Concrete Calculator

One cubic metre of concrete requires approximately 93 standard 20kg bags of pre-mix, or about 62 bags of 30kg pre-mix. That is a lot of bags to open, mix, and pour — which is why most tradies order ready-mix delivery for anything over half a cubic metre. Ready-mix also gives you a more consistent result since the batching plant controls the water-cement ratio.

20kg bags are easier to handle, especially if you are working alone or on a ladder. They are a good choice for small repairs, post holes, and light jobs. 30kg bags are more cost-effective per cubic metre and mean fewer bags to open and mix — better for larger areas like shed slabs or garden paths. Either way, both produce the same concrete once mixed. The main difference is convenience versus value.

In Australia, typical depths are: 100mm for garden paths and patios, 100-125mm for garage floors, and 125-150mm for driveways that need to support vehicle weight. Footings for sheds, retaining walls, and house slabs vary depending on soil type and engineering requirements — always follow your engineer's specification for structural work. Your local council may also have minimum depth requirements.

Concrete reaches its initial set in 24-48 hours — you can walk on it carefully after this point. At 7 days it reaches about 70% of its design strength, which is enough for light use. Full design strength is reached at 28 days. During the first week, keep the surface moist by misting with water or covering with plastic sheeting — this prevents surface cracking. Avoid heavy loads for at least 7 days, and keep vehicles off for the full 28 days.

Light rain is generally manageable if you can cover the freshly poured concrete with plastic sheeting straight away. The real risk is heavy rain — it dilutes the cement paste on the surface, which leads to a weak, dusty finish that flakes and scales over time. If heavy rain is forecast, it is better to postpone the pour. If you are caught out mid-pour, cover the surface immediately and avoid working water into the finish. Once concrete has set (24+ hours), rain will not affect it.