Concrete Calculator

How Much Concrete Do I Need?

Whether you are pouring a patio, laying a garden path, or setting fence posts, knowing the exact amount of concrete saves money and prevents mid-project shortages. If you are setting posts for a new boundary, our fence calculator works out the panels, posts, and Postcrete you need. This guide covers the calculation formula, bag counts for common project sizes, and how to choose between premix bags and ready-mix delivery.

The Concrete Volume Formula

Volume (m3) = Length (m) x Width (m) x Depth (m)

Always convert depth from centimetres to metres first. A 10 cm slab is 0.10 m. Then add 10% for waste (spillage, uneven subbase, formwork irregularities).

Example: A 4m x 3m patio slab at 10 cm depth:

• Volume = 4 x 3 x 0.10 = 1.2 m3
• With 10% waste = 1.32 m3
• Premix bags (25 kg) = 1.32 / 0.012 = 110 bags
• Ready-mix delivery = 1.4 m3 (rounded up to nearest 0.1)

Premix Bag Count by Slab Size

The table below shows 25 kg bag counts for common slab sizes at three depths. All figures include 10% waste.

For anything over 50 bags, seriously consider ready-mix delivery. Mixing 100+ bags by hand is exhausting and time-consuming.

Recommended Depth by Project Type

Choosing the right depth ensures your concrete is strong enough for its purpose without wasting material.

Premix vs Ready-Mix: When to Choose Each

The crossover point is roughly 0.5 m3 (about 42 bags). Below that, premix is simpler. Above that, ready-mix saves significant time, effort, and money.

Concrete Mixing Ratios

If you are mixing concrete from raw materials (cement, sand, gravel) rather than using premix bags, use these standard ratios by volume:

The water-to-cement ratio should be around 0.45-0.55 by weight. Too much water weakens the concrete. The mix should hold its shape when squeezed, with a slight sheen of moisture on the surface.

Step-by-Step: Pouring a Concrete Slab

1. Excavate — Dig to the required depth plus 10-15 cm for a compacted sub-base (gravel or hardcore).
2. Compact the sub-base — Use a plate compactor or hand tamper. A solid base prevents cracking from settlement.
3. Build formwork — Use timber boards (25 mm thick) staked every 60-90 cm. Check levels and add a slight fall (1-2%) for drainage on outdoor slabs.
4. Lay reinforcement — Place wire mesh on spacers (chairs) so it sits in the lower third of the slab, not resting on the ground.
5. Pour and spread — Work in sections. Pour concrete into the formwork and spread with a rake or shovel.
6. Level and screed — Drag a straight edge (screed board) across the formwork rails to level the surface.
7. Finish — Use a bull float or hand trowel for a smooth surface. For a non-slip outdoor finish, drag a broom across the surface.
8. Cure — Cover with plastic sheeting or spray with curing compound. Keep moist for at least 7 days.

If you are pouring foundation pads for a raised deck, use our decking calculator to work out the boards, joists, and screws for the structure above. For an indoor floor slab that will be finished with laminate or vinyl, our flooring calculator helps estimate the covering material.

Common Mistakes to Avoid

• Adding too much water — Makes mixing easier but dramatically weakens the concrete. A stiff mix is stronger.
• Pouring on hot days without shade — Concrete that dries too fast cracks. Pour in the morning or erect shade cloth.
• Skipping the sub-base — Concrete poured directly on soil will crack from uneven settlement within 1-2 years.
• Forgetting expansion joints — Slabs wider than 3 metres need expansion joints every 2.5-3 metres to control cracking.
• Underestimating quantity — Running out mid-pour creates a cold joint (weak seam). Always round up and add 10% waste.