Concrete Calculator

Concrete is ordered by volume. Ready-mix plants in the United States sell it by the cubic yard; suppliers in the UK, Canada and Australia sell it by the cubic metre. This calculator turns the dimensions of the space you are filling into that order volume.

It handles the two shapes that cover most residential pours:

• Rectangular pours – slabs, footings, pads, steps treated as blocks, and walls.
• Round pours – columns, piers, cardboard tube forms and post holes.

For several identical pieces, such as eight deck-post footings, set the quantity instead of repeating the calculation. For an L-shaped or irregular slab, split it into rectangles and add the results together.

1. Pick the shape: a rectangle (slab, footing or wall) or a round column or hole.
2. Enter the length and width (or diameter) and the thickness or depth. Each box has its own unit menu, so you can give the length in feet and the thickness in inches, or use ft + in for measurements like 12 ft 6 in.
3. Set the quantity if you are pouring several identical pieces.
4. Keep or change the waste allowance. The default is 10%.
5. Optionally enter your supplier’s price per cubic yard or cubic metre to estimate the material cost.

For a rectangular slab, footing or wall:

Volume = Length × Width × Thickness

For a round column or hole, where the radius is half the diameter:

Volume = π × (Diameter ÷ 2)² × Height

Then multiply by the number of pieces and add the waste allowance:

Volume to order = Volume × Quantity × (1 + Waste % ÷ 100)

All measurements are converted to the same unit before multiplying. A common mistake is multiplying feet by inches: a 4-inch slab is 4 ÷ 12 = 0.333 ft thick, not 4 ft. To convert, 1 cubic yard = 27 cubic feet and 1 cubic yard = 0.7646 cubic metres.

A 12 ft × 10 ft patio slab, 4 inches thick:

• Thickness in feet: 4 ÷ 12 = 0.333 ft
• Volume: 12 × 10 × 0.333 = 40 cubic feet
• In cubic yards: 40 ÷ 27 = 1.48 yd³
• With 10% waste: 1.48 × 1.10 = 1.63 yd³ (about 1.25 m³)

The same slab in metric (3.6 m × 3.0 m × 100 mm): 3.6 × 3.0 × 0.1 = 1.08 m³, or 1.19 m³ with 10% waste.

Six round footings, each 10 inches across and 4 ft deep: π × (5 ÷ 12)² × 4 = 2.18 ft³ each, so 13.09 ft³ (0.48 yd³) in total before waste.

• Concrete to order – the total including your waste allowance, in cubic yards (US) or cubic metres (metric). This is the figure to give a ready-mix supplier.
• Net volume – the exact geometric volume with no allowance. Useful for comparing quotes.
• Waste allowance – the extra volume added for uneven ground, form movement and spillage.
• Estimated material cost – volume to order × your price. Delivery, short-load and pump fees are not included.

• Measure the inside of the forms, not the outside of the boards.
• Check the depth in several places. Subgrade is rarely perfectly flat, and a slab planned at 4 in that averages 4½ in uses about 12% more concrete.
• For thickened slab edges or footings under a slab, calculate them separately and add them in.
• Round the order up, not down. Running short means waiting for a second truck and risks a cold joint where fresh concrete meets concrete that has started to set.

• Subgrade variation – low spots in compacted base fill with concrete.
• Form deflection – forms that bow outward under load hold more concrete.
• Spillage and pump line – some concrete is always left in the truck chute, wheelbarrows or pump hose.
• Supplier minimums – many plants deliver in fixed increments and charge extra for small loads.

For small pours, compare the cost of ready-mix with bags using the concrete bag calculator. For patios and shed bases, the concrete slab calculator also gives you the area and the length of edge forms.