Concrete Calculator — Slab, Column, Footing & Stairs

Real-world use cases

• Pour a backyard slab without over-ordering

  You are laying a 4 m × 3 m shed slab at 100 mm thick. Pick the slab shape, type 4, 3 and 0.1 in metric, and read 1.2 m³ before waste, 1.28 m³ with the default 7% margin. The tool also tells you a 1:2:3 hand mix needs roughly 11 bags of 50 kg cement plus the matching sand and gravel volumes. Now you can decide between bagging it yourself for a weekend or calling a small ready-mix load — and you will not be the person standing in wet concrete realizing you are two bags short with the mixer already going off.

• Estimate strip and pad footings for an extension

  A house extension needs four pad footings, each 0.6 m square and 0.8 m deep. Use the slab shape for one pad (0.6 × 0.6 × 0.8 = 0.288 m³), or switch to round footings if you are using form tubes. Bump the count to 4 and the tool totals all of them at once, then gives the cement-bag count for a stronger 1:1.5:3 mix that suits load-bearing footings. Pair it with the imperial toggle when your ready-mix supplier quotes in cubic yards.

• Build concrete steps for a porch

  Three steps up to a porch, each 280 mm run and 170 mm rise, 1.1 m wide. The stairs shape sums the triangular step blocks for you — three steps is six (run × rise) blocks — so you get the true solid volume instead of guessing a box. The result feeds straight into a 1:2:3 bag count so you can buy cement, sand and gravel in one trip rather than two.

• Size columns for a carport or pergola

  Six 250 mm square columns, each 2.7 m tall, holding up a carport roof. Choose the square-column shape, enter 0.25, 0.25 and 2.7, set the count to 6, and the tool returns the combined 1.01 m³ plus the 1:1.5:3 materials for a C25-class pour appropriate to slender load-bearing columns. The shared link lets you send the exact spec to whoever is buying the materials.

• Convert a contractor's cubic-yard quote to bags you can buy

  Your contractor says the driveway needs "about 2 cubic yards." Flip the units to imperial, work backward from the slab dimensions until the volume matches 2 yd³, then read the metric-equivalent bag count if you are tempted to hand-mix part of it. Seeing 2 yd³ ≈ 1.53 m³ ≈ roughly 70 bags of cement in a 1:2:3 mix usually settles the buy-vs-bag question in favor of the truck.

Common pitfalls

• Entering the radius where the tool asks for the diameter. A round footing uses V = π × (diameter/2)² × height, so putting the radius in the diameter field quadruples the volume. Always type the full width of the tube or hole.

• Forgetting to convert thickness into the same unit. A 100 mm slab is 0.1 m, not 1 m and not 100. In metric, every linear field is meters; in imperial, every field is feet. Mixing millimeters or inches straight in gives a wildly wrong volume.

• Ordering the exact theoretical volume with no waste. Real pours lose 5–10% to spillage, over-dig and the wheelbarrow you cannot empty. Skipping the waste margin is how a slab finishes one bag short and gets a cold joint. Keep at least 5–7%.

• Treating the nominal ratio as additive water-and-all. 1:2:3 is cement:sand:gravel by dry volume; water is added separately to workability. The 1.54 dry-volume factor is already applied for you, so do not also pad the dry quantities by hand.

Privacy