Post-hole concrete quantity depends on the space that will receive concrete and any post, sleeve, or hardware inside that same space. Hole diameter alone can overstate the net concrete when a post occupies part of the filled zone.
This calculator handles one group of identical straight round, tapered round, or rectangular holes. It subtracts a verified round or rectangular post displacement, keeps the measured and planning quantities separate, and converts the result to whole packages using the current mixed yield you enter. Read the post-hole measurement and volume guide before calculating mixed hole groups, collars, gravel zones, or irregular excavation.
How much concrete do your post holes need?
Enter the approved concrete-filled geometry, the number of identical holes, and the exact portion of the post inside the concrete. Add the selected product's current mixed yield to calculate whole packages.
The result is a quantity record. Drawings, permits, product instructions, soil, frost, wind and structural loads, post connection, drainage, utilities, and local requirements determine the acceptable foundation detail.
What does each result mean?
| Result | Calculation | Use |
|---|---|---|
| Gross hole volume | Selected hole geometry before deductions | Checks the concrete-filled boundary |
| Post displacement | Post cross-section × its depth inside concrete | Shows the occupied volume being deducted |
| Net concrete per hole | Gross hole volume − post displacement | Concrete space for one hole |
| Total measured concrete | Net volume × identical hole count | Unrounded geometric quantity |
| Planning concrete | Measured quantity after the entered allowance | Documented planning basis |
| Whole packages | Planning volume ÷ current mixed yield, rounded up once | Package-purchase estimate |
Which hole shape should you select?
Use straight round for a cylindrical concrete zone with one diameter. Use tapered round when the approved top and bottom diameters differ and the sides change at a constant rate. Use square or rectangular when length and width remain constant through the filled depth.
A bell, flare, step, collapsed side, rock pocket, irregular auger path, above-grade collar, or several depth zones needs separate valid solids. Calculate those sections outside this one-group tool, then use the Concrete Bags Calculator in known-volume mode when one package basis applies.
Which depth belongs in the calculation?
Enter the vertical depth that will receive concrete. Total excavation depth can include gravel, drainage space, soil backfill, an unfilled top zone, or other material.
Enter post depth only for the part of the post that overlaps the concrete. A post may stop above the hole bottom, extend into a separate base zone, sit in a bracket, or continue through a collar. The tool rejects a post depth greater than the concrete fill depth.
What formulas does the Concrete Post Hole Calculator use?
Straight round hole = π × (diameter ÷ 2)² × concrete depth
Tapered round hole = π × depth ÷ 12 × (top diameter² + top diameter × bottom diameter + bottom diameter²)
Rectangular hole = length × width × concrete depth
Round post displacement = π × (post diameter ÷ 2)² × post depth in concrete
Rectangular post displacement = post length × post width × post depth in concrete
Whole packages = ceiling(total net volume × allowance factor ÷ current mixed yield)
Metric horizontal dimensions use metres and depths use centimetres. Imperial horizontal dimensions use feet and depths use inches. The tool retains full precision through the group calculation before it rounds the package quotient upward.
Worked imperial example: 12 straight round holes
Twelve approved holes are 10 in in diameter with 24 in of concrete fill. Each installed timber post measures 3.5 in × 3.5 in and occupies 22 in of the concrete depth. The selected 80 lb product currently lists about 0.60 ft³ of mixed yield per package.
- Gross volume per hole: π × (10 ÷ 2)² × 24 = 1,884.956 in³.
- Post displacement per hole: 3.5 × 3.5 × 22 = 269.5 in³.
- Net per hole: (1,884.956 − 269.5) ÷ 1,728 = 0.934870 ft³.
- Total measured volume: 0.934870 × 12 = 11.218442 ft³, or 0.415498 yd³.
- Exact package quotient: 11.218442 ÷ 0.60 = 18.697403.
- Whole-package result: 19 bags.
The combined quotient rounds once. Rounding 1.558 bags per hole to 2 and multiplying by 12 would produce 24 bags, 5 more than this compatible group calculation.
Worked metric example: 6 tapered holes
Six approved holes have a 0.35 m top diameter, a 0.45 m bottom diameter, and 80 cm of concrete fill. Each 0.10 m × 0.10 m post occupies 75 cm of that filled zone.
- Gross frustum per hole: π × 0.80 ÷ 12 × (0.35² + 0.35 × 0.45 + 0.45²) = 0.101055 m³.
- Post displacement: 0.10 × 0.10 × 0.75 = 0.0075 m³.
- Net per hole: 0.101055 − 0.0075 = 0.093555 m³.
- Total measured quantity: 0.093555 × 6 = 0.561327 m³, or 561.327 L.
- An entered 5% project allowance gives 589.394 L.
- A current 17 L package yield gives 589.394 ÷ 17 = 34.670, rounded to 35 packages.
The 5% allowance and 17 L yield are example inputs. Replace both with records that apply to the project and exact product.
Why does post displacement matter?
A post inside the concrete occupies volume. The change can be material across many holes, especially when the post is large relative to the hole or the embedment is deep.
Deduct displacement only when the object occupies the same concrete zone. Do not subtract a post attached to an above-grade base, a void that will fill with concrete, or an item whose dimensions and embedment remain unknown.
The fit checks prevent a round post from equalling or exceeding the narrowest hole width. For a rectangular post in a round hole, the tool checks the post diagonal against the narrowest diameter. These checks identify impossible entered geometry. They do not approve concrete cover, constructability, placement, or structural performance.
How should tapered and irregular holes be measured?
Use tapered round only when a frustum describes the concrete boundary closely enough for the approved takeoff. Measure both end diameters at their actual fill levels and record the vertical distance between them.
Split a changed shape into non-overlapping sections. A straight upper cylinder and widened lower frustum need 2 lines. A measured local collapse needs a dated field-change record. Keep water, loose soil, rock, utilities, unstable sides, and unapproved geometry outside the quantity until the responsible process resolves them.
How should different post-hole groups be combined?
One calculator run applies to identical holes with the same geometry, post profile, concrete fill depth, package yield, and allowance basis. Give line posts, corners, ends, gates, signs, and structural posts separate runs when any of those inputs change.
| Condition | Treatment |
|---|---|
| 12 identical line-post holes | Use quantity 12 and round the compatible package quotient once |
| Corner holes have another diameter | Run a separate corner group |
| Two concrete products or yields | Keep separate package calculations |
| One hole has accepted overbreak | Record and calculate the changed hole separately |
| Ready-mix supply | Combine checked compatible volumes, then use the ready-mix workflow |
Use the Ready-Mix Concrete Calculator when a producer will supply the concrete. Supplier selling increments, mixture requirements, delivery access, placement rate, minimum orders, and charges require current supplier and project records.
Why must package yield come from the selected product?
Package mass and mixed yield measure different properties. Mass identifies dry packaged material. Mixed yield states the approximate finished volume produced under the named product's instructions.
QUIKRETE Concrete Mix No. 1101 lists approximate yields of 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, and 0.60 ft³ for 80 lb packages. Sakrete publishes the same values for named packages of its High-Strength Concrete Mix. These are current product examples, not universal conversions for every concrete mix or market.
Copy the exact product name, item, package, market, mixed yield, technical-data revision, and access date. Follow its water, mixing, placement, temperature, curing, application, and safety instructions.
When should you enter a planning allowance?
The calculator starts at 0%. Enter a percentage only when drawings, specifications, measured tolerance, accepted field conditions, placement method, or another documented project basis supports it.
Keep measured geometry visible. An allowance cannot define unknown fill depth, repair a collapsed hole, select a product, compensate for an incompatible post, or prove that every package will achieve its listed approximate yield.
Common post-hole calculation mistakes
- Using total excavation depth as concrete fill depth.
- Entering a generic hole size without the approved project detail.
- Using nominal post size when the installed section differs.
- Subtracting the post below its actual concrete overlap.
- Applying a cylinder formula to a tapered, stepped, or collapsed hole.
- Combining line, corner, gate, and end posts under one geometry.
- Rounding every hole to whole bags before combining identical work.
- Using bag weight as mixed yield.
- Adding an automatic allowance without recording its basis.
- Treating a bag count as approval of foundation design or installation.
What safety and design limits apply?
Locate expected underground utilities before excavation. OSHA 29 CFR 1926.651 sets this requirement for covered United States construction work and requires safe location steps as work approaches an installation. Follow the utility-owner process, local law, site controls, and responsible supervision for the project location.
Wet portland cement can cause caustic burns and dermatitis. Follow the product safety data sheet and applicable controls for skin, eyes, dust, lifting, mixing equipment, water, and washing facilities.
The calculator does not size a post foundation, assess soil, frost, wind or structural loads, choose a post connection, select reinforcement, approve excavation, or certify completed work. Use current drawings, specifications, permits, product data, site conditions, local requirements, and qualified professionals.
What should the calculation record contain?
- Hole group, post type, count, drawing or permit revision, and measurement date.
- Hole shape, top and bottom dimensions, concrete fill depth, units, and measurement points.
- Post profile, actual dimensions, concrete overlap, and other verified occupied volume.
- Gross, displacement, net, measured, and planning volumes.
- Allowance, reason, and source.
- Product, package, mixed yield, exact quotient, whole packages, and capacity above planning quantity.
- Supplier, delivery or collection plan, field changes, unresolved conditions, and checker.
Use the Concrete Bags per Post Hole Chart for diameter-and-depth reference rows. Use the Post Hole Concrete Quantity Worksheet for line, corner, end, gate, and changed-hole groups. Use Oversized Post Hole Concrete to compare an accepted wider or locally changed boundary with the original quantity. Return to the concrete planning hub for packaged concrete, ready-mix, reinforcement, block, and joint tools. The calculation methodology explains unit conversion, assumptions, precision, and rounding.
Sources and scope
- NIST Handbook 133 (2026), Appendix E: volume-unit relationships used in conversion.
- QUIKRETE Concrete Mix No. 1101 SPEC-DATA and Sakrete High-Strength Concrete Mix: named-product package-yield examples.
- QUIKRETE, Setting Posts without Mixing: one named manufacturer's post-setting workflow and distinct fill zones.
- OSHA 29 CFR 1926.651 and OSHA concrete-products hazard guidance: the stated United States workplace safety context.
Source scope: NIST supports unit relationships. Manufacturer records support only their named products and stated workflows. OSHA supports the cited United States workplace boundary. These sources do not set a universal post-hole diameter, depth, allowance, product, yield, frost depth, soil capacity, connection, or structural design.
Review note: Saleem Sial owns the research and editorial record. Formula fixtures, source checks, build validation, and rendered QA form the internal publication gate. Waseem Sial, External Reviewer and Engineer, is listed for ongoing external review; no completed review date is claimed.