General

Concrete Calculator

Estimate concrete volume and bag count for slabs, footings and columns in yards or cubic meters.

Imperial Metric

Shape

Length ft

Width ft

Thickness in

Bag size

Concrete needed

—

Enter dimensions to estimate.

Includes a 10% waste allowance so the truck or pallet does not arrive short.

What this calculator does

Concrete is sold by volume, but volume is awkward to estimate from a sketch. Pick a shape, type the dimensions and the calculator works out cubic yards, cubic meters and the equivalent number of pre-mix bags at standard sizes. A 10% waste allowance is added because spillage, over-pour and uneven subgrade are facts of life.

How to use it

Pick the shape that matches your job. For a slab use length × width × thickness. Footings are usually rectangular trenches; columns are square or round. Multiple identical pieces, like deck post footings, can be entered with a count multiplier so you do not have to add them up by hand.

Volume formulas

Slab and rectangular footing: L × W × T. Round footing or round column: π × (d/2)² × depth. Square column: side × side × height. All inputs use the same unit system; the calculator handles the conversion to cubic yards (1 yd³ = 27 ft³) and cubic meters.

Bag yield reference

Bag	Yield
40 lb (18 kg)	0.30 ft³ (~0.0085 m³)
60 lb (27 kg)	0.45 ft³ (~0.0127 m³)
80 lb (36 kg)	0.60 ft³ (~0.0170 m³)
20 kg	~0.33 ft³ (0.0094 m³)
30 kg	~0.49 ft³ (0.0140 m³)

Frequently asked questions

Bags or ready-mix?

Below about 1 cubic yard (27 cu ft) bags are usually cheaper and more convenient. Above that, ready-mix from a truck wins on both price and labor. The break-even varies by region; price two 80 lb bags vs. a quarter yard at your local plant to be sure.

How much does an 80 lb bag yield?

About 0.6 cubic feet (0.017 m³) of mixed concrete. So 45 bags fill 1 cubic yard. A 60 lb bag yields 0.45 cu ft and a 40 lb bag yields 0.30 cu ft.

Why a 10% waste factor?

Subgrade is never perfectly flat, formwork loses some to spillage and you almost always over-fill the last few feet. Estimators routinely add 10% on small jobs and 5% on large pours to keep the truck from running out.

Does it account for rebar?

No. Rebar takes up roughly 1% of slab volume, well within the 10% waste allowance. For heavily reinforced footings, treat the rebar as zero volume rather than trying to subtract it.

What is a sonotube?

A cardboard tube used as a form for round concrete columns and footings. Pick the round-column or round-footing shape and use the tube's inside diameter.

Cubic yards or cubic meters?

Cubic yards in the US and Canada, cubic meters everywhere else. 1 cubic yard equals 0.7646 cubic meters. The calculator shows both so you can quote the correct unit when you call the supplier.

Concrete Calculator