How the calculator works it out
Most post hole calculators work out the volume of the hole and stop. Two things in the hole are not concrete. A 100 × 100 mm post takes up 6 litres of a 600 mm hole. The gravel at the bottom, which lets water drain away from a timber post, takes up another 7 litres of a 300 mm hole. Counting the whole hole as concrete overstates it by about 40 %: 42 litres instead of 30.
round hole area = π × d² ÷ 4 · bags = ⌈ concrete × posts × (1 + allowance) ÷ litres per bag ⌉
The last step uses the yield the maker prints, not the weight of the bag. A 20 kg bag of Cement Australia concrete is stated at 110 bags per m³, about 9.1 litres. Australian Builders Quick Set is stated at about 100 per m³, 10 litres. The difference is two or three bags on a ten-post fence.
Bags per post for common set-ups
At 110 bags per m³ (9.1 L per 20 kg bag), with the concrete finished at ground level and no allowance added:
| Post | Hole | Gravel | Concrete | 20 kg bags |
|---|---|---|---|---|
| 75 × 75 mm timber | 250 mm round × 450 mm | 100 mm | 15.2 L | 1.7 |
| 100 × 100 mm timber, fence to 1.2 m | 300 mm round × 450 mm | 100 mm | 21.2 L | 2.3 |
| 100 × 100 mm timber, fence to 1.8 m | 300 mm round × 600 mm | 100 mm | 30.3 L | 3.3 |
| 125 × 125 mm timber | 350 mm round × 600 mm | 100 mm | 40.3 L | 4.4 |
| 50 × 50 mm steel post | 200 mm round × 600 mm | – | 17.3 L | 1.9 |
| Rotary clothesline socket | 250 mm round × 600 mm | 150 mm | 21.2 L | 2.3 |
Buy for the whole job and share the part bags between holes. Rounding every post up separately is the most common way to end up with a stack of spare bags. One exception: rapid set concrete is poured dry into each hole and can't be saved once wet. Pour whole bags where you can, and keep an opened bag sealed and dry.
How deep and how wide?
- Timber fences. Timber Queensland's fence guide asks for about 450 mm in the ground for fences up to 1.2 m high and 600 mm up to 1.8 m. Holes should be at least 300 mm across, with at least 100 mm of coarse gravel in the base.
- Steel sheet fences. Lysaght's installation guide specifies 200 mm wide footings. Depth runs from 100 mm to 900 mm depending on wind region, terrain, soil and panel run. On stiff clay in a typical Sydney suburb it is 200 × 400 mm.
- The three-times rule. Dricon in New Zealand suggests a hole about three times the post width, with at least a quarter of the post in the ground. That is 600 mm for a post standing 1.8 m above the ground.
- Clotheslines. A common installation is a 250 mm hole, 600 mm deep, on 150 mm of gravel, which is about two 20 kg bags.
- Pergolas and carports. Posts that hold up a roof are structural. Follow the footing sizes in your plans. Timber Queensland prefers galvanised stirrups set in the concrete, with a 75 mm gap under the post, over burying the timber.
Soil changes everything. In Lysaght's tables, soft clay or loose sand can need twice the depth of firm ground. Never set posts into loose fill. The post depth guide goes through soil, timber treatment and finishing in detail.
Rapid set or ordinary concrete?
Rapid set post mixes go into the hole dry. Brace the post, pour in the water the bag asks for (about 2–3.5 litres per bag, depending on the brand), then pour the dry mix in and tamp it. It hardens in about 15 minutes, and the Australian and New Zealand makers say the braces can come off after about an hour. It sets too fast to mix in a barrow, and standard rapid set is sold for non-structural uses, although high-strength versions exist. Ordinary concrete mix is mixed with water in a barrow first. It is stronger and gives you time to adjust the post, but the post needs bracing until it has set. Either way, too much water weakens concrete. Use the amount on the bag.
Finishing the top
Don't leave a flat or dished top, because it holds water against the timber. Either slope the concrete away from the post at ground level, as Cement Australia's instructions show, or stop it 100 mm below the surface and backfill with soil, as Dricon's guide does. The calculator's "top of the concrete" setting takes that soil cap out of the concrete volume.
Post hole questions
How many bags of concrete per fence post?
For a 100 × 100 mm post in a 300 mm hole 600 mm deep with 100 mm of gravel, about 3.3 bags of a 20 kg mix that yields 9.1 L. In a 450 mm hole for a fence up to 1.2 m, about 2.3 bags.
How much concrete does a 20 kg bag make?
About 8 to 10 litres, depending on the product. Stated yields range from 110 bags per m³ (9.1 L) to 100 bags per m³ (10 L) for standard and quick-set mixes. Dricon's high-strength rapid set is 8 L per 20 kg.
Do I put gravel in the bottom of a post hole?
For timber posts, yes: about 100 mm of coarse gravel lets water drain from the end grain. Steel posts are usually set in concrete with concrete underneath instead.
Sources and method
- Cement Australia, Concrete Mix product data sheet (110 bags per m³), PROStrength Rapid Set and How to set a fence post.
- Boral, Concrete Mix product data sheet (108 bags per m³); Australian Builders, Quick Set Concrete data sheet (about 100 bags per m³).
- Dricon (Firth), technical data sheets for RapidSet, High Strength RapidSet and HandiCrete – yields, hole size, soil cap.
- Timber Queensland, Technical Data Sheet 20: Residential timber fences and TDS 19: Pergolas and carports – depths, gravel, stirrups.
- Lysaght (BlueScope), Fence and gate installation guide, non-cyclonic (January 2026) – footing sizes by wind, terrain and soil.
- Volumes are geometry. The 10 % allowance for hand-dug holes is our own default, not a manufacturer figure; set it to zero if your holes are augered to size.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error