Estimate How Much Concrete You Need For Post Holes Before You Mix Or Order. Enter Hole Dimensions, Choose The Hole Shape, Add The Number Of Posts And A Practical Waste Allowance, Then Estimate Cubic Metres, 20 Kg Bags And Concrete-Only Cost. All Numeric Entries Start At Zero So You Can Build The Estimate From Your Own Measurements.
Select The Hole That Best Matches The Excavation. The Calculator Converts Millimetres To Metres, Calculates The Concrete Volume For All Holes, Then Applies Your Waste, Bag-Yield And Cost Inputs.
Your Result Will Appear Here After You Press Calculate Post Hole Concrete.
The PDF Includes The Site Name, Hole Shape, Measurements, Number Of Holes, Waste Allowance, Base Volume, Rounded Order Volume, Bag Estimate And Concrete-Only Cost.
This Figure Keeps The Three Main Inputs Separate: Hole Diameter, Concrete Depth And Number Of Holes. All Labels Stay Inside The Figure And Remain Readable On Smaller Screens.
The Result Separates The Exact Geometric Volume From Your Contingency, Bag Estimate And Editable Cost So You Can See Where Each Number Comes From.
Choose Round, Square Or Rectangular Excavations Instead Of Forcing Every Post Hole Into One Formula.
Use The Dedicated Round-Post Mode When You Want To Subtract The Space Occupied By A Round Post.
See The Base Concrete Volume For All Identical Post Holes In Cubic Metres.
Add Your Own Contingency For Irregular Excavation, Spillage Or Small Measurement Differences.
Enter The Actual Bag Yield From The Product Data Sheet To Estimate Whole Bags.
Enter A Local A$/m³ Rate To Create A Simple Concrete-Only Budget For The Post Holes.
A concrete post hole calculator turns the dimensions of a hole into a volume of concrete. That sounds simple, but the shape of the hole matters. A round augered hole is calculated as a cylinder, a square hole uses width × width × depth, and a rectangular hole uses length × width × depth. If you have many identical holes, the volume for one hole is multiplied by the number of posts.
The most important part of the estimate is not the arithmetic; it is using realistic measurements. Excavated holes are rarely perfect. An auger can wander, loose material can widen the top, hard ground can create a tapered profile and hand digging can make a rectangular excavation larger than the intended nominal size. Measure the real hole where practical and use an appropriate planning allowance instead of relying only on a nominal auger diameter.
This calculator uses millimetres for post-hole dimensions because that is convenient for many site measurements, but the final concrete quantity is shown in cubic metres. It converts millimetres to metres before applying the volume formula. For other concrete shapes, you can also use the Concrete Calculator Australia or the broader Concrete Volume Calculator.
Volume = π × Radius² × Depth × QuantityFor a round hole, first divide the diameter by two to get the radius. If the diameter is entered in millimetres, convert it to metres before using the formula. A hole with a 300 mm diameter has a 150 mm radius, which is 0.15 m. The same conversion applies to the depth.
Volume = Width × Width × Depth × QuantityA square hole is straightforward when all sides are the same width. Measure the width and depth, convert both to metres and multiply them. If the hole is noticeably longer in one direction, use the rectangular option instead of averaging the dimensions.
Volume = Length × Width × Depth × QuantityRectangular holes are useful for posts installed in excavations that are not circular or square. The calculator keeps the length and width separate, which helps avoid underestimating a hole that has been enlarged in one direction.
A post takes up space inside the hole, so technically it displaces some concrete. Whether it is worth subtracting that volume depends on the hole size, post size and the level of accuracy needed for planning. The calculator includes a dedicated mode for a round hole with a round post. It calculates the cylindrical hole volume and subtracts the cylindrical post volume for the same concrete depth.
The amount of concrete depends on the void around the post, not simply the post dimensions. Two identical posts can require very different concrete quantities if one is placed in a narrow augered hole and another in a wide hand-dug excavation. Measure hole diameter or width at several points when the shape is irregular and choose dimensions that reasonably represent the actual concrete space.
Depth is multiplied by the full cross-sectional area of the hole. If every other dimension stays the same, increasing the depth by 25% also increases the geometric concrete volume by 25%. Before changing hole depth for cost reasons, remember that required depth is a design and site decision, not something an online concrete quantity calculator should choose.
Bagged concrete is convenient for many small post installations because it can be carried to individual holes and mixed in manageable batches. The number of bags cannot be determined accurately from bag weight alone. Different products can have different stated yields, so the calculator asks you to enter the actual cubic-metre yield per bag from the manufacturer. If the bag yield is left at zero, the calculator deliberately reports zero bags instead of inventing an assumption.
Once a valid yield is entered, total concrete volume is divided by the yield and rounded up to the next whole bag. For bag-focused planning, see the Concrete Bag Calculator. Product directions, water requirements, placement limits and curing instructions should be taken from the product information rather than from a generic volume calculator.
When a project has many large holes, ready-mix can become more practical than mixing a large number of bags. If you are comparing the two approaches, first calculate the total cubic metres, then ask suppliers about minimum order quantities, delivery, access, waiting time and any small-load charges. The concrete rate field on this page is editable so you can use the actual quoted A$/m³ value rather than a fixed national price.
For a broader project budget that includes delivery, pumping, labour, reinforcement or other extras, use the Concrete Cost Calculator.
There is no single percentage that is correct for every post-hole job. A neat machine-augered hole in stable ground may be close to its nominal size, while hand-dug holes, loose ground or collapsed sides can create noticeably more volume. The calculator therefore starts the waste field at zero and leaves the decision to you. Add an allowance that reflects the accuracy of the actual holes and the way the concrete will be mixed, carried and placed.
Every numeric input starts at zero. This avoids presenting an example hole size, bag yield or concrete rate as if it were suitable for your project. Enter the values from your own measurements, product information and supplier quote. The result remains hidden until you press the Calculate button, keeping the page clean before a real estimate has been entered.
These examples show the geometry only. They are not recommendations for hole size, post size, depth or structural design.
| Example | Dimensions | Quantity | Base Volume | With 10% Example Allowance |
|---|---|---|---|---|
| Small Round Holes | 250 mm Ø × 600 mm Deep | 6 | 0.177 m³ | 0.194 m³ |
| Round Fence Holes | 300 mm Ø × 600 mm Deep | 8 | 0.339 m³ | 0.373 m³ |
| Square Holes | 300 × 300 × 700 mm | 6 | 0.378 m³ | 0.416 m³ |
| Rectangular Holes | 350 × 300 × 700 mm | 8 | 0.588 m³ | 0.647 m³ |
A useful post-hole estimate separates quantity from specification. First work out the physical concrete volume using the measured hole dimensions. Next add any reasonable contingency. Only after that should you translate the quantity into bags or a ready-mix budget. Keeping these steps separate makes it easier to see whether a cost change comes from more concrete, a different bag yield or a different supplier rate.
Group holes only when their dimensions are genuinely similar. If a gate post, corner post or pergola support uses a larger excavation than the remaining fence posts, calculate that group separately and add the totals. Treating every hole as identical can hide a meaningful volume difference.
If the holes have already been dug, measure them rather than relying solely on the nominal auger size. If they have not been excavated yet, use dimensions from the appropriate project documents or installation requirements and recognise that real excavation can vary.
For a relatively narrow post inside a wide hole, subtracting displacement may make only a small difference. For a large round post inside a tight round hole, the difference may be more noticeable. The calculator lets you compare the full-hole and displaced volumes without forcing one method.
For bagged concrete, enter the manufacturer-stated yield. For ready-mix, enter the A$/m³ concrete rate from a local quote. Do not assume the rate includes every service or fee. Delivery, small-load, waiting-time and access-related charges can be separate depending on supplier and job conditions.
If you are comparing contractor quotes, confirm what each quote includes. One may include excavation, post setting and clean-up while another may cover materials only. The concrete quantity from this calculator can act as a common reference point, but the cheapest price per cubic metre is not automatically the cheapest installed outcome.
After calculation, use the PDF button to save the key dimensions and result. The PDF is intended as a planning record, not a structural or ordering certificate. If the holes change on site, update the measurements and calculate again before buying the final quantity.
Answers To Common Questions About Hole Volume, Post Displacement, Bagged Concrete, Waste And Cost.
Convert the hole diameter and depth to metres. Divide the diameter by two to get the radius, then calculate π × radius² × depth. Multiply by the number of identical holes.
Convert the width and depth to metres, then calculate width × width × depth. Multiply the result by the number of holes.
Convert length, width and depth to metres, then multiply all three dimensions. Multiply by the number of identical holes.
The post displaces some concrete. The dedicated displacement mode can subtract a round post from a round hole. Whether you use the full-hole or displaced volume for planning depends on your project and required accuracy.
Calculate the total concrete volume, enter the manufacturer-stated yield per 20 kg bag and divide the volume by that yield. The calculator rounds up to a whole bag when a valid yield is entered.
Bag yield varies by product. Starting at zero prevents the calculator from presenting an assumed yield as if it applied to every 20 kg concrete mix.
There is no universal percentage. The appropriate contingency depends on the accuracy of the excavation, ground conditions, hole shape, placement method and measurement uncertainty. Enter your own allowance.
Yes, for concrete quantity estimating when you know the relevant hole dimensions and number of fence-post holes. The calculator does not decide the required hole size, depth or structural detail.
Yes for volume estimating if the excavation fits one of the supported shapes. Structural requirements, footing design and post embedment should come from appropriate project information.
No. The cost result is simply the rounded concrete volume multiplied by the A$/m³ rate you enter. Delivery, labour, pumping, small-load fees and other project costs are separate unless already included in your entered rate.
Zero defaults keep the calculator neutral. Enter your actual hole measurements, quantity, bag yield, waste allowance and supplier rate instead of relying on example values.
The result appears only after you press Calculate Post Hole Concrete so the result reflects the measurements you have entered rather than a pre-filled example.
Use Current Product Data, Supplier Information And Appropriate Safety Guidance Alongside Any Online Quantity Estimate.
Australian industry information and technical concrete resources.
Visit CCAANational information about silica hazards and workplace safety.
Read Silica GuidanceUse the exact manufacturer yield, mixing, placement and curing information for the product you buy.
Confirm current concrete rate, delivery access, minimum loads and other service charges before ordering.