How Many 20 kg Bags of Concrete in a Cubic Metre?

The Cement Australia sell sheet describes two 20 kg bags covering an area 450 mm by 500 mm at 75 mm thickness. Multiplying those dimensions gives 0.016875 cubic metres for two bags, or 0.0084375 m³ per bag. Dividing one cubic metre by that per-bag yield gives about 118.5 bags, so a whole-bag purchase for a full cubic metre rounds up to 119 bags before any allowance for waste. The maker presents the coverage as approximate.

That calculation is specific to the cited product and coverage statement. It is not a claim that every 20 kg bag makes the same volume. The bag weight alone does not define yield because mix proportions, aggregate grading and stated coverage differ. Confirm that the bag you buy is the same mix and check its current label and data sheet.

Some manufacturers publish a volume per bag, some give a coverage area at a stated thickness, and others give only a project example. The source used here gives an approximate coverage statement, so the calculator derives a volume from the three published dimensions. It does not use an older third-party figure that could not be confirmed against the current manufacturer document.

Water added during mixing, material left in the mixer or barrow, uneven excavation, leakage from formwork and an irregular base can all affect how far a batch goes on site. Those are practical reasons to measure carefully and decide whether your project needs an allowance. The calculator defaults to zero because the checked manufacturer sheet does not set a universal waste percentage. Enter an allowance that reflects your formwork and conditions.

For a 3 m by 3 m slab at 100 mm thick, volume is 3 × 3 × 0.1 = 0.9 m³. Dividing by 0.0084375 m³ per bag gives 106.67 bags, which rounds up to 107 bags with no waste allowance. At a user-entered 10% allowance, the order volume becomes 0.99 m³ and the calculator estimates 118 bags. These are quantity estimates for this example product, not structural design advice.

Use the same sequence for a footing: length × width × depth, with all three dimensions expressed in metres. For circular holes or piers, the calculator uses the cylinder volume. A post hole subtracts the area occupied by a square post standing on the bottom of the hole. If you fill the hole before placing the post, enter zero post width to match that sequence.

A full cubic metre is a large bag order. Compare the total bag price, mixing effort, access and delivery options before choosing a supply method. The calculator can compare bag cost with a ready-mix price and delivery fee when you enter current local quotes. That comparison uses your inputs and does not include pump hire, minimum loads, waiting charges or every supplier fee, so ask suppliers for a complete delivered quote.

Concrete products are heavy and cement dust can harm skin, eyes and breathing. The cited Cement Australia safety data sheet calls for avoiding dust and wearing protective gloves, clothing and eye protection. Follow the instructions and safety data sheet for the product actually supplied, plan lifting and mixing safely, and do not treat a material take-off as a footing or slab specification.

Before placing a concrete order, write down the dimensions and the source used for each product yield. If the supplier changes the mix or bag size, recalculate with that item’s yield instead of adjusting only the bag weight. Ask whether the quote includes delivery, unloading and any minimum quantity, then compare like with like. For a continuous pour, the supplier can also advise on dispatch timing and access, but the structural specification should come from the project designer.

For a non-rectangular pour, split the shape into simple measured sections and add their volumes. Avoid counting the same area twice and distinguish the volume of a footing from any post or reinforcement that occupies it. Keep a copy of the calculations with your shopping list so the assumptions are visible when measurements or products change.

For this Australian example, the product sheet supports a derivation from a small coverage area, not a directly declared cubic-metre yield. That distinction matters if another retailer quotes a different bag count. Ask the seller to identify the exact mix and show its current yield basis. If it differs from the cited Cement Australia product, use the seller’s verified figure in a fresh calculation rather than trying to reconcile different formulations by weight alone.