Post Hole Concrete Calculator – Bags for Fence Posts
Estimate Concrete for Post Holes
Enter actual hole diameter and concrete-filled depth, then choose the number of holes. If the posts occupy concrete-filled space, optionally subtract the embedded post volume.
1. Project & measurement setup
📘 Try a post-hole example
Load examples of empty holes, holes with round posts, or metric dimensions. These are quantity estimates, not installation-design recommendations.
2. Project sections
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Name each pour, select the shape that represents the space concrete will fill, then enter dimensions and units. Use Add concrete for new pours and Deduction / void to subtract real empty space. The diagram beside each section result helps confirm the geometry.
Examples: a circular solid slab is a short cylinder (round shape with slab thickness as height). For a hollow circular ring choose Tube / ring. Solid steps are different from supported structural stair flights. For a variable-thickness ramp, use its verified measured volume if a suitable ramp shape is unavailable.
This estimates concrete volumes only; it does not choose footing width, depth, reinforcement or bearing capacity. Those depend on structural loads, soil conditions, applicable code and competent engineering design.
3. Order allowance
Editable planning allowance for uneven grade, form variation, spillage or placement loss. It is not a structural safety factor.
A planning default only. Replace it with your supplier’s actual ordering increment.
4. Ready-mix supplier & cost settings
Supplier delivery minimums, short-load billing rules, truck sizes and rounding vary. Enter actual supplier rules. A minimum billed volume is not necessarily a minimum delivered volume.
5. Premix bag settings
Use the yield printed on the exact bag or product data sheet. Bag weight alone does not reliably determine mixed-concrete volume.
Concrete order result
Calculation steps with your values
Quick volume converter
Your take-to-the-store checklist
Concrete Order Summary
Your final purchasing checklist. Choose one supply option below and verify quantities before buying.
Project sections and deductions
| # | Section | Shape | Qty | Use | Volume (m³) | Volume (yd³) |
|---|
A. Delivered ready-mix concrete
B. Premixed concrete bags
Use the exact bag product and its stated mixed yield. Bag quantities are a separate alternative to ordering ready-mix.
C. Buy cement, sand & aggregate separately
Prefer to mix materials on site? Select a nominal planning ratio or enter your approved proportions. Your estimated shopping quantities will appear below.
Enter your material-mixing ratio
Planning assumption; adjust to your estimating method.
Leave blank unless supplied by your approved mix/design basis.
Important: nominal ratios are estimating examples, not strength guarantees. For M25/M30 and other design-mix concrete, use approved project or supplier proportions.
Important: A 1 : 2 : 3 ratio describes proportions by volume—not automatically one bag of cement plus two bags of sand and three bags of stone. The result gives cement mass / estimated 50 kg cement bags and sand and aggregate volumes. Supplier bag sizes, loose-material bulk densities and moisture differ; verify them before ordering. Mix proportions do not certify concrete strength.
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Post Hole Concrete Calculator: practical measurement guide
Read guide
Measure the hole that concrete fills
For a round excavation, enter the actual concrete-filled diameter and depth, not simply the nominal auger size. Soil collapse, enlarged bases and irregular excavation can change the amount of concrete required.
Posts may displace some of the concrete
An embedded round post occupies physical volume. If you enter its outside diameter and embedded depth, the tool subtracts that cylindrical displacement. Leave post diameter blank if there is no displaced post or if the full hole volume is wanted. This is a geometrical estimate, not a guarantee of actual yield.
Depth of concrete may be less than excavation depth
Where gravel or another material forms the lower part of the hole, do not include that portion in the concrete-filled depth. The calculator does not determine safe installation depth, frost depth, structural embedment or drainage requirements.
Square posts and belled holes
For square or irregular posts, use rectangular deductions through the flagship Concrete Calculator or a verified volume from your project drawing. For belled or noncylindrical holes, divide the geometry into valid sections rather than assuming every hole is a straight cylinder.
For a round hole, set the post cross-section to No post, Round post or Square post. The concrete-filled depth excludes any optional gravel below it. A square post with side w has diagonal w × √2; that diagonal must fit inside the hole diameter.
📷 Reference pictures (open — click to collapse)
These reference photos support the calculator. They are illustrations, not project-specific drawings. You can collapse the entire picture guide whenever you prefer more space.
Assumptions & Limitations
These estimates depend on the dimensions, units, mix proportions, bag-yield and supplier information you provide. Please review the points most relevant to your purchase.
📐 Measurements & geometry
Dimensions are assumed to describe the actual volume to fill. Slabs are treated as uniform-thickness solids. Round shapes and tube/ring sections use the indicated geometry. Variations in ground level, excavation, irregular profiles, embedded members and unmodelled voids are not automatically measured.
➕ Allowance & deductions
Percentage allowance is applied to the calculated net volume. It is a user-selected planning allowance, not a guaranteed waste percentage or structural safety factor. Only the deductions that you enter are subtracted. Inspect all dimensions and ensure no area is counted twice.
🧱 Bags & supplier ordering
Bag count is rounded upward using the actual mixed yield of the selected product. Supplier order increment, minimum load, vehicle capacity, prices and extra charges vary and must be confirmed directly. A supplier minimum billed quantity can differ from the concrete actually delivered.
🪨 Mixing your own ingredients
Nominal volumetric ratios estimate cement, sand and coarse aggregate quantities using assumed dry-volume and bulk-density factors. They do not account automatically for aggregate grading, moisture correction, air content, admixtures, mixing losses or strength verification. Water demand and structural performance require an approved mix/design basis.
🛠️ Practical site considerations
Allow for safe formwork, compaction, reinforcement and cover, site access, truck/pump availability, labor, placing sequence, weather, curing, quality control and local regulations. These are not checked by the volume calculation.
✅ Before making a payment
Compare measurements with approved drawings and actual site conditions, confirm the concrete specification, ask suppliers for written product yields and quotation terms, and have a qualified professional review structural or safety-critical work.
Post Hole Concrete Calculator: Questions & Answers
Practical answers and worked examples. Expand a question when you need more detail.
How much concrete do I need for one round post hole?
For a round hole without a post, volume is π × (hole diameter ÷ 2)² × concrete-filled depth. For instance, a 12 in diameter and 2 ft deep round hole holds about 1.571 ft³ before allowance. Multiply by the number of identical holes.
Does a post reduce the amount of concrete needed?
Yes, if part of the post occupies the concrete-filled space. The calculator can subtract a round post volume using its outside diameter and length inside concrete. For a 12 in diameter, 2 ft deep hole around a 4 in round post embedded 2 ft, net concrete is about 1.396 ft³ per hole, versus 1.571 ft³ for the empty hole. Eight identical holes require about 11.17 ft³ before allowance.
How do I calculate ten post holes using centimetres or millimetres?
Choose the metric example or enter dimensions independently in cm, mm or metres. Ten 300 mm diameter holes with 900 mm concrete fill, minus 100 mm diameter round posts embedded 600 mm, require approximately 0.589 m³ net before allowance. Check that the post diameter is an accurate representation of the displaced volume.
Should I measure hole depth from ground level or the bottom of the gravel?
Use only the depth that is actually filled with concrete. If a 1.0 m excavation has a 0.1 m gravel layer and 0.9 m of concrete, enter 0.9 m as concrete-filled depth. Other specifications may require different treatment; follow the approved construction method.
Can I calculate concrete for a square wooden post?
The optional displacement field represents a solid round post. A square post does not have the same cross-sectional area as a round post of the same width. For square posts, use the full hole volume conservatively or calculate a correct rectangular deduction in the flagship Concrete Calculator; do not enter square width as round diameter.
Does this calculator design safe fence-post footings?
No. The tool estimates quantity only. Suitable hole diameter, depth, frost protection, anchorage and post design depend on loading, soil, wind, local regulations and competent design. Obtain suitable specifications rather than using a material calculator to select them.
How many premix bags should I buy for several post holes?
The calculator sums the measured concrete volume, adds any selected allowance, divides by the mixed-concrete yield of your chosen bag product and rounds upward to full bags. Check the manufacturer’s yield, not just the bag weight. The estimate for post displacement is geometrical and may not represent every site condition.
What if the hole is wider at the bottom or irregular?
The post-hole model assumes a straight, circular concrete-filled cylinder. For bell bottoms, stepped geometry or sloughing earth, use valid separate shapes or a site-measured/approved volume. A straight-cylinder estimate can understate irregular excavations.
How much concrete surrounds a square post over a gravel base?
Enter the round hole diameter, excavated depth and gravel base thickness. Select Square post and enter the post side and embedded length within concrete. The calculator subtracts gravel from filled depth, then subtracts square-post displacement. For an excavated 12 in round hole, 30 in deep, with 6 in gravel and a 4 in square post embedded for 24 in, one hole needs approximately 1.349 ft³ of concrete before allowance. Confirm that the square-post diagonal fits within the hole and that your approved installation method permits the gravel layer.
Methodology & formulas
Page focus: Post Hole Concrete Calculator. Results depend on the user-selected physical shape and measured quantities. Calculations are for material estimation only, not construction design certification.
Geometry
- Rectangular: V = length × width × depth.
- Round solid: V = π × (diameter ÷ 2)² × height.
- Tube / ring: V = π × [(OD ÷ 2)² − (ID ÷ 2)²] × height.
- Solid steps: V = width × tread × rise × n(n+1)/2, plus optional platform depth × total stair height × width.
- Curb + gutter: curb section + gutter/flag section along the entered length.
Ordering sequence
Added sections − deductions = net measured volume. Allowance is then added. Supplier increment/minimum is applied only afterward, so purchasing rules never change the underlying measured need.
Bags
Whole bags = required volume ÷ manufacturer-stated mixed yield per bag, rounded up. Bag weight is used only for handling/total dry weight, not to infer yield.
Site-mix materials
Optional material quantities use the selected parts and dry-volume factor. Cement mass uses an estimating bulk density of 1440 kg/m³. Real mix design depends on materials, grading, water, admixtures, exposure, workability and performance requirements.
Important Disclaimer
Planning and educational estimates only. CalcTypes is a calculation aid, not a survey, engineering design, specification, safety assessment, construction instruction, quality certification, or guarantee that your purchase quantity will be sufficient. Results are based entirely on the information entered and on stated assumptions. Errors in measurement, units, geometry, voids, material yield, site conditions, calculation inputs or supplier practices can cause shortages, surplus material, cost overruns and construction defects.
You are responsible for independent verification. Before ordering, mixing, placing or building, check all measurements and actual site conditions; validate the mix grade, strength and exposure requirements; confirm bag yields, supplier ordering/billing rules and prices; and obtain review and approvals from qualified engineers, contractors, local authorities and suppliers whenever appropriate. Do not use this calculator as the sole basis for structural or safety-critical decisions.
No professional relationship or warranty. Information is provided on an “as available” basis for general planning, without a promise of accuracy, completeness, suitability or fitness for a particular project. To the extent permitted by applicable law, CalcTypes does not accept responsibility for losses arising from reliance on unverified estimates, including material shortage, over-ordering, delays, damages or improper construction. Nothing here excludes liability that cannot lawfully be excluded.
Read our full website disclaimer: CalcTypes Disclaimer. This notice should be reviewed alongside your site’s legal terms by a qualified adviser; wording alone does not guarantee legal protection.
Sources
- QUIKRETE Concrete Mix 1101 data sheet — published approximate yields for common US bag sizes.
- QUIKRETE Mixing & Placing Concrete training — volume and bag-yield example.
- IS 456:2000 material reproduced in Quality Management Guidelines — nominal-mix provision for M20 or lower.
- Bureau of Indian Standards — current standard catalogue/reference.
- ASTM C94/C94M — Ready-Mixed Concrete.