Concrete calculator

Concrete is sold by the cubic yard from a ready-mix truck or by the bag at the store. This calculator gives you both, so you can see where bagged mix stops making sense.

How the calculator works

Every shape comes down to volume in cubic feet, divided by 27 to get cubic yards:

• Slab: length x width x thickness (inches / 12).
• Wall footing: length x footing width x footing depth.
• Round column or tube: 3.1416 x radius² x height, times the number of columns.
• Steps: each step is a block as wide as the stair, as deep as its tread and as tall as its height above the ground. The calculator adds them up, plus a top landing if you enter one.

Bag counts use the yields printed on Quikrete Concrete Mix: an 80 lb bag makes about 0.60 cu ft, a 60 lb bag 0.45 cu ft and a 40 lb bag 0.30 cu ft. That means about 45 bags of 80 lb mix per cubic yard.

A worked example

A 10 x 10 ft patio, 4 in thick, is 33.3 cu ft. With 10% extra that is 36.7 cu ft, or 1.36 cu yd. In bags, that is 62 bags of 80 lb mix or 82 bags of 60 lb mix.

Why add extra

Ground is never perfectly flat, forms bow a little and some concrete stays in the wheelbarrow. Most people add 5 to 10%. Running short in the middle of a pour leaves a cold joint, which is weaker and shows.

Bags or a truck?

Past about a yard, bags get heavy: a yard of 80 lb bags is 45 bags and well over a ton and a half to lift and mix. Ready-mix suppliers can deliver small loads, often with a short-load fee, so ask before you decide.

Sources

• Quikrete Concrete Mix (No. 1101) data sheet: bag yields 0.30, 0.375, 0.45 and 0.60 cu ft for 40, 50, 60 and 80 lb
• US Navy construction training manual: conventional concrete weighs about 150 lb per cubic foot

Sources last checked 1 November 2026.