Underestimating concrete for post holes is one of the most frustrating on-site mistakes. Making extra runs to Bunnings or the local trade yard for three bags of Rapid Set burns billable hours. This technical guide explains how to calculate exact net concrete volume, select the right bag formulation, and comply with AS 1604 and AS 2870 footing standards across Australia.
1. How to Calculate Post Hole Concrete Volume & Bag Counts
Calculating concrete for post footings requires finding the gross volume of the excavation, subtracting the volume displaced by the embedded post, and converting the net volume into standard Australian concrete bags or raw batching ratios.
Step 1: Calculate Gross Hole Volume (Round Auger Hole vs Square Footing)
Most residential post holes in Australia are drilled using a petrol or hydraulic post hole auger (round hole), while trench footings and pier pads are boxed square.
Round Cylinder Hole Formula (Auger)
Gross Volume (m³) = π × (Diameter ÷ 2000)² × (Depth ÷ 1000)
Example: A 250mm diameter auger hole drilled 600mm deep equals: 3.1416 × (0.125)² × 0.60 = 0.0295 m³ (29.5 Litres).
Square / Boxed Footing Formula
Gross Volume (m³) = (Width ÷ 1000) × (Length ÷ 1000) × (Depth ÷ 1000)
Example: A 300mm × 300mm square footing dug 600mm deep equals: 0.30 × 0.30 × 0.60 = 0.0540 m³ (54.0 Litres).
Step 2: Subtract Post Volume Displacement
When a timber post or steel pipe is submerged into the concrete hole, it displaces wet mix. Subtracting this displacement gives you the exact net concrete required.
For a 100mm × 100mm square timber post embedded 600mm into the hole: 0.10 × 0.10 × 0.60 = 0.0060 m³ (6.0 Litres).
Net Concrete Volume per Hole: 29.5L (Gross) - 6.0L (Post) = 23.5 Litres (0.0235 m³).
Step 3: Convert Net Volume to Standard 20kg Bags (with Wastage)
One standard 20kg bag of dry premix concrete yields approximately 0.0091 m³ (9.1 Litres) of mixed wet concrete. Dividing your net volume by bag yield gives the required bag count:
Total 20kg Bags = [Net Volume (m³) × 1.10 Wastage Allowance] ÷ 0.0091 m³/bag
For 10 fence post holes requiring 23.5L net concrete each, total net concrete is 235 Litres (0.235 m³). Adding 10% for hole irregularities equals 0.2585 m³. Dividing by 0.0091 yields 28.4 → 29 bags of 20kg Rapid Set (an average of 2.9 bags per hole).
2. 20kg Rapid Set vs Post Hole Mix vs General Purpose (GP) Concrete Bags
Not all bagged concrete products sold at Australian suppliers (Bunnings, Bowens, Dingo, Boral, Bastion) are interchangeable. Selecting the wrong bag type can cause structural failure or fail council certification.
| Concrete Bag Type | Yield per 20kg Bag | Initial Set Time | Compressive Strength | Recommended Australian Trade Application |
|---|---|---|---|---|
| 20kg Rapid Set Concrete | ~9.1 Litres (0.0091 m³) | 15 – 20 mins | 12 – 15 MPa | Non-structural: Timber & Colorbond boundary fence posts, clotheslines, letterboxes. |
| 20kg Post Hole / Fence Mix | ~9.0 Litres (0.0090 m³) | 1 – 2 hours | 15 MPa | Economical line posts, rural farm fences, farm gate posts. |
| 20kg General Purpose (GP) | ~9.25 Litres (0.00925 m³) | 4 – 6 hours | 20 – 25 MPa | Structural: Deck subfloor posts, pergola beams, carports, retaining wall posts (AS 2870). |
| 30kg Maxi-Crete / Heavy Duty | ~13.8 Litres (0.0138 m³) | 3 – 5 hours | 25 – 32 MPa | Heavy pole barns, rural shed columns, commercial shade sail posts, high-load anchor pads. |
Important NCC Structural Compliance Warning:
3. Post Volume Displacement & Why It Saves Money
A common estimating error is calculating concrete bags based solely on hole diameter without subtracting the embedded post. On large boundary fences or multi-bay pole barns, failing to deduct post displacement leads to buying 15% to 25% more concrete bags than needed.
Consider a 50-metre boundary fence requiring 22 posts (100mm × 100mm H4 treated pine posts in 250mm × 600mm augered holes):
- Gross Hole Volume: 22 holes × 29.5 Litres = 649 Litres (72 bags of 20kg Rapid Set).
- Post Displacement Deducted: 22 posts × 6.0 Litres = 132 Litres (15 bags saved).
- Actual Net Concrete Required: 517 Litres (57 bags of 20kg Rapid Set).
Deducting post volume saves 15 full bags of concrete (~$142 AUD on a single fencing job), keeping your quote competitive and reducing unnecessary ute payload.
4. Recommended Post Hole Dimensions by Project Type
Post hole diameter and depth depend on the wind classification (AS 4055 N1–N4), soil classification (AS 2870 Class A to Class P), and total structure height above ground.
| Project Application | Standard Post Size | Recommended Hole Diameter | Recommended Hole Depth | Avg. 20kg Bags per Hole |
|---|---|---|---|---|
| 1.8m Timber Paling Fence | 100 × 100mm or 125 × 75mm | 250 mm | 600 mm (min AS 1604) | 2.5 – 3.0 Bags (Rapid Set) |
| 1.8m Colorbond Steel Fence | 50 × 50mm C-Post | 200 mm | 600 mm | 1.8 – 2.2 Bags (Rapid Set) |
| Low-Level Timber Deck (<1.0m H) | 90 × 90mm or Gal Stirrup | 300 mm | 450 – 600 mm | 3.5 – 4.5 Bags (20MPa GP) |
| Elevated Deck / Balcony (>1.0m H) | 100 × 100mm / 125 × 125mm | 350 – 400 mm | 750 – 900 mm | 5.5 – 7.5 Bags (20MPa GP) |
| Freestanding Pergola / Carport | 125 × 125mm H4 Pine | 350 mm | 750 – 900 mm | 6.0 – 8.0 Bags (20MPa GP) |
| Rural Pole Barn / Farm Shed | 150 × 150mm / 150mm Round Pole | 450 – 500 mm | 1000 – 1200 mm | 12.0 – 18.0 Bags (30kg Maxi) |
| Timber Sleeper Retaining Wall (≤1.0m) | 100UC Steel or 200×75 Hardwood | 300 – 350 mm | 900 – 1200 mm (1:1 Ratio) | 6.5 – 9.5 Bags (20MPa GP) |
Planning a larger build? Use our dedicated Fence Material Calculator, Decking Calculator, Retaining Wall Calculator, or Carport Calculator for comprehensive member take-offs.
5. On-Site Concrete Batching Recipe (1:2:3 GP Structural Mix)
When pouring dozens of post holes or large pole barn footings exceeding 1.0 m³, buying 20kg bags becomes expensive. Mixing raw materials on-site using a portable electric cement mixer reduces material costs by up to 50%.
Standard 1 : 2 : 3 Structural Concrete Mix Ratio
To achieve an Australian Standard 20MPa compressive strength, use the trade-proven 1:2:3 batching recipe by volume:
- 1 Part General Purpose (GP) Portland Cement
- 2 Parts Clean Coarse Concrete Sand (Washed River Sand)
- 3 Parts 10mm to 20mm Crushed Blue Metal Aggregate (Gravel)
- 0.5 Parts Clean Potable Water
Raw Material Quantities for 1.0 m³ of Mixed Concrete
- Cement: 320 kg (16 × 20kg bags of GP Cement)
- Concrete Sand: 650 kg (~0.45 m³ loose sand)
- 10-20mm Blue Metal Gravel: 1,200 kg (~0.85 m³ gravel)
- Water: 160 – 180 Litres
6. Australian Standards & Embedment Rules (AS 1604 & AS 2870)
Australian building certifiers and council inspectors check post holes for two main compliance items: timber hazard rating and embedment depth ratio.
1. Timber Preservative Treatment Classes (AS 1604)
Under AS 1604, timber is graded into Hazard Classes (H1 through H6) depending on moisture exposure and termite risk:
- H3 Treated Pine: Suitable for above-ground exterior exposure (e.g. decking boards, pergola rafters). Never place H3 timber directly into soil or wet concrete.
- H4 Treated Pine: Mandatory for exterior in-ground contact (fence posts, retaining wall posts, pergola posts). Chemically treated with CCA, ACQ, or Copper Azole to resist ground rot and subterranean termites.
- H5 Treated Pine: Required for posts subject to constant wetting or critical structural house stumps in damp soil.
2. The "1/3 Post Length in Ground" Rule of Thumb
For non-engineered structures like boundary fences and privacy screens, the minimum embedment depth into stable ground must be at least 1/3 of the total post length (or 1/2 of the above-ground post height).
For a 1.8m fence, you need a 2.4m post with 600mm in ground. For a 2.4m high pergola, you need a 3.2m post with at least 800mm in ground.
7. Step-by-Step: Digging, Setting & Pouring Post Holes
Follow this trade-tested procedure on-site for rock-solid, plumb posts that will not lean or rot prematurely:
Step 1: Check Underground Services (BYDA)
Always submit a free enquiry to Before You Dig Australia (BYDA — dial 1100) before augering post holes. Striking underground electrical, NBN, or gas mains leads to heavy fines and extreme safety hazards.
Step 2: Dig and Bell-Bottom the Hole
Auger the hole to your calculated depth. In reactive clay soils (AS 2870 Class M or H), use a hand shovel or crowbar to widen the bottom 150mm of the hole into a bell shape (under-reaming). This creates a concrete plug that prevents ground frost or clay expansion from lifting the post.
Step 3: Add a 50mm Crushed Gravel Drainage Bed
Pour 50mm of clean 10–20mm gravel or blue metal into the bottom of the hole and tamp it down. The post sits on top of this gravel bed rather than resting directly on soil, allowing rainwater soaking down the timber to drain out freely.
Step 4: Plumb and Two-Way Timber Bracing
Position the post in the centre of the hole. Use a spirit level on two adjacent faces (at 90 degrees to each other) to verify vertical plumb. Secure the post with two diagonal timber braces staked firmly into the ground.
Step 5: Pour Concrete and Dome the Top Surface
If using Rapid Set Concrete: Pour approximately 2.5 litres of clean water into the hole first, empty half the bag around the post, rod the mix with a stake to remove air voids, add another litre of water, and pour the rest of the bag.
Finish the top of the concrete 25mm above ground level and use a hand trowel to slope the wet concrete outwards in a dome shape away from the post. This prevents surface rainwater from collecting around the timber base.