Estimate concrete for a slab or post holes

The job

Concrete comes as dry bags you mix — the question is always 'how many'. Slabs are L×W×D; post holes are cylinders × the hole count. The yield per bag varies by brand, so it is an input, not a guess.

Compute it

Pick slab or post-hole shape, enter the dimensions, and read the yield off your bag (≈9–14 L for typical 20–30 kg bags — it is on the label). Extra allowance covers spillage and a slightly-deep dig.

• Slab: length, width, thickness.
• Post: hole diameter, depth, count.
• 10% extra is the usual buffer for uneven ground.

Worked example

Four post holes at 200 mm diameter × 500 mm deep = 0.063 m³ ≈ 63 L. At 10% extra that's ~70 L; at 12 L per bag, 6 bags. The two-bag spare you nearly bought stays at the store.

Limits

Treats holes as perfect cylinders and slabs as perfect boxes — real ground is neither, so keep the extra allowance honest. Up to 1,000 m³ and 50% extra. Bag yields differ; always check the label.