Concrete Calculator

How to use the concrete calculator

Choose the shape you are pouring — a flat slab (patio, shed base, driveway), a strip footing, a square column, or a round column, pier or circular slab such as a cardboard form tube. Enter each dimension and pick its unit; you can mix units freely, so a slab can be measured in feet with the thickness in inches, or in metres with the thickness in centimetres. Add the number of identical pours (for example eight deck piers) and a waste allowance, and the calculator returns the volume in cubic yards, cubic metres and cubic feet plus the number of premix bags to buy.

Concrete volume formulas

The most common mistake is mixing units — multiplying feet by inches. Always convert every dimension to the same unit first. A 4-inch slab is 4 ÷ 12 = 0.333 ft thick; a 10 cm slab is 0.10 m thick. The calculator does this conversion for you and shows the working.

Worked example: a 10 × 10 ft patio

A 10 ft × 10 ft slab poured 4 inches thick contains 10 × 10 × 0.333 = 33.3 ft³, or 33.3 ÷ 27 = 1.23 cubic yards (0.94 m³). Adding 10% for waste brings the order to 36.7 ft³, about 1.36 yd³. An 80 lb bag of premix yields roughly 0.60 ft³, so you would need 36.7 ÷ 0.6 = 61.1, rounded up to 62 bags — over 2.2 tonnes of material to lift and mix. That is exactly the point where a small ready-mix delivery starts to make sense.

How many bags of concrete do I need?

Bagged concrete is sold by weight, but what matters is the volume it produces once mixed. Typical yields are:

Yields vary slightly between brands and mixes (fast-setting post mix often yields less), so check the bag. As a rule of thumb, bags are economical for post holes, small pads and repairs up to about one cubic yard. Beyond that, ready-mix is cheaper per unit, more consistent and saves a huge amount of labour — but many suppliers charge a "short load" fee for orders under a few yards.

Typical thicknesses and depths

• Walkways and patios: 4 in (100 mm).
• Driveways and garage floors: 4–6 in (100–150 mm), thicker for heavy vehicles.
• Shed or hot tub bases: 4–6 in (100–150 mm), often with mesh or rebar.
• Strip footings: commonly 8–12 in deep and 16–24 in wide, and always below the local frost line.
• Deck piers: 8–12 in (200–300 mm) diameter tubes set below frost depth.

Requirements depend on soil, load and local building codes, so confirm with your building department or engineer for structural work.

Results are estimates for planning; actual quantities depend on site conditions, form accuracy and product yield.