Everyday
Concrete Calculator
Enter your slab, post, footing, step, or wall dimensions to get cubic yards, bag count, mixer batches, and a cost estimate before you buy.
How it works
Behind the numbers
Concrete volume is length times width times thickness, with inches converted to feet. The calculator handles four project shapes: a rectangular slab, round post holes or footings (cylinder volume), a wall, and a flight of steps, where each step is treated as a solid block 11 inches deep by 7 inches tall. Dividing cubic feet by 27 gives cubic yards, the unit ready-mix is sold in.
Bagged concrete is sold by weight, but what matters is the volume each bag fills when mixed. An 80 lb bag fills about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30, from published manufacturer yields. The calculator multiplies your volume by your waste allowance, divides by the bag yield, and rounds up, because you cannot buy part of a bag. Brand yields for the same bag size are close enough that one set of numbers covers them.
Bags make sense for small pours, and at some point a ready-mix truck costs less per yard and saves hours of mixing. A common rule of thumb is to price a truck once you reach about a cubic yard. This calculator shows both totals side by side with editable prices, so you can put in a local quote and decide for yourself.
FAQ
Questions, answered
How many 80 lb bags of concrete make a cubic yard?
A cubic yard is 27 cubic feet, and an 80 lb bag fills about 0.60 cubic feet, so 45 bags make one cubic yard before waste. With the standard 10% waste allowance, plan on 50 bags.
How many 60 lb bags of concrete are in a cubic yard?
A 60 lb bag fills about 0.45 cubic feet. 27 divided by 0.45 is 60 bags per cubic yard, or 66 with a 10% waste allowance.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, a 10 by 10 foot slab is 10 x 10 x (4/12) = 33.3 cubic feet, about 1.23 cubic yards. That works out to 62 bags of 80 lb concrete with 10% waste, or a small ready-mix load.
Should I buy bagged concrete or order a ready-mix truck?
Bags work for small pours like a few fence posts or a small pad. Once you reach about a cubic yard, which is 45-plus bags of 80 lb mix, a ready-mix truck usually costs less per yard and pours in minutes instead of hours. The calculator shows both totals side by side so you can compare with local prices.
How much extra concrete should I order for waste?
10% is the standard default, and it covers spillage, over-excavation, and slightly uneven forms. You can set it anywhere from 0 to 20% in the calculator.