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holes
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Leave at zero when the entered hole depth is entirely concrete.
Leave at zero when the concrete quantity stops at finished grade.
Zero omits cost; enter an optional local pre-tax USD price per bag.
$per bag
Concrete volume, selected yield, rounded bags, surplus, and optional cost from the canonical estimate.
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Field review

These are planning checks and measurement prompts, not structural or code approval.

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Concrete Calculator Estimate concrete for slabs and other common pours with waste, whole-bag counts and ready-mix rounding, plus optional material cost and weight.
Fence Materials Calculator Estimate fence posts and style-specific infill from a measured run with gates and corners while accounting for waste, footing concrete, and material cost.
Deck Materials Calculator Estimate deck boards and framing alongside footings and railing, stairs and hardware with an illustrative budget for a rectangular layout.
Gravel Calculator Estimate gravel volume and mass from area and depth using bulk density, then apply supplier minimums, order increments and material costs.

A post hole is usually treated as a round cylinder, but the concrete fills only the space around the post. The post cross-section displaces part of the mix, compacted gravel removes depth from the concrete zone, and an above-grade collar adds volume back. Multiplying that net volume by the number of matching holes gives the material quantity before waste.

Real excavations are rarely perfect cylinders. Augers wander, loose soil caves in, bell-shaped bottoms widen, and hand-dug holes vary from one location to the next. A waste allowance covers those irregularities as well as spills and finishing loss. It should not be used to hide dimensions that are known to be larger than the estimate.

Factor Effect on concrete Field check
Hole diameter Changes area by the square of the diameter. Use a measured average for rough or enlarged holes.
Concrete height Equals hole depth minus gravel plus any collar. Measure from finished grade and confirm the intended top level.
Post displacement Subtracts the square or circular post area. Use the actual post or pipe size, not its nominal label.
Bag yield Converts purchase volume into whole bags. Use the mixed yield printed on the exact product.

Bag weight is not a universal volume. Different mixes and package sizes produce different approximate mixed yields, and excess water can change strength and handling without creating reliable extra concrete. The bag count must be rounded up because a fractional final bag still requires opening another bag.

Hole geometry is a structural decision before it is a quantity decision. Fence line posts, gate posts, mailboxes, pergolas, and deck supports face different loads. Frost depth, soil bearing, wind, slope, uplift, hardware, and local inspection rules can require a different diameter or depth from a generic rule of thumb.

Locate underground utilities before digging and keep the manufacturer's safety data and mixing instructions available. A volume estimate can plan materials; it cannot certify a footing or replace site-specific design.

How to Use This Tool:

Group only holes that share the same diameter, depth, post size, and base detail. Estimate gate, corner, or structural posts separately when their geometry differs.

  1. Choose a Job preset as a starting example or select custom measurements, then set the number of Matching holes.
  2. Enter Hole diameter and Hole depth with their units. Use the excavated dimensions rather than the intended auger size when the holes have widened.
  3. Choose Post displacement and enter the square width or round diameter. Select No post displacement only when the concrete volume should not subtract a post.
  4. Select the exact Premix bag yield, or enter a Custom mixed yield from the bag or current product sheet. Add a realistic Waste and irregularity allowance.
  5. Open Advanced for a gravel base, above-grade collar, or optional bag price. Read the whole Bags to buy value and review geometric or field warnings before purchasing.

Interpreting Results:

Net concrete volume is the modeled void around the post before waste. Purchase volume adds the selected allowance. Exact bags shows the mathematical division, while Bags to buy rounds up to a whole package.

  • Surplus volume is the nominal yield left after whole-bag rounding, not a promise that all mixed material will be recoverable.
  • Estimated cost uses the optional USD price per bag and excludes tax, delivery, tools, gravel, reinforcement, and disposal.
  • The clearance and embedment notes are review cues. They do not establish code compliance or structural adequacy.
  • A large change in bag count between nearby waste percentages indicates that the estimate sits close to a whole-bag boundary.

Technical Details:

All lengths are converted to metres before area and volume are calculated. The hole is circular. Post area is either square, circular, or zero, and that area is treated as constant through the full concrete height.

Formula Core:

Concrete height subtracts the gravel base from the below-grade depth and adds any same-diameter collar above grade.

h=d−g+c

For N matching holes, net volume subtracts post area from round-hole area before multiplying by concrete height.

Vnet=N× [π(D2)2−Apost]×h

Square post area is s²; round post area is π(s ÷ 2)². Purchase volume adds waste, and bag count uses the mathematical ceiling so any positive fraction rounds upward.

Bags=⌈ Vnet×(1+w100) y ⌉

Lookup Core:

The built-in bag choices use approximate mixed yields from the named product data. A custom yield replaces the lookup for other products.

Bag profile Mixed yield
25 kg fast-setting mix11.6 L
30 kg fast-setting mix14 L
50 lb concrete mix0.375 ft³
60 lb concrete mix0.45 ft³
80 lb concrete mix0.60 ft³

Rule Core:

Check Exact rule
Gravel depthMust be strictly less than hole depth.
Round post fitPost diameter ≤ hole diameter.
Square post fitSquare width × √2 ≤ hole diameter, so the corners fit inside the round hole.
Whole bagsExact bags are rounded up; an exact integer remains unchanged.
Waste range0% through 50%, inclusive.

Accuracy and Safety Notes:

  • Measure representative holes after excavation when soil collapse, roots, rocks, or auger wander have changed the intended diameter.
  • Product yields are approximate. Batch handling, water amount, spills, and incomplete placement can reduce usable volume.
  • Presets and embedment ratios are estimating examples, not structural recommendations. Verify local frost, wind, soil, footing, and inspection requirements.
  • Contact the local utility-locate service before digging and follow the product's current safety data and placement instructions.

Worked Examples:

Eight square fence posts

Eight 300 mm diameter holes, each 900 mm deep around a 90 mm square post, require about 0.4506 m³ before waste. A 10% allowance raises purchase volume to about 0.4957 m³. At 11.6 L per bag, the exact result is 42.73 bags, so the purchase quantity is 43.

Round post with gravel and collar

A 250 mm hole around a 60 mm round post uses 550 mm of below-grade concrete after a 50 mm gravel base, plus a 25 mm collar. With 8% waste and a 14 L yield, the estimate is 2.052 bags and rounds to three. At $6.50 per bag, optional cost is $19.50.

References: