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Concrete Calculator

Estimate how much concrete you need for a slab, footing or round column, including a waste allowance. The result shows ready-mix volume, premix bags, and the cement, sand and aggregate for a nominal 1:2:4 site mix.

Units
Shape
m
m
cm
%
Concrete needed1.32 m³1.73 cubic yards
Before waste
1.2 m³
80 lb premix bags (0.6 ft³)
78
40 kg premix bags (≈0.019 m³)
70
1:2:4 mix: cement (50 kg bags)
9
1:2:4 mix: sand
0.58 m³
1:2:4 mix: aggregate
1.16 m³
Step-by-step working
  1. Volume = length × width × thickness
  2. Net = 1.2 m³; with 10% waste = 1.32 m³
  3. Site mix: dry volume = wet × 1.54 = 2.033 m³; cement = dry × 1/7 ÷ 0.0347 m³ per bag = 8.4 bags

Bag yields vary by brand. The 1:2:4 figures are a nominal mix (roughly M15) for estimating; structural work should follow an engineer’s mix design.

How to use the Concrete Calculator

  1. Choose units and shape.
  2. Enter dimensions, how many of them, and a waste allowance (5–10% is usual).
  3. Press Calculate.

Formula

Slab: V = L × W × T. Column: V = π r² h. Total = V × quantity × (1 + waste).

Site mix 1:2:4: dry volume ≈ 1.54 × wet volume; cement = dry ÷ 7, sand = 2 × dry ÷ 7, aggregate = 4 × dry ÷ 7. One 50 kg cement bag ≈ 0.0347 m³.

Examples

  • 4 × 3 m slab, 10 cm thick, 10% waste: 1.2 m³ → 1.32 m³, about 9 bags of cement for a 1:2:4 mix.

Frequently asked questions

Should I order exactly what is calculated?

Order a little extra. Running short during a pour causes weak joints; 5–10% extra is standard.

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