Volume Required

m³

Bag Size

Concrete Bag Formula

                Bags Required: ceil(Volume / Bag Yield)

                Bags + Waste: ceil(Bags × 1.1)

Australian Construction Industry Stats

Reference figures for Australian builders. All numbers cited from authoritative public sources. Built Simple updates these as official data is released.

~171,000new dwellings approved in Australia per year (12-month rolling, ABS 8731.0)
0k - 0kaverage new home build cost ex-land in Australia (HIA, 2025-26)
15 - 25%typical residential builder gross margin on cost (Master Builders Australia)
10 - 15 daysstatutory window for a payment schedule under most AU Security of Payment Acts
6+ yearsstatutory warranty for major defects from Practical Completion (most AU states)
8 - 15 days/yeartypical residential build days lost to weather on outdoor trades (industry average)
10 - 20%recommended contingency budget for owner-builds (HIA guidance)
125mmmaximum balustrade gap under NCC (designed to prevent a child's head fitting through)

Sources: Australian Bureau of Statistics (abs.gov.au), Housing Industry Association (hia.com.au), Master Builders Australia (masterbuilders.com.au), Australian Building Codes Board (abcb.gov.au). Figures are reference guidance only - verify current values against the source for legal or commercial decisions.

Concrete Bag Calculator — How Many Bags Do You Need?

Australian hardware stores sell over 12 million bags of pre-mixed concrete annually (ABS Retail Trade, 2024). A standard 20kg bag yields just 0.009 m³, meaning you need approximately 108 bags per cubic metre — getting the count right saves trips and money.

Bags Per Cubic Metre — Quick Reference

Bag Size	Yield per Bag	Bags per m³	Avg. Price per Bag	Cost per m³
20 kg	0.009 m³	108	$9–$12	$970–$1,296
25 kg	0.012 m³	84	$11–$14	$924–$1,176
40 kg	0.019 m³	53	$16–$20	$848–$1,060

Coverage Table — Bags for Common Slab Sizes

Area × Depth	Volume (m³)	20 kg Bags	25 kg Bags	40 kg Bags
1m × 1m × 50mm	0.05	6	5	3
1m × 1m × 100mm	0.10	11	9	6
2m × 2m × 100mm	0.40	44	34	21
3m × 3m × 100mm	0.90	100	75	48
4m × 4m × 100mm	1.60	178	134	85

Bags vs Ready-Mix — Cost Comparison

Method	Cost per m³	Best For
20 kg bags	$970–$1,296	Posts, small repairs (<0.25 m³)
40 kg bags	$848–$1,060	Footings, small pads (0.25–0.5 m³)
Ready-mix delivery	$250–$380	Slabs, driveways (>0.5 m³)

When to Use Bags vs Ready-Mix

Use bags for fence posts, small footings, letterbox bases, and repairs under 0.5 m³.
Use ready-mix for slabs, driveways, footpaths, and any pour over 0.5 m³.
Ready-mix is 3–4× cheaper per cubic metre — the crossover point is around 0.5 m³.
Most ready-mix suppliers have a minimum order of 0.2–0.5 m³ with a small-load surcharge under 1 m³.

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Frequently Asked Questions

How many 20kg bags of concrete do I need per m3?

You need approximately 108 bags of 20kg concrete to make one cubic metre. Each 20kg bag yields about 0.009 m³ of mixed concrete. For larger pours, 40kg bags (53 per m³) are more efficient.

How many bags of concrete for a fence post?

A standard fence post hole (200mm diameter × 600mm deep) requires about 0.019 m³ of concrete — that is 2 bags of 20kg concrete or 1 bag of 40kg. For a 10-post fence, budget 20 × 20kg bags.

Is it cheaper to buy bags or ready-mix concrete?

Bags cost $850–$1,300 per m³ while ready-mix costs $250–$380 per m³. Ready-mix is 3–4 times cheaper per cubic metre. The break-even point is around 0.5 m³ when you factor in small-load delivery fees.

How long does bagged concrete take to set?

Pre-mixed bagged concrete is typically walkable in 24–48 hours and reaches full strength (20–25 MPa) at 28 days. Rapid-set varieties can be walkable in 2–4 hours for post-hole applications.

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Concrete Pier Calculator — pier-specific bag counts.