Concrete Calculator

Enter the shape and dimensions, set the thickness, and you get the volume in cubic yards, cubic feet and cubic metres, the number of bags at any common bag size, the approximate weight, and an optional cost.

A waste allowance is included by default, because subgrade is never perfectly flat and running short mid-pour is the one mistake worth paying to avoid.

How to use the concrete calculator

Choose your shape and enter the dimensions in any units.

Set the slab thickness and a waste allowance.

Read the cubic yards, bag count and estimated cost.

Why use the Build Tally concrete calculator

Any shape

Rectangle, circle, triangle, or enter a known area directly.

Mixed units

Feet, inches, yards, metres, centimetres — each dimension has its own selector.

Bags and yards

See both the ready-mix volume and the bag count at five common bag sizes.

Cost estimate

Add a price per cubic yard and get a total in any currency.

How concrete volume is calculated

Volume is area × thickness. The arithmetic is simple; the units are what trip people up. Thickness is usually in inches while length and width are in feet, so a 10 × 10 ft slab at 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Because there are 27 cubic feet in a cubic yard, small changes in thickness move the total more than people expect — going from 4 to 6 inches adds 50% to the order.

Choosing a thickness

Four inches is the standard for residential patios, walkways and shed bases. Driveways carrying cars are usually 4 inches over a compacted base, and 5–6 inches if a truck or RV will park on them. Garage floors are typically 4–6 inches. Anything structural — footings, suspended slabs, load-bearing pads — is not a thickness you should pick from a table; it needs to come from your local code or an engineer.

Why the waste allowance matters

The theoretical volume assumes a perfectly level, perfectly excavated base. In practice the subgrade dips, the forms bow slightly under the weight of wet concrete, and some is lost in the chute and the wheelbarrow. Five percent is the minimum on a well-prepared base; ten percent is sensible on rough ground. If the calculator says 3.2 cubic yards, order 3.5 — the surcharge for a short load usually exceeds the cost of the extra concrete.

Read the guide: How Much Concrete Do I Need? A Straight Answer for Slabs, Footings and Posts

Frequently asked questions

How much concrete do I need for a 10x10 slab?

A 10 × 10 ft slab at 4 inches thick needs about 1.23 cubic yards, or roughly 56 of the 80 lb bags. Add 5–10% for waste.

How many bags of concrete make a cubic yard?

About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb. Each 80 lb bag yields around 0.6 cubic feet.

How thick should a concrete slab be?

Four inches for patios, walkways and standard driveways. Five to six inches where heavy vehicles will park. Structural slabs need engineering.

How do I calculate concrete for a circle?

Choose the circle shape and enter the diameter. The calculator uses π × r² × thickness.

How much does a cubic yard of concrete weigh?

About 4,000 lb, or two US tons, at a typical density of 145 lb per cubic foot.

Should I use bags or ready-mix?

Under about half a cubic yard, bags are practical. Beyond one cubic yard, ready-mix is usually cheaper once you value your own labour, and it gives a consistent mix.