Concrete Calculator

The formula

Slab and footing: volume = length × width × thickness (or depth), converted to the same unit. Square column: side × side × height × number of columns. Round column: π × (diameter ÷ 2)² × height × number of columns. A wastage percentage is added on top to account for uneven ground, spillage, and formwork variance — concrete is typically ordered a little over the exact calculated volume rather than under, since running short mid-pour is far more disruptive than having a small surplus.

Worked example

A 6m × 4m slab at 10cm thick: 6 × 4 × 0.10 = 2.4 m³ before wastage. With a 10% wastage allowance, that's 2.64 m³ (about 3.45 yd³) of concrete to order.

Why order extra

• Ground is rarely perfectly level, so actual thickness varies across the slab
• Some concrete is lost during mixing, transport, and pouring
• Running short mid-pour can create a weak seam where the pour is interrupted

A strip footing example

A foundation strip 12 m long, 0.5 m wide and 30 cm deep holds 12 × 0.5 × 0.30 = 1.8 m³. Adding 10% brings the order to 1.98 m³, or about 2.59 yd³. Choose Footing and enter the length and width in meters and the depth in centimeters (or feet and inches with Imperial selected), as the field labels say, because mixing the units is the most common way to get a wrong answer. A footing is calculated with the same box formula as a slab; only the labels differ.

Turning volume into something you can order

Ready-mix suppliers sell by the cubic meter in most countries and by the cubic yard in the US, which is why both figures are shown. Many suppliers have a minimum load and may charge extra for small or partial loads, so ask before assuming a tiny pour costs proportionally less. For small jobs, bagged mix is common, but the yield per bag varies by brand and is printed on the bag; divide your volume by that yield and round up.

Fresh concrete weighs roughly 2,400 kg per cubic meter. The 2.64 m³ slab in the worked example is therefore about 6.3 tonnes, which matters for access, wheelbarrow runs and the strength of any formwork.

A round column example

Four round columns, each 30 cm in diameter and 3 m high, with the wastage set to 0, hold 4 × π × 0.15² × 3 = 0.848 m³, or about 1.109 yd³. With the default 10% allowance the order becomes 0.933 m³. For a square column of 30 cm side and 3 m height the volume is 0.3 × 0.3 × 3 = 0.27 m³ each.

The cement bag estimate

Choose a bag size to add a rough cement figure. It assumes a 1:2:4 mix by volume (1 part cement, 2 sand, 4 coarse aggregate). Wet concrete needs about 1.54 times its volume in dry materials, cement is 1 part in 7 of that, and loose cement weighs about 1,440 kg per cubic meter, so the estimate is 1.54 ÷ 7 × 1,440 = 316.8 kg of cement per cubic meter of concrete. It is applied to the volume including wastage and rounded up to whole bags. For the 2.64 m³ slab above that is about 836 kg, or 17 bags of 50 kg.

A 94 lb US bag is taken as 42.64 kg. Other mixes use more or less cement, and strength grades, admixtures and bag sizes differ by supplier, so follow the mix design your project specifies. The Concrete Mix Calculator lets you set the ratio and also gives sand and aggregate.

What this does not cover

• Sub-base such as compacted gravel under the slab, which is calculated and ordered separately
• Sloped, tapered or hollow shapes, and rectangular columns with different sides; use the volume of the matching solid or split the shape into simple pieces
• Sand, aggregate and water quantities; the Concrete Mix Calculator covers those
• Structural requirements such as depth, mix strength and joint spacing, which depend on the load and on local building rules