the same diameter.

Formula Core

The net concrete volume per hole is the concrete area times the concrete-filled height:

V = (π r^{2} - A) h

V is net concrete per hole, r is hole radius, A is post area, and h is hole depth minus gravel plus any above-grade collar. Total concrete before waste is V multiplied by the number of holes. Purchase volume is total concrete multiplied by 1 + waste percent / 100.

Post hole concrete calculation stages
Stage	Rule	Result effect
Unit conversion	Convert millimeters, centimeters, meters, inches, feet, liters, and cubic feet to meters and cubic meters.	Keeps mixed unit entries comparable.
Hole area	Use pi times radius squared from the entered hole diameter.	Diameter changes volume strongly because radius is squared.
Post displacement	Subtract square or round post area over the concrete height.	Prevents overbuying when the post occupies a meaningful part of the hole.
Waste margin	Multiply total net concrete by the selected waste percentage.	Covers rough holes, spills, over-excavation, and mounded tops.
Bag rounding	Divide purchase volume by selected bag yield, then round up to a whole bag.	Creates the buy count and surplus volume.

The post-fit check differs by shape. A round post fits when its outside diameter is less than the hole diameter. A square post has corner-to-corner diagonal width, so the check uses post face width multiplied by the square root of 2. That catches the case where a square post face looks narrow enough but the corners cannot clear the round hole.

The waste ladder recalculates purchase volume, exact bags, whole bags, and estimated cost at 0, 5, 10, 15, 20, and 25 percent waste. It is useful when one more bag is triggered by a small margin change.

Accuracy Notes:

This calculator estimates material quantity only. It does not design a footing, certify a deck or fence post, or replace local building rules.

Confirm frost depth, soil, wind load, gate load, deck load, setbacks, and permit requirements before digging structural holes.
Use actual hole diameter and depth after digging when buying concrete for a critical job.
Follow the bag label for water amount, mixing method, placement, set time, and curing instructions.

Worked Examples:

Eight fence posts in 300 mm diameter holes, 900 mm deep, with 90 mm square posts and no gravel base use the annular area around each post. With a 10 percent waste margin and 25 kg bags yielding 0.012 m3 each, the Bag Takeoff row rounds the exact-bag result up to a whole-bag purchase count.

A mailbox round post in a 250 mm diameter hole, 600 mm deep, with a 60 mm round post subtracts much less displacement than a square post. The Hole Checks row should still be reviewed because shallow holes may fail the depth-versus-exposed-height comparison even when the concrete volume looks modest.

A deck or pergola post with a 140 mm square post inside a 450 mm hole has more volume but also stronger clearance. If a 100 mm gravel base is entered, the concrete depth is reduced by that amount, and the Gravel base row notes that the gravel volume has been excluded from concrete.

If the summary says Enter hole dimensions, check for zero or invalid hole diameter, hole depth, or bag yield. If it says Review post clearance, increase the hole diameter or reduce the post size before trusting the material takeoff.

FAQ:

Why does the calculator subtract the post volume?

The post occupies part of the hole, so that space is not filled with concrete. Square and round post shapes use different displacement areas.

Should I use the exact-bag number or bags to buy?

Use Bags to buy for purchasing because bags are whole units. The exact-bag number explains how close the estimate is to the next whole bag.

Why does a small diameter change affect the result so much?

Hole volume depends on radius squared. A modest increase in diameter can add much more volume than the same-looking increase in depth.

Can I use this for deck footings?

Use it only for bag quantity. Deck footings may need code-specific depth, width, bearing, frost, and inspection requirements beyond the material estimate.

Glossary:

Hole diameter: The actual round width of the dug or augered hole.
Post displacement: The volume occupied by the post inside the concrete-filled height.
Gravel base: Stone or gravel placed at the bottom of the hole and excluded from concrete volume.
Waste and irregularity: A percentage buffer for rough holes, spills, over-digging, and mounded tops.
Bag yield: The approximate mixed concrete volume produced by one bag.
Surplus volume: The concrete volume left after rounding exact bags up to whole bags.

References:

Concrete Calculator - How Much Do I Need?, QUIKRETE.
QUIKRETE Concrete Mix No. 1101 Spec Data, QUIKRETE.
High-Strength Concrete Mix, Sakrete.
How to Install a Chain Link Fence, The Home Depot.