Concrete Calculator

Concrete is estimated by volume and ordered by the cubic yard: measure the shape, multiply out the volume and divide by 27 to get yards. Work out a slab, footing or round column in cubic yards, cubic metres and cubic feet, plus how many bags of concrete you would need and the ready-mix cost — with a waste allowance built in so you order enough.

%
$
Concrete needed — cubic yards
Cubic feet — ft³
Cubic metres — m³
Bags of concrete — at selected size
Estimated cost — ready-mix

How to Calculate Concrete

Pick the shape, measure, and read the yards.

1

Choose the shape

Use Slab for a floor, driveway or patio, Footing / Wall for a strip footing or wall, and Column for a round pier, post hole or tube where you enter a diameter and height.

2

Enter the dimensions

Measure length and width in feet and the thickness or depth in inches — the two unit selectors handle the mix, so you never convert by hand. Set a quantity if you are pouring several identical pieces.

3

Read yards, bags and cost

You get cubic yards, cubic metres and cubic feet, the number of bags for your bag size, and — with a price per yard — the ready-mix cost. A waste allowance is added on top so you order enough.

Frequently Asked Questions

How much concrete do I need for a slab?
Concrete for a slab is length times width times thickness, with every dimension in the same unit, then converted to cubic yards because ready-mix concrete is ordered by the yard. Measure the slab in feet and the thickness in inches, and the calculator does the conversion for you: it turns the thickness into feet, multiplies the three figures to get cubic feet, and divides by 27 to get cubic yards. For example a 10 by 10 foot slab at 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, which is about 1.23 cubic yards before any waste allowance. Add a waste margin of five to ten percent so you are not caught short when the forms flex or the sub-base is uneven, and round up when you order.
How many bags of concrete make a cubic yard?
It depends on the bag size, because a bag is sold by weight but concrete is placed by volume. A cubic yard is 27 cubic feet, and a pre-mixed bag yields roughly 0.60 cubic feet for an 80 lb bag, 0.45 cubic feet for a 60 lb bag and 0.30 cubic feet for a 40 lb bag. That works out to about 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb to fill one cubic yard. The calculator does this for you: pick your bag size and it divides the total volume, including the waste allowance, by that bag's yield and rounds up to whole bags. Bagged concrete is convenient for small jobs, but once you are past roughly a cubic yard it is usually cheaper and far less work to order ready-mix delivered.
Should I order extra concrete for waste?
Yes — a small overage is normal and worth it, because running out mid-pour is far more costly than a little leftover. Sub-grades are never perfectly flat, forms bow outward under the weight of wet concrete, and some is always lost to spillage and to what stays stuck in the truck or mixer, so the volume you actually place is a bit more than the neat geometric figure. A waste allowance of five to ten percent covers this for most slabs and footings; go toward the higher end on rough ground or narrow footings where a slumped edge eats volume quickly. The calculator adds your chosen waste percentage on top of the measured volume and shows both figures, so you can see the raw number and the amount to actually order.
How do I calculate concrete for a round column or post hole?
A round column, pier or filled post hole is a cylinder, so its volume is pi times the radius squared times the height — π × r² × h — with the radius being half the diameter. Switch the calculator to Column mode and enter the diameter and the height; it converts both to feet, works out the cylinder volume in cubic feet, then gives you cubic yards, cubic metres and the number of bags. A 12-inch diameter tube filled to 4 feet, for instance, is π × 0.5² × 4 = 3.14 cubic feet, about 0.12 cubic yards. If you are setting several identical posts, set the quantity so the total covers every hole in one order rather than calculating them one at a time.
Is ready-mix by the yard or bagged concrete cheaper?
For anything beyond a small job, ready-mix delivered by the cubic yard is almost always cheaper per yard and dramatically less labour than mixing bags by hand. Bagged concrete makes sense for footings, a few post holes, a small pad or a repair — jobs under roughly a cubic yard — where a delivery would carry a minimum-load or short-load fee that dwarfs the material cost. Once you need a cubic yard or more, the per-yard price of ready-mix, even with delivery, usually beats buying, hauling, mixing and placing dozens of bags. Use the calculator's cost field to price the ready-mix option by entering your local price per cubic yard, then compare that against the bag count it shows for the same volume.

More home & DIY calculators