Concrete Calculator

What this concrete calculator does

This is the general-purpose tool on the site. It turns the measurements of one or more parts of a pour into a single concrete volume, then presents that volume in the units your supplier is most likely to use. Because each section carries its own shape, unit and quantity, a job made of a slab, two strip footings and six post holes can be estimated in one pass instead of four separate sums.

How to measure your project

1. Split the job into simple shapes. Anything that is not a neat rectangle can usually be broken into rectangles, circles and strips.
2. Measure each shape in the same place you will pour it, not on the drawing. Ground is rarely as flat as a plan suggests.
3. Take the depth in several spots and use the deepest realistic figure if the base dips.
4. Count identical items rather than measuring each one: six matching holes are one section with a quantity of six.

The formula in plain language

For a rectangle the calculator multiplies length by width to get the area, then multiplies that area by the depth. For a circle it halves the diameter to get the radius, multiplies the radius by itself, multiplies by π (roughly 3.14159) and then multiplies by the depth. A post hole starts as a plain hole volume and then has the space taken up by the post removed. Everything is converted to metres first, so mixing millimetres and metres, or inches and feet, never changes the answer.

Metric worked example for the UK, Ireland, Australia, New Zealand and Canada

A shed base 3 m long, 2.5 m wide and 100 mm thick works out at 3 × 2.5 × 0.1 = 0.75 m³. That is 750 litres, or about 0.98 cubic yards. Adding a 10% waste allowance gives 0.825 m³ as the figure to order, which most suppliers would round to a quoted quantity when you call.

Imperial worked example for the United States

A driveway bay 10 ft long, 10 ft wide and 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet. Divided by 27 that is 1.23 cubic yards, or 0.94 m³. With 10% waste the order figure becomes 1.36 cubic yards. Suppliers usually sell in quarter-yard steps, so you would discuss 1.5 yards with the batching plant.

Common mistakes to avoid

• Entering a slab thickness in metres when the value on the plan is in millimetres.
• Using the radius where the calculator asks for the diameter, which quarters the result.
• Forgetting that a post hole loses volume to the post itself.
• Adding waste twice, once here and again when phoning the supplier.
• Assuming every bag of the same weight produces the same finished volume.

When to speak to a supplier or a qualified professional

Talk to your ready-mix supplier about access, minimum loads, pour windows and pump hire before you commit to a delivery date. Talk to a structural engineer, building control officer or equivalent local authority when the pour supports a structure, sits near a boundary, carries vehicles, or falls under building regulations where you live. Requirements differ by use, soil, climate and local rules, so this calculator never tells you how thick or deep a pour should be.